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THE AM BULLETIN BOARD => Technical Forum => Topic started by: N0WVA on September 01, 2013, 11:28:23 PM



Title: National NCX3 Speaker Buzzing
Post by: N0WVA on September 01, 2013, 11:28:23 PM
So its an annoying buzz, and present with the 6aq5 pulled. The buzz starts as soon as the rig is turned on. It is being picked up through two capacitors c37 and c38. Lifting them makes it quiet. But then theres no VOX. Im about at the end of my knowledge here.


Title: Re: National NCX3 Speaker Buzzing
Post by: wa3dsp on September 02, 2013, 12:41:37 AM
I think you mean C97 and C98???   37 and 38 are trimmers in the VFO.

Anyhow.... C97 is a bypass on the plate to the screen for high frequencies and I would suspect it is the one that fixes the problem. C98 is the coupling to the antivox detector. It is unlikely there would be any buzz coming from there. Did you try just removing one cap at a time?

I suspect that there is noise, AC ripple on the supply and it is being coupled by C97. If you have a scope you could check for AC on the B+ line at the 6AQ5 screen.  You may need to replace the electrolytics in the power supply.

While you are recapping it would not be a bad idea to replace C97 and also C96A in the cathode of the 6AQ5. It is probably in a can now but if the rest of the can sections are OK you could disconnect it from the can and put a stand alone cap in.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 02, 2013, 08:50:04 AM
I think you mean C97 and C98???   37 and 38 are trimmers in the VFO.

Anyhow.... C97 is a bypass on the plate to the screen for high frequencies and I would suspect it is the one that fixes the problem. C98 is the coupling to the antivox detector. It is unlikely there would be any buzz coming from there. Did you try just removing one cap at a time?

I suspect that there is noise, AC ripple on the supply and it is being coupled by C97. If you have a scope you could check for AC on the B+ line at the 6AQ5 screen.  You may need to replace the electrolytics in the power supply.

While you are recapping it would not be a bad idea to replace C97 and also C96A in the cathode of the 6AQ5. It is probably in a can now but if the rest of the can sections are OK you could disconnect it from the can and put a stand alone cap in.

Yes c97 and c98! It was getting late last night, I must have misread the 3 for a 9.

Yep, I snipped both cap leads that go to the plate of the 6aq5. The buzzing stopped, but it continues when I replace either caps, they both contribute to the problem. I also tried temporarily replacing them...same thing.

I did remove the 280v wire that comes in from the PS and heads for the 0a2 and associated voltage divider, the noise then stopped!

I jumped the electrolytics in c96 with a brand new cap, no change.

This morning I see there is yet another filter cap in the PS on the 280 v line as you suggested. I will jump it with a cap and see what happens.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 02, 2013, 09:08:47 AM
Well, I got into the PS and then I remember I did a complete re-cap a couple years ago with brand new caps. I was trying to fix this problem then, to no avail. I did see a .01 bypass cap was blown in two, so I replaced it, same problem .

I have a B&K 30mhz scope here, but never had to use it on high voltages. Would I need a special probe?

Is it possible for a rectifier diode to not work properly?


Title: Re: National NCX3 Speaker Buzzing
Post by: wa3dsp on September 02, 2013, 09:24:19 AM
It could be a noisy rectifier or maybe the OA2 is oscillating. A scope would give you a better idea of what it is and where it is coming from. Your probe should work AC coupled. If in doubt you could use a .01 500V cap in series with the end of the probe.

You don't want to put too much capacity across the gas regulators or they will oscillate.

Does it sound like 60 or 120hz? The scope would tell you what the freq. is.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 02, 2013, 10:14:40 AM
Sounds like 60  to my ear, its pretty raspy. Turned the scope on and the trace flies off the screen, cant get it to show, only when I turn it on and off for a second. One thing leads to another.


Title: Re: National NCX3 Speaker Buzzing
Post by: gerry_w1id on September 02, 2013, 10:32:27 AM
Others have dispensed good advice on the subject but let me throw in something else. Perhaps there is a bad return ground somewhere in the system. The NCX-3 I used to own had riveted sockets. Maybe one or more of those have poor conductivity. Another problem I had was one of the filter chokes developed a mechanical resonance and had to be replaced. I supposed I could have tried adding varnish to secure the I lamination but I had a junker Eico 751 power supply for the infamous 753 and canibalized it. As an aside, there were many similarities between the NCX-3 and the 753 except the NCX-3 was way better.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 02, 2013, 10:35:36 AM
Ok, seeing how my scope is down, I just replaced the rectifier diodes, same problem.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 02, 2013, 10:41:27 AM
Others have dispensed good advice on the subject but let me throw in something else. Perhaps there is a bad return ground somewhere in the system. The NCX-3 I used to own had riveted sockets. Maybe one or more of those have poor conductivity. Another problem I had was one of the filter chokes developed a mechanical resonance and had to be replaced. I supposed I could have tried adding varnish to secure the I lamination but I had a junker Eico 751 power supply for the infamous 753 and canibalized it. As an aside, there were many similarities between the NCX-3 and the 753 except the NCX-3 was way better.

I was just looking at those very same rivets! WA3DSP recommended checking for oscillation from the OA2, but Im not sure how to do that, but will suspect the grounding rivets next. I might have to break out the Weller.

You mentioned the choke may be bad. What clued you in to replace it? Was it also making a mechanical buzz? My supply seems to be noisier than it needs to be when on a wooden table.

I also have a 753, and would like to do a complete tear down and rebuild, and shore up the VFO. Could you imagine getting rave comments on a 753?


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 02, 2013, 11:12:36 AM
Soldered a common ground wire to all ground terminals in the power supply.....speaker still buzzing.

Also, the same effect can be achieved by again desoldering c97 and c98 then grounding that side of the audio transformer with a .01 cap at the plate terminal. So there is some sort of potential being set up in radio chassis itself.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 02, 2013, 12:56:41 PM
Temporarily lifted L2 in the power supply and put in a 10 watt 1.2k resistor. everything is absolutely hum free now, perhaps it is the choke after all?


