National HRO-60 Hum

[W1TTL]

Hi everyone,

I am banging my head against solving this issue, so I'd appreciate any pointers in the right direction.

I have an HRO-60 that I recently recapped.  I had to do 'em all because so many of the original dipped paper caps had split down the middle.  Now that I've replaced the electrolytics and dipped paper caps, I am still having a hum issue.  It is a hum that increases and decreases in volume as I turn the AF Gain.  When I switch off the B+, the hum goes away.  The hum is prominent on CW and AM modes.  

I was actually able to align one of the coil packs as long as I put the RF Gain all the way up so the hum was drowned out, but I'd like to fix the hum first before I align the other coil packs!

I've tested all the tubes and none of them appear to have shorts.  

Any tips as to where I should start?  Should I start with the audio amp tube (V13) with an oscilloscope and see if there is AC on the circuit and follow it back?  I am looking for some sage advice before I continue.

Thanks and 73.

[W1TTL]

I am thinking that it might be in the audio section.  When I switch the input to phono, the hum is still there.  Looking at the schematic, it appears that switching to phono will switch out the RF section and connect the RCA phono jack in the back to the input of the audio section.

[Pete, WA2CWA]

It would be helpful and courteous to either supply a schematic or point to where readers can view a schematic or the manual when asking for help about the circuitry in a piece of equipment.

Did it hum before you changed out all the capacitors? It can help determine if the problem existed before replacements were made or afterward. Maybe you put in a cap that was defective, or wrong polarity, or poor solder connection, or in the wrong place.

You might want to check the cathode capacitor on the 6V6's. Should be an electrolytic 25 Mfd as a start. Also check, remove, or lift one leg of the capacitor across the primary of the audio transformer. A scope is very useful for tracing down bad hum (good hum - it depends on the tune and weigh the advantage)

[W3SLK]

Have scope, will find it!!!! Just follow the signal stream from beginning to end. If the hum tends to increase with volume control then I would look around that 6SJ7 at the first stage. Make sure those coaxial wires are grounded well also. Even though you have selected the 'PHONO' mode, be mindful that the RF and IF stages are still running.

[W1TTL]

Pete,

Thanks -- you're right.  I should've posted a schematic, or at least the section I was looking at -- especially since there are two versions of the schematics that I am aware of.  (I have a SAMS version of the HRO-60 schematic and it has V13 as V12 and things are a little wired differently than the HRO-60 that I have...)

I didn't power on the receiver until I had replaced all of the paper caps as the majority of them had split.  I didn't want to take a chance.

Mike -- thanks.  I started probing with my scope at the start of the audio section (V13, the 6SJ7).  Sure enough, on pin 4, there was a 60Hz AC waveform.  

So, I went to the start of the RF section.   On all the pins I saw a 60Hz AC waveform.  See the attached photo from scoping pin 6.

I then scoped pin 4 on V14... I thought maybe there was AC getting through the LC circuit at L1/C65A/C65B.  And, I saw what appears to be a 60Hz signal... see attached.

Does anyone have any thoughts on what to check next?  My guess is that maybe AC is getting through L1... I have replaced C65A/C65B with new electrolytic capacitors.  The waveforms you see are in the AM position of the input switch.  The hum is louder in CW and AM mode compared to phono mode.

73

[Pete, WA2CWA]

I hope you checked the tubes.

National never included a Block Diagram in the manual for the HRO-60.
So someone has drawn one up and posted it on the web. (ATTACHMENT BELOW)
http://emmittsfixitshop.com/files/Download/Projects_HRO_original_block_diagram.pdf

A rather crude method for possibly finding the area where the problem lies would be to use the Block Diagram.
To start all tubes in - do you hear the hum
Pull the first tube in the signal flow starting at the antenna input
Do you still hear the hum; if yes pull the next tube
Do you still hear the hum; if yes pull the next tube
Etc.
If you pull a tube and the hum is no longer there, the circuitry around that tube would be a good starting place to check components, tube pin connections, solder connections, etc.
Did you check the electrolytic capacitor from the cathodes of the 6V6's? It's a polarized capacitor; make sure negative goes to ground.

