National NCX3 Speaker Buzzing

[N0WVA]

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.

[wa3dsp]

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.

[N0WVA]

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.

[N0WVA]

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?

[wa3dsp]

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.

[N0WVA]

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.

[gerry_w1id]

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.

[N0WVA]

Ok, seeing how my scope is down, I just replaced the rectifier diodes, same problem.

[N0WVA]

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?

[N0WVA]

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.

[N0WVA]

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?

[N0WVA]

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.

[wa3dsp]

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.

[KA2DZT]

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  

[Pete, WA2CWA]

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.

[flintstone mop]

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

[wa3dsp]

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.

[flintstone mop]

Buzz in the speaker with the 6AQ5 (audio output) removed is definitely the power supply.

[KA2DZT]

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

[wa3dsp]

>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. 

[flintstone mop]

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.

[AB2EZ]

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

[Pete, WA2CWA]

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.

[N0WVA]

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.

[wa3dsp]

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.