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 02, 2013, 01:37:50 PM
Substituted the choke with one just slightly smaller in physical size. Hum clears up considerably. Anyone have a direct replacement? Not sure of the specs, the manual doesnt mention them.


Title: Re: National NCX3 Speaker Buzzing
Post by: wa3dsp on September 02, 2013, 09:53:27 PM
I am not sure why the choke would cause the buzz. I can see it causing audible noise at the choke but its purpose is to improve filtering. Are you sure the electrolytic caps in the power supply are good? You really need to have a scope to check this out otherwise you are just shooting in the dark.


Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 03, 2013, 01:22:37 AM
Check the ground connections in the power supply.  You can measure the AC ripple on the B+ with a digital volt meter.  Just use the AC function, the meter will block the DC.  The AC component should be very low, maybe only a volt or less.  Make sure the VR tube does not have any filter caps on the output.  VR tubes shouldn't have more than about .05ufd on the output.

Make sure the rectifiers are good.  I'm thinking while I writing... If there are any caps across the AC line, get rid of them.  Seems like some ground is floating above ground.  Could be the whole chassis is floating above ground..... get rid of any AC caps across the incoming AC line.

You say it's a buzz, do you mean a hum??  Does the volume control affect the hum??

Another thing is the power xfmr or the choke may be leaking AC to the core.  This will put AC on the chassis.  Your set should have a three wire grounding AC cord.  If it doesn't, install one.  Do the testing without any antenna connected.

 Let us know what you find.  These problems are not a mystery, there is an answer.

I'm not familiar with the NCX3, but reading the posts, I guess it's a xmtr.

Fred  


Title: Re: National NCX3 Speaker Buzzing
Post by: Pete, WA2CWA on September 03, 2013, 03:20:55 AM
I'm not familiar with the NCX3, but reading the posts, I guess it's a xmtr.

Fred  

It was National's first attempt at a sweep tube transceiver. It covered 80, 40, and 20 meters. It had issues. National issued several bulletins on it. It even lacked a RIT control. AM sounded horrible on receive and transmit but I still have mine even after 40 years.

(http://images.marketplaceadvisor.channeladvisor.com/hi/15/15147/nationalncx3_unit1a.jpg)


Title: Re: National NCX3 Speaker Buzzing
Post by: flintstone mop on September 03, 2013, 06:52:10 AM
Be careful working with tube sets and the high voltage used inside.
"Jumping" the  filter caps is not a good way to troubleshoot the problem. You will have to lift one lead and get the substitute cap completely in the circuit.
If the buzz is heard when the volume is completely turned CCW; then I suspect something in the Power supply.
With your 'scope out of service, it's gonna be hit-and-miss to find the culprit. Sometime the AC bypass capacitors, where the power cord enters into the unit, can cause strange things in the radio.
Good to read that you have a schematic.
Fred


Title: Re: National NCX3 Speaker Buzzing
Post by: wa3dsp on September 03, 2013, 07:20:13 PM
I looked at the schematic again. When I looked at it last night I was tired. Anyhow it makes perfect sense with the tube removed if you added the cap coupling from the top of the output transformer you would couple the power supply noise across the primary of the transformer and hear it on the speaker. You definitely have noise (ripple) on your power supply. Possibly the choke is shorted and therefore would not be doing its filtering job. Putting in the resistor would have helped the filtering but definitely not the regulation. I think you should check and or replace the filter caps and check and/or replace the choke.


Title: Re: National NCX3 Speaker Buzzing
Post by: flintstone mop on September 04, 2013, 08:37:32 PM
Buzz in the speaker with the 6AQ5 (audio output) removed is definitely the power supply.


Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 04, 2013, 11:35:47 PM
Buzz in the speaker with the 6AQ5 (audio output) removed is definitely the power supply.

The output tube pulled results in no current through the output xfmr which should leave no audio in the speaker.  So, if there is ripple on the B+ it still wouldn't cause the hum to reach the speaker with the output tube removed.

The buzz is being caused by something but not sure what.  Should check wiring for a short on the output circuit wiring.

Fred


Title: Re: National NCX3 Speaker Buzzing
Post by: wa3dsp on September 05, 2013, 01:13:06 AM
>The output tube pulled results in no current through the output xfmr which should >leave no audio in the speaker.  So, if there is ripple on the B+ it still wouldn't cause >the hum to reach the speaker with the output tube removed.

Well not exactly true. B+ is on the bottom end of the audio output transformer and the two caps he mentioned are on the top[ side and would couple the energy across it. That is why removing the caps stopped the buzz.

So where is the scope? That would tell you whats up in an instant. 


Title: Re: National NCX3 Speaker Buzzing
Post by: flintstone mop on September 05, 2013, 09:52:42 AM
In my early early electronic days, removing the audio output tube and still hearing hum pointed right to the PS.
I think the OP's 'scope is not goodness and he can't see the problem. It will have to be old fashioned temporary connecting electrolytic caps into the PS and hopefully it goes away.


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 05, 2013, 12:19:10 PM
Ok, seeing how my scope is down, I just replaced the rectifier diodes, same problem.

I agree with all of the other comments. Every symptom and every experiment you have reported on here strongly suggests that there is 60Hz (and/or 120Hz) hum on the 280V (nominal) power supply output.

If the full wave rectifier for the 280V supply is not balanced, then one would expect a large amount of extra 60Hz at the input to L2.

The could be particularly significant if one of the diodes on the right side of the 4-diode bridge rectifier was installed backwards.


From the BAMA schematic, the power supply uses a single center tapped HV transformer to drive two separate rectifiers that share some of the same diodes. One is a bridge rectifier that (of course) employs all four diodes to produce the +700V (nominal) HV for the transmitter. The other is a full wave rectifier that uses the center tap of the transformer, and also shares two of the diodes from the HV bridge rectifier.

I suggest that you check (again) that all four diodes are: of the same type (to achieve good 60Hz balance), and not open (i.e. each diode, when measured individually, should conduct in one direction).