[W4AMV]

In doing some simple searches on the HRO60 and considering isolating the power supply, divorce it from the rest of the Rx and load it appropriately... I came across this item. Does it apply to your case?
==========================================
Hi all you HRO60 admirers.
I Am an electronic engineer with vacuum tube Radios as a hobby.
I have studied the HRO60 schematic carefully. and the now famous C118 should be
removed. It tries to correct a factory wiring mistake. My schematic clearly shows that the rectifier tube is incorrectly wired. The wires to the two heater pins 2 and 8 of V17 should be swapped. As it is incorrectly wired there will be about 5V extra line-frequency hum on C65A due to the miswiring of V17. C118 tries to correct that the wrong way. It puts an extra load on the high voltage secondary. I can see no other reason for C118, so cut it out and rewire V17 correctly.
Hans J Weedon.

[W1DAN]

Hi W1TTL!

How far down is the hum from normal audio? Are you using the associated National table speaker, a hi-fi speaker or headphones? If a hi-fi speaker or 'phones, then you will hear hum on most tube receivers. The table speaker has some attenuation at 60Hz.

Because the hum varies with the volume control, I am thinking you may have a tube heater to cathode short before the audio power stage. Also it could be a wiring mistake where a new cap is tied to a tube filament pin.

Please report...

Thanks,
Dan
W1DAN

[W1TTL]

Pete, thanks for the block diagram.  By removing the tubes, I noticed that upon removing V10, the hum/buzz decreased.  So, I put all the tubes back in except for V10 and still the decreased hum/buzz.  Putting V10 in made the hum/buzz come back.

I should note that I hear the humming/buzzing when the AF gain is at 6 and above.  Thinking about what Dan said (I have the matching National speaker, btw), I wondered if I should expect to hear some hum on the HRO-60... I am just used to my National NC-300 that has no hum when you crank up the AF gain.  Maybe the HRO-60 is just a different beast.

At any rate, I checked the circuitry around V10 (the B half -- see attached schematic) and all the capacitors and resistors checked out OK.  I also made sure that the new caps were soldered correctly and that all electrolytics are soldered in the correct polarization.  I did the mod that W4AMV shared from Hans and that didn't have too much of a dramatic effect on the hum.  

The 6SN7GT at V10 tested in my B&K tester as good, but I tried another 6SN7GT that had tested good and no change in hum.  

I decided to pop in my A coil set, since that is the only one I have aligned so far.  The AM signals just blew the doors of the HRO-60 with the RF gain all the way up. You really don't hear any hum under the static.  Which makes me wonder if I am just a dog chasing my tail here.  I am curious as to what you guys think since this is my first time using/working on an HRO-60.

There is something up with my RF gain, though.  It only works from 9 to 10 on the knob.  Anything below 9 and there is no static.  I saw that Terry posted a video on his D-Lab channel about something similar but with the AF gain on an NC-125: https://www.youtube.com/watch?v=m56FnPYPCG4

I noticed that there was ~22VDC on the RF gain on my rig and the DC voltage dropped to zero as I got to 9-10 and I could hear signals.  Terry mentions to look and see if the capacitors between the sections are allowing DC to come across, which I did and things appear to be fine. The SAMS schematic says I should expect ~32V at the RF gain... so it's confusing to say the least.  Has anyone had this problem where you don't hear any static/signals until the RF gain reaches 9 or 10?

73,
Tony W1TTL

[Jim, W5JO]

When you are searching for this problem by pulling tubes, do not pull  the 1st 6BE6 or 6C4.  The two tubes are side by side and those directions are stamped on the top of the chassis.

[KA3EKH]

Not trying to be ignorant or offensive,but there are a couple procedures involved in superheterodyne troubleshooting and wanted to review them with you.
Step one, confirm the power supply is working correctly. If the output of the DC power supply is low or has excessive ripple (AC voltage) it can show up not only as noise but as a number of other issues. Always confirm its DC and percent of AC is correct. Anything below a third of a volt peak to peak is usually good.
Step two,try to determine what works and what don't. every system can be broken down into smaller sub systems. In the superheterodyne you have a RF input stage, mixer oscillator, fixed intermediate frequency (IF) stage, detector and audio stage. There are also some other things you can throw in there like AVC, product detectors and filters but let?s try to keep it simple first.
Already addressed the power supply. Let?s work backwards! On that radio if you select the mode switch to the phono position do you still have hum? If you inject audio at the phono connector at a very low level do you have lots of clean output? That radio has a high power push pull amplifier and generates tons of gain when working right.
I tend to treat the detector and the IF section as one block so let?s look at that next. Try injecting 455 KHz at a very low level on the grid of the first IF amplifier(V6) that will tell you the status of the IF chain and detector