If you installed one of these diodes backwards, it could quickly be damaged by excess current... and open up, or worse still for hum, if it didn't open up, it would produce lots of extra 60Hz ripple on the power supplies. Therefore, make sure that you installed each of the new diodes in the correct direction. You should use the power supply schematic available from BAMA. You should not assume that the diodes were in the correct directions when you started your trouble shooting.

http://bama.edebris.com/manuals/national/ncx3/

The power supply schematic is on the 10th page of the 3rd file in the list of files: NCX3_schematic_notes

If the diodes are all good and installed in the correct directions, then I suggest that you try the following (carefully... letting the capacitors bleed down). Leaving everything else in stock configuration... disconnect the 2 diodes that connect to the left side of L1.

If the hum goes away when you disconnect those two diodes... then you may want to check (again) that all four diodes are good, of the same type (you might want to use 1N4007's if you haven't already used those), and pointed in the correct directions.

Good luck
Stu



Title: Re: National NCX3 Speaker Buzzing
Post by: Pete, WA2CWA on September 05, 2013, 01:55:03 PM
While I agree that the problem sounds like it's power supply related, I would not overlook inspecting the cinch plug connections from the power supply/speaker cabinet. That collection of wires in the cable include high voltage, low voltage, filaments, speaker, and AC connections. The cable is relatively stiff and intermittent or major type problems at the pins is not uncommon. I would also check the matting connector connections in the NCX-3. I had a similar type speaker buzzing sound problem with my NCX-5 years ago. The problem was found when I ran a separate ground connection from the power supply/speaker cabinet to the chassis of the NCX-5. Buzzing stopped. I pulled the protective hat off the power supply cable and re-soldered all the pin connections. The problem went away.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 05, 2013, 08:33:49 PM
Thank you for all the suggestions everyone, and also taking the time to analyze my situation.

My scope is still down, I need to get it up and running again, it worked last time I used it!

This radio has had the annoying buzz/hum since being bought 5 years or so ago. After putting it back on the operating bench, I am sort of picking up where I left off a few years back. Taking the power supply apart, I then realized that I did indeed replace those old filter capacitors with brand new ones, although they were computer type in series, and I put them in under the chassis. And I done a pretty good job, if I say so myself. All three supply caps were replaced ( the -45 , 280, and 700v lines). And I remember being frustrated after all that work that it didnt eliminate the problem back then, so I just run it like that ( annoying as it is on RX, hardly detectable on TX).

But I did NOT replace the three section electrolytic in the radio itself which goes to the OA2. So I did solder some good capacitors of value underneath, but it didnt change a thing.

I have replaced all the rectifier diodes, taking care to get them oriented properly, same noise afterward.

Also I have run a common wire to each of the grounds on all the terminals.

The only thing that has improved the situation so far has been messing with the choke in the 280v line in the PS. Subbing the resistor for the choke got rid of the problem completely, but I know I cant be using that because of voltage drop. I did replace the choke with one of slightly smaller physical size and the hum went down to about half or more. I think it may not be enough henries to do the job completely.

I feel bad for just "throwing parts" at it, as thats not the way of a good tech and realize I should just get my scope going. 

So I will do that and check back here to let you all know whats going on with the NCX3 hopefully in a few days. I am going to take a break from it as its driving me crazy.

Thanks again and I will be back.


Title: Re: National NCX3 Speaker Buzzing
Post by: wa3dsp on September 05, 2013, 09:07:27 PM
Did you measure the resistance of the original choke? I suppose it could be shorted which would negate its effect. I don't know the value and I have not seen the choke so I can't guess what a right resistance would be but I would expect it to be something greater than 30-50 ohms.


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 05, 2013, 11:52:10 PM

I did remove the 280v wire that comes in from the PS and heads for the 0a2 and associated voltage divider, the noise then stopped!


The voltage divider chain leading to the OA2 is drawing 80ma (I.e. a 60V drop across the 750 ohm resistor) from the capacitors across the 280V supply. This current would cause a 120 Hz voltage ripple having amplitude of 8.3V across a parallel pair of 40uF capacitors. The choke L2 should be providing a steady charging current to offset this discharging current, and therefore to reduce this ripple.

If disconnecting the voltage divider chain makes the hum go away, this would confirm that L2 is not doing its job. I.e. it doesn't have enough inductance to provide a smooth charging current of approximately 100mA to the parallel pair of 40 uF capacitors.

You could reduce the ripple by increasing the inductance and/or adding more capacitance (e.g. adding 100uF) across the 280V supply.

Separately, if the hum is at 60Hz (rather than 120 Hz), there is almost certainly  a defective diode, or an improperly installed diode in the 4-diode bridge.

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 06, 2013, 05:27:04 AM
Didn't know that the set has interconnecting Cinch plugs.  I'm not familiar with the NCX3 but if the interconnecting cables uses a single ground wire for filament, B+ and the speaker return, this can be a problem.  The speaker should have a separate set of wires, to the spk and back to the output xfmr.

I have to look at the schematic, but from what Stu has mentioned, this set uses a single set of 4 FW bridge rectifiers.  The center tap of the HV xfmr is where 1/2 the HV is pulled from.  So, you get 700 volts from the output of the FWB rectifiers and the lower 350 volt B+ from the xfmr CT.  Each of these two B+ outputs needs to be filtered separately.

Fred


Title: Re: National NCX3 Speaker Buzzing
Post by: MikeKE0ZUinkcmo on September 06, 2013, 07:00:51 AM
Here are some schematics of the subject.

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/NCX-3/NCXA1_zps9ae4896d.jpg)  (http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/NCX-3/NCXA2_zps5d350f43.jpg)

As mentioned, the two units are interconnected with about a 3' cable, partially visible in this picture

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/NCX-3/NCXA3_zps73b59a30.jpg)


Title: Re: National NCX3 Speaker Buzzing
Post by: wa3dsp on September 06, 2013, 12:27:34 PM
It looks like there is a good amount of filtering - 40uf in the supply and another 40uf in the transceiver on the 280V line.

Both the low and high voltages are choke input. Specs are shown for L2 on the schematic but not L1 in the low voltage supply.

If you want to be sure that this is not being caused by the regulator pull when end of R107 which feeds B+ to it and see what happens. I doubt it is causing the problem but easy to try.  If the choke were shorted I would expect the B+ to be on the high side, possibly 330 volts or more. If you have a good digital meter you could measure the AC ripple on these lines. I would expect it to be less than a volt and preferably <100 millivolts on the low voltage lines.


Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 06, 2013, 12:54:02 PM
Mike,

Thanks for posting the schematics.  Take a close look at the Cinch connector on the PS.  Seems that the spk ground return is jump to the main ground return even though it shows having a separate pin connection on the other connector.  I have to take another look (can't do it while I'm typing) but there something there that needs a closer look.  Running the spk grd return with the filament return will cause a problem.

Fred



Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 06, 2013, 12:59:13 PM
Mike,

Just took another look,  maybe that jumped ground is at the return chassis end.  Could be the cables need to be checked for any problems or mods.

Fred


Title: Re: National NCX3 Speaker Buzzing
Post by: Pete, WA2CWA on September 06, 2013, 01:11:41 PM
That's why I posted to check the interconnecting cable plug on the power supply and the mate in the NCX-3. This type of wiring harness and typical connections were very common on a a number of HF and VHF transceivers back in the 60's and 70's. The cable stiffness exerts a lot of stress on the pins and the soldered connections. In almost all cases, there is no strain relief(clips, etc.) to hold the cable plug firmly into the mating connection on the rig. Poor soldered connections, oxidation or crud on the pins, and, since all the individual cable leads are stranded wire, slivers of stranded wire can bridge or break, causing ground connections to be marginalized. I suspect a redesign of the power supply is not necessary to solve the problem.


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 06, 2013, 01:16:33 PM
Look at the schematics that Mike posted above.

The voltage across C3 should be about 350V (I.e. about half of the 700V B+, as Fred points out).

The voltage across C96B is supposed to be 280V.

Therefore the total current being drawn by the transceiver from the 350V supply is: (350V - 280V)/ 660 ohms = approximately 100mA

Likewise, the current flowing through R106 is supposed to be (280V - 220V) / 1000 ohms = 60mA.

Likewise, the current flowing into the voltage regulator is supposed to be (220V - 150V) / 2500 ohms = 28mA.

Therefore, if you want to partially unload the 350V power supply, to see if that reduces the ripple/hum, you should disconnect one end of R106 (rather than R107).

If the inductor L2 is not doing its job, then disconnecting one end of R106 should reduce the ripple/hum on the 280V (nominal) line.

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 06, 2013, 01:31:16 PM
That's why I posted to check the interconnecting cable plug on the power supply and the mate in the NCX-3. This type of wiring harness and typical connections were very common on a a number of HF and VHF transceivers back in the 60's and 70's. The cable stiffness exerts a lot of stress on the pins and the soldered connections. In almost all cases, there is no strain relief(clips, etc.) to hold the cable plug firmly into the mating connection on the rig. Poor soldered connections, oxidation or crud on the pins, and, since all the individual cable leads are stranded wire, slivers of stranded wire can bridge or break, causing ground connections to be marginalized. I suspect a redesign of the power supply is not necessary to solve the problem.

Pete

This is good advice, but if a loose ground is the problem, it is hard for me to see why replacing L2 with a different choke would reduce the hum; or why some of the other tests that were done would produce the results that were posted.

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: Pete, WA2CWA on September 06, 2013, 01:32:34 PM
I just pulled my data on the NCX-A power supply. Looks like there was at least three different series numbers assigned to the product over its life but all circuitry looks the same. The basic circuit design of the NCX-A P.S./Speaker was used for the NCX-3, NCX-5, NCX-200, and the NCX-500.

For L1 Specs. - Swinging Choke - 60 ohms DC, 25/300 MA, 12/2 HY
For L2 Specs. - Filter Choke - 120 ohms DC, 200 MA, 8 HY

In the past, I've used a Clegg Venus P.S./speaker and the Swan 117-X P.S./speaker in place of the NCX-A. The designs were pretty much identical.


Title: Re: National NCX3 Speaker Buzzing
Post by: Pete, WA2CWA on September 06, 2013, 01:52:04 PM
That's why I posted to check the interconnecting cable plug on the power supply and the mate in the NCX-3. This type of wiring harness and typical connections were very common on a a number of HF and VHF transceivers back in the 60's and 70's. The cable stiffness exerts a lot of stress on the pins and the soldered connections. In almost all cases, there is no strain relief(clips, etc.) to hold the cable plug firmly into the mating connection on the rig. Poor soldered connections, oxidation or crud on the pins, and, since all the individual cable leads are stranded wire, slivers of stranded wire can bridge or break, causing ground connections to be marginalized. I suspect a redesign of the power supply is not necessary to solve the problem.

Pete

This is good advice, but if a loose ground is the problem, it is hard for me to see why replacing L2 with a different choke would reduce the hum; or why some of the other tests that were done would produce the results that were posted.

Stu

I'm only saying, based on my past experiences with a number of NCX-3's, 5's, 200's, and 500's, Clegg Venus, and various Swans, any speaker hum/buzzing, or similar anomaly, was found to be related to interconnection between the rig and the power supply/speaker. A simple way to test the integrity of the power supply/speaker and cable would be to derive the necessary receiver voltages from some other source (another power supply/another rig, etc.) using the classic clip lead approach directly to the NCX-3. The high voltage connection would not be necessary for this test. A separate speaker would also be necessary. The circuit design of the NCX-A power supply/speaker was around for years. While component breakdown is still a possible solution, sometimes a more obvious solution is sometimes overlooked.


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 06, 2013, 02:33:10 PM
Pete

Yes... we are, I believe, in 100% agreement. Sometimes the simple solution is overlooked... Like a cable with a poorly soldered connection.

Separately, 8H sounds about right for 100mA and 350V. I wonder if the choke that is in the power supply is even the right choke (maybe replaced some time in the past with a smaller value choke)

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: gerry_w1id on September 06, 2013, 07:12:50 PM
I have been asked how I knew the filter choke had a loose lamination. I was referring to the LV choke and the HV choke. There was a persistent loud buzzing which I identified as a mechanical resonance of some kind. By listening closely, it was determined to come from the LV choke though I didn't know the cause. So I took the choke out and measured 2.4H and about 45 Ohms DCR. Visual inspection showed the I lamination was loose and was therefore the source of the loud buzzing. Rather than fix this choke I replaced it with a 10H potted unit rated for 150 ma continuous current. At the same time, I relocated the new choke away from the venting slots over the B+ bleeder resistors. The original choke location did not allow the HV bleeder resistors to properly cool and contributed to overheating the low B+ choke. Since the new choke had more DC resistance than the original, only one of the 330 Ohm 10W  resistors (either R5 or R6) was used. As I went through the bottom of the chassis, I also noticed some of the riveted terminal strips had gotten loose. Unfortunately, the ground lug of the terminal strip to which the ground wire going to the transceiver was soldered to was loose. At this point, I made a decision to drill out all the power supply rivets and secure the terminal strips with 6-32 hardware. As I mentioned previously, there are a lot of similarities between the NCX-3 and the Eico 753. A couple of articles have been published on improving the 753 but they do not address the fundamental problem: there is only one VFO inductor. Consequently calibration will not track for all 3 bands. You can calibrate the VFO to read accurately on either 80 and 20 but not 40 and vice versa.


Title: Re: National NCX3 Speaker Buzzing
Post by: MikeKE0ZUinkcmo on September 06, 2013, 07:27:00 PM
 The National P/Ns B50863(swinging choke), B50864 may be stamped on the top cover.   Don't have any idea who they bought them from but Carl (KM1H) probably knows.   if the P/Ns are on the chokes it would at least answer your questions about replacements.



Title: Re: National NCX3 Speaker Buzzing
Post by: Pete, WA2CWA on September 06, 2013, 09:01:25 PM
The National P/Ns B50863(swinging choke), B50864 may be stamped on the top cover.   Don't have any idea who they bought them from but Carl (KM1H) probably knows.   if the P/Ns are on the chokes it would at least answer your questions about replacements.


Actually, the swinging choke could be B508063 or B51781 or B52068 or something else.
The filter choke could be B50864 or B52067 or something else.


Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 07, 2013, 04:46:34 AM
I went back and re-read the details of what the OP has told us so far.  Two very important details, 1) he claims the buzz starts as soon as the set is turned on. 2)  he claims the buzz in there even with the output tube (6aq5) removed.

These two points tell me that the problem is not the power supply.  If you look close at the way the Cinch plug on the main chassis is wired.  The spk ground return and the ground pin are jumped together and then grounded to the chassis.  If the ground connection to the chassis is not 100%, what would happen?  Some ground current would travel through the spk coil to the secondary of the output xfmr, through the secondary winding to ground.  This would cause the buzz to begin as soon as the set is turned on and it would still be there with the output tube removed.

Fred

       


Title: Re: National NCX3 Speaker Buzzing
Post by: MikeKE0ZUinkcmo on September 07, 2013, 08:03:45 AM
Pete said;

Quote
Actually, the swinging choke could be B508063 or B51781 or B52068 or something else.   The filter choke could be B50864 or B52067 or something else.

As you know, the other numbers you mention depend upon which power supply you're looking at.   The AC-200 and AC-500 supplies were for the NCX-200 and NCX-500 transceivers respectively, and provided significantly more power than the NCX-A, shipped with the NCX-3s or NCX-5s.

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/NCX-500/NCX50000.jpg)

Fred,

I think your hypothesis may very well be correct, and the input/output grounding points at both the radio and power supply, as well as the cable itself, should be thoroughly checked.


Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 07, 2013, 11:28:53 AM
>The output tube pulled results in no current through the output xfmr which should >leave no audio in the speaker.  So, if there is ripple on the B+ it still wouldn't cause >the hum to reach the speaker with the output tube removed.

Well not exactly true. B+ is on the bottom end of the audio output transformer and the two caps he mentioned are on the top[ side and would couple the energy across it. That is why removing the caps stopped the buzz.

So where is the scope? That would tell you whats up in an instant. 


What you're saying may also be correct.  The set uses solid state rectifiers, so the ripple (if any) would be present at start-up (no time delay waiting for tube rectifiers).  I also see where those caps could possible cause some energy to appear across the output xfmr with the output tube removed.  I'll have to take a closer look at the circuit around the 6aq5.  That cap between the plate and screen should be a rather small value, only used to limit higher freq audio.

Fred


Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 07, 2013, 11:39:53 AM
I looked at the schematic,  that .01 cap to the vox circuit would caused some ripple current to flow through the output xfmr all the time, even with the 6aq5 removed.  So, the problem may very well be poor filtering in the PS.

I guess we'll have to wait for the OP to tell us more.

Fred


Title: Re: National NCX3 Speaker Buzzing
Post by: pa0ast on September 07, 2013, 11:45:55 AM
How about a magnetic coupling betwee the trafo and the coil. Is the position of the coil or the trafo changed in the past ?
Anton


Title: Re: National NCX3 Speaker Buzzing
Post by: Pete, WA2CWA on September 07, 2013, 01:44:42 PM
Pete said;

Quote
Actually, the swinging choke could be B508063 or B51781 or B52068 or something else.   The filter choke could be B50864 or B52067 or something else.

As you know, the other numbers you mention depend upon which power supply you're looking at.   The AC-200 and AC-500 supplies were for the NCX-200 and NCX-500 transceivers respectively, and provided significantly more power than the NCX-A, shipped with the NCX-3s or NCX-5s.


Actually, the part numbers referenced in my previous post came directly from several original NCX-A manuals (tagged with different series numbers). And, if you review the NCX-A and AC-200 schematics and power supply output specs, they are virtually identical.  The NCX AC-500 uses pretty much the same parts as the other supplies except they moved both chokes (swinging and filter) to the low voltage leg, added an additional filter cap and bleeder to the high voltage side, and some other minor things. I ran a NCX-500 with an NCX-A for several years. The only difference was 700 volts on the HV side versus 1100 volts and a minor difference in the bias voltage. I even ran my NCX-5 with a Clegg Venus power supply for awhile until I found another NCX-A and it was only acquired for cabinet matching purposes.


Title: Re: National NCX3 Speaker Buzzing
Post by: MikeKE0ZUinkcmo on September 07, 2013, 07:03:49 PM
Pete said;

Quote
And, if you review the NCX-A and AC-200 schematics and power supply output specs, they are virtually identical.

Ooops, you're right.   The only significant difference is the 115/230 primary and switch in the AC-200 and 500.   Aside from the higher B+ the AC-500 didn't have an internal speaker either.

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/NCX-500/AC-500PS_zps189d0aee.jpg)

I also ran mostly National radios for some time, but had the associated power supplies for each. 
(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/Station%20KE0ZU/All%20National/NATStation1.jpg)


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 08, 2013, 10:30:20 AM
N0WVA wrote:
"This radio has had the annoying buzz/hum since being bought 5 years or so ago. After putting it back on the operating bench, I am sort of picking up where I left off a few years back. Taking the power supply apart, I then realized that I did indeed replace those old filter capacitors with brand new ones, although they were computer type in series, and I put them in under the chassis. And I done a pretty good job, if I say so myself. All three supply caps were replaced ( the -45 , 280, and 700v lines). And I remember being frustrated after all that work that it didnt eliminate the problem back then, so I just run it like that ( annoying as it is on RX, hardly detectable on TX).

But I did NOT replace the three section electrolytic in the radio itself which goes to the OA2. So I did solder some good capacitors of value underneath, but it didnt change a thing."

Note:

If the old electrolytic capacitors are leaky, then putting new capacitors in parallel with them may not result in an improvement in their effectiveness as charge storage devices.
You might want to be sure that you have disconnected one end (or both ends) of each old capacitor that has been "replaced".

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: WD5JKO on September 08, 2013, 11:10:43 AM

If the hum comes on immediately with or without tubes present, or before the tubes warm up, then there is either magnetic coupling, or there is high ripple between the 220v DC screen supply (6AQ5) and the 280V source for the screen supply. Removing cap C97 eliminating the buzz confirms this. The buzz must therefore be high amplitude and narrow pulse width, and likely at a 120 hz rate. Too bad the scope is crapped out as this could confirm the problem.

My bet is the diodes have a different reverse recovery time then the ancient originals and they are shorting the transformer secondary for a tiny amount of time each half cycle, perhaps 100 us every zero crossing. When the slow to turn off rectifiers in the bridge turn off, the transformer generates a back emf pulse so narrow in pulse width, that it goes right through L2 unimpeded.

So as a test, try taking a series R-C snubber from the transformer secondary center tap to ground. Try something like a 3K 2 watt (not critical) and a .01 uf 1000 V capacitor...See if it has any effect. If so, perhaps try different values, or another R-C across the entire transformer secondary. The Cap would need to be rated higher there, at least 2 kv.

I cover the diode hash / buzz issue in more detail here in yesterday's post:
http://amfone.net/Amforum/index.php?topic=34793.0

Jim
WD5JKO


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 08, 2013, 11:24:27 AM
So its an annoying buzz, and present with the 6aq5 pulled. The buzz starts as soon as the rig is turned on. It is being picked up through two capacitors c37 and c38. Lifting them makes it quiet. But then theres no VOX. Im about at the end of my knowledge here.

As has been previously posted, these capacitors are C97 and C98. They are both supposed to be 0.01uF capacitors. Therefore, the impedance of each should be 132k ohms at 120Hz and 265k ohms at 60Hz.

If the audio output tube is disconnected, either of these capacitors could (as previously posted) conduct current from the 280V B+ through the primary of the audio output transformer to ground (if the capacitor is connected between the top of the primary and ground)

However, given the high impedance of either capacitor at 60Hz and 120Hz, any hum between the 280V B+ line and ground should produce only a small current through the primary of the audio output transformer. The hum should be greatly attenuated because the input impedance of the audio output transformer (with the speaker attached to the secondary of the transformer) should be much less than the impedance of the capacitor.

Nevertheless, N0WVA reports that the hum goes away when both of these capacitors are disconnected at one end (with or without the audio output tube in place).

I suggest the following experiment:

Disconnect both ends of the audio output transformer's primary (i.e. disconnect everything connected to the top side of the primary, and disconnect the 280V B+ from the bottom of the primary). Place one of the 0.01uF capacitors across the primary.

This will result in a 133k ohm load across the primary (at 120Hz), and a corresponding much lower impedance looking into the secondary (where the speaker is connected). I.e. the impedance looking into the secondary of the audio transformer at 120Hz will be 133k ohms divided by the square of the transformer's turns ratio (ignoring the transformer's magnetizing inductance).

If there is a 60Hz voltage from the filament supply being coupled directly into the speaker leads (e.g. due to some problem in the cabling between the power supply/speaker and the transceiver)... and if that is the cause of the hum that is being heard, then the hum should still be present with the primary of the audio output transformer having only a .01uF capacitor across it (nothing else connected to either end). It should disappear (or be greatly reduced) when the .01uF capacitor across the primary is removed.

If there is no hum when the capacitor is across the primary of the audio output transformer (with nothing else connected to the primary) then try placing a short circuit across the primary of the audio output transformer (with nothing else connected to the primary). If there is still no hum, then it is unlikely that the hum is associated with 60Hz (e.g. filament voltage/current) being coupled from the filament supply into the speaker leads inside the cable between the power supply/speaker and the transceiver.

Given N0WVA's reported result that the hum goes away when L2 is replaced with a 1.2k ohm resistor, I would (again) be surprised if the hum is being introduced in the cable between the power supply/speaker and the transceiver.

 Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 08, 2013, 12:23:54 PM
Another thought:

Are you sure that the hum is coming from the speaker... and not from a transformer or a choke vibrating?

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 08, 2013, 12:44:13 PM
Ok, I got the B&K 1474 scope going again, I just had to clean the edge connectors joining the pc boards.

I have a .01 1kv disc in series with my probe, the probe is set on 1X, aliigator clip grounded to the supply, and the scope voltage input is on 5v. With radio on and in receive, power supply open and flipped upside down in front of me, and checking pin 9 on the cinch connector ( 280v line) I get a very clean signal with no noticeable AC until I get down to .1 on the voltage switch and even then its hardly a ripple. So for reference I checked at the junction of L2 and c3, before the 2 330 ohm 10 watt resistors, and then it showed about 4v ac ripple. So a little more ripple before the two 330 ohm resistors and regulator circuitry, which is to be expected, but it looks clean at pin 9 with radio running.

Also I used a separate ground wire  between radio and PS, it did change the hum a little, but its still there.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 08, 2013, 12:56:21 PM


So as a test, try taking a series R-C snubber from the transformer secondary center tap to ground. Try something like a 3K 2 watt (not critical) and a .01 uf 1000 V capacitor...See if it has any effect. If so, perhaps try different values, or another R-C across the entire transformer secondary. The Cap would need to be rated higher there, at least 2 kv.

I cover the diode hash / buzz issue in more detail here in yesterday's post:
http://amfone.net/Amforum/index.php?topic=34793.0

Jim
WD5JKO

Tried this test with a 1.5k/.01  disc. With receiver on, grounding the .01 made no difference in hum level.


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 08, 2013, 12:58:06 PM
When you looked at the voltage across C3 (4 volts ac):

What was the frequency (60Hz repetition rate or 120Hz repetition rate)?

Did it look like a sine wave or a saw tooth wave?

Was 4V the peak-to-peak voltage?

Was the scope input coupling set to DC (sometimes AC coupling will introduce a high pass filter at the input to the scope that will attenuate low frequencies, and produce an inaccurate reading). With the .01uF capacitor present, DC will be blocked, and it will be okay to have the scope set for DC coupling on its input.

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 08, 2013, 01:15:34 PM
When you looked at the voltage across C3 (4 volts ac):

What was the frequency (60Hz repetition rate or 120Hz repetition rate)?

Did it look like a sine wave or a saw tooth wave?

Was 4V the peak-to-peak voltage?

Was the scope input coupling set to DC (sometimes AC coupling will introduce a high pass filter at the input to the scope that will attenuate low frequencies, and produce an inaccurate reading). With the .01uF capacitor present, DC will be blocked, and it will be okay to have the scope set for DC coupling on its input.

Stu
It was a nice clean sine wave, around 4v p2p.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 08, 2013, 01:20:19 PM
Another thought:

Are you sure that the hum is coming from the speaker... and not from a transformer or a choke vibrating?

Stu

There is some vibration, but the previous owner put a 1/8 headphone plug in the back of the PS, I am using phones to monitor.


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 08, 2013, 01:25:33 PM
So its an annoying buzz, and present with the 6aq5 pulled. The buzz starts as soon as the rig is turned on. It is being picked up through two capacitors c37 and c38. Lifting them makes it quiet. But then theres no VOX. Im about at the end of my knowledge here.

As has been previously posted, these capacitors are C97 and C98. They are both supposed to be 0.01uF capacitors. Therefore, the impedance of each should be 132k ohms at 120Hz and 265k ohms at 60Hz.

If the audio output tube is disconnected, either of these capacitors could (as previously posted) conduct current from the 280V B+ through the primary of the audio output transformer to ground (if the capacitor is connected between the top of the primary and ground)

However, given the high impedance of either capacitor at 60Hz and 120Hz, any hum between the 280V B+ line and ground should produce only a small current through the primary of the audio output transformer. The hum should be greatly attenuated because the input impedance of the audio output transformer (with the speaker attached to the secondary of the transformer) should be much less than the impedance of the capacitor.

Nevertheless, N0WVA reports that the hum goes away when both of these capacitors are disconnected at one end (with or without the audio output tube in place).

I suggest the following experiment:

Disconnect both ends of the audio output transformer's primary (i.e. disconnect everything connected to the top side of the primary, and disconnect the 280V B+ from the bottom of the primary). Place one of the 0.01uF capacitors across the primary.

This will result in a 133k ohm load across the primary (at 120Hz), and a corresponding much lower impedance looking into the secondary (where the speaker is connected). I.e. the impedance looking into the secondary of the audio transformer at 120Hz will be 133k ohms divided by the square of the transformer's turns ratio (ignoring the transformer's magnetizing inductance).

If there is a 60Hz voltage from the filament supply being coupled directly into the speaker leads (e.g. due to some problem in the cabling between the power supply/speaker and the transceiver)... and if that is the cause of the hum that is being heard, then the hum should still be present with the primary of the audio output transformer having only a .01uF capacitor across it (nothing else connected to either end). It should disappear (or be greatly reduced) when the .01uF capacitor across the primary is removed.

If there is no hum when the capacitor is across the primary of the audio output transformer (with nothing else connected to the primary) then try placing a short circuit across the primary of the audio output transformer (with nothing else connected to the primary). If there is still no hum, then it is unlikely that the hum is associated with 60Hz (e.g. filament voltage/current) being coupled from the filament supply into the speaker leads inside the cable between the power supply/speaker and the transceiver.

Given N0WVA's reported result that the hum goes away when L2 is replaced with a 1.2k ohm resistor, I would (again) be surprised if the hum is being introduced in the cable between the power supply/speaker and the transceiver.

 Stu

With only the .01 cap across the primary there is hum in the phones/speaker. It goes away when the cap is disconnected.

But just as we are getting this thing nailed down and Im typing this, I just heard the supply bog down and it blew a fuse. Now I get tot troubleshoot two things at once.



Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 08, 2013, 02:30:41 PM
When you looked at the voltage across C3 (4 volts ac):

What was the frequency (60Hz repetition rate or 120Hz repetition rate)?

Did it look like a sine wave or a saw tooth wave?

Was 4V the peak-to-peak voltage?

Was the scope input coupling set to DC (sometimes AC coupling will introduce a high pass filter at the input to the scope that will attenuate low frequencies, and produce an inaccurate reading). With the .01uF capacitor present, DC will be blocked, and it will be okay to have the scope set for DC coupling on its input.

Stu
It was a nice clean sine wave, around 4v p2p.

4V p2p across C3 seems about right... given the current that is being drawn from this point by the load (around 100mA). The fact that it looks like a sine wave (rather than a sawtooth wave) suggests that the choke is doing its job. If the choke were shorted, the waveform across 40uF capacitor C3 would look like a saw tooth with a peak to peak amplitude of around 21V. I.e. if the choke were shorted, and since Q=CV, the voltage across C3 would drop roughly linearly by:  0.1A x (1/120)s x (1/0.000040F) every (1/120)s, and then jump back up to full value when the diode bridge conducts.

The ripple from pin 9 to ground should be around 5% of the ripple across C3, if the ripple frequency is 120Hz. I.e. the impedance of the capacitor from pin 9 to ground, at 120Hz, is around -j33 ohms. With the pair of 330 ohm resistors in series, the capacitor from pin 9 to ground works in conjunction with the resistors to form a 33/660 = .05 voltage divider that attenuates the 120 Hz ripple.

Therefore, if you see a 4V peak-to-peak 120Hz sine wave across C3, you should see a .2V peak to peak sine wave from pin 9 to ground.

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 08, 2013, 03:37:59 PM
Another thought:

Are you sure that the hum is coming from the speaker... and not from a transformer or a choke vibrating?

Stu

There is some vibration, but the previous owner put a 1/8 headphone plug in the back of the PS, I am using phones to monitor.

In the schematic, there is a headphone jack in the transceiver itself. I think you are saying that someone installed an additional headphone jack inside the power supply. Please clarify... is this additional headphone jack across the two speaker wires in the power supply, but isolated from power supply ground; or does it introduce a path inside of the power supply that connects one side of the speaker to power supply ground?

If you connect one side of the speaker to ground inside the power supply... that could be the problem.  

Even with a wire (having some amount of resistance) running between transceiver ground and power supply ground... the filament current flowing between the transceiver ground and the power supply ground will cause a 60Hz voltage to exist between the transceiver ground and the power supply ground. There also will be a 60Hz voltage magnetically induced around any closed loop that encloses the power supply's transformer and the transceiver.

One side of the speaker (located in the power supply) is connected (via a wire in the cable) to transceiver ground, at the point where the cable connector is located on the back of the transceiver. The audio output transformer is located in the transceiver. One side of the audio output transformer's secondary winding is also connected to transceiver ground. The other side of the audio transformer's secondary winding is connected to the other side of the speaker (via a wire in the cable). There is no direct connection between either side of the speaker and power supply ground.

If one adds a path to power supply ground from the same side of the speaker that connects (via the cable) to transceiver ground... then you will have 60Hz current flowing through the speaker (provided there is some load across the primary of the audio output transformer). I.e. 60Hz current will flow from power supply ground, to the grounded side of the speaker, through the speaker, across the cable to the audio output transformer's secondary winding, through the audio output transformer's secondary winding, to transceiver ground, and back to the power supply ground via the path through the effective 60Hz voltage source that exists between power supply ground and transceiver ground.

Stu



Title: Re: National NCX3 Speaker Buzzing
Post by: AB2EZ on September 08, 2013, 05:30:58 PM
The wiring of the added headphone jack might explain why replacing L2 with a 1.2k ohm resistor made the hum go away.

If the associated wiring was not done using a balanced - twisted pair approach, then there might be 60Hz being induced into the added headphone jack wiring by the time varying magnetic field of L2.

It would probably be best to remove this added headphone jack, and to return the wiring between the speaker and the multi-pin connector on the back of the power supply to stock form (using a twisted pair of wires).

Stu


Title: Re: National NCX3 Speaker Buzzing
Post by: N0WVA on September 08, 2013, 08:20:51 PM
The wiring of the added headphone jack might explain why replacing L2 with a 1.2k ohm resistor made the hum go away.

If the associated wiring was not done using a balanced - twisted pair approach, then there might be 60Hz being induced into the added headphone jack wiring by the time varying magnetic field of L2.

It would probably be best to remove this added headphone jack, and to return the wiring between the speaker and the multi-pin connector on the back of the power supply to stock form (using a twisted pair of wires).

Stu

And there it is.......The previous owner put a 1/8 headphone in the power supply/speaker unit. Disconnecting the wire that is grounded to the jack solved the problem. Well, I pulled the whole mess out of there and put the original choke back in....The only hum is a slight buzz from the choke, not really objectionable compared to the raspy speaker buzz I was getting earlier.

I can believe this run me in such a large circle.

 Thanks to everyone who responded here, if nothing else, I at least learned a few things from all your knowledge.


Title: Re: National NCX3 Speaker Buzzing
Post by: KA2DZT on September 09, 2013, 12:00:21 AM

 Thanks to everyone who responded here, if nothing else, I at least learned a few things from all your knowledge.

Study, because we'll be given you a test on Friday.

Seems the problem proved to be a grounding issue or as in this case, the ground from the added headphone jack.

Fred


Title: Re: National NCX3 Speaker Buzzing
Post by: MikeKE0ZUinkcmo on September 09, 2013, 12:43:19 AM
Fred,

If you really want to keep the head phones, just use an insulated phone jack and things will be fine.    You just want to keep the speaker/phones ground return line separate from the PS chassis ground.   What you were hearing was the AC voltage drop across the PS to radio cable ground wire.   The buzz was from the load of all the filaments in the radio.

If you connect your scopes ground to the radio chassis, and put the probe(no ground) on the power supply chassis, you'll see the AC.

Good detective work Stu.

Love them ground loops. ;)
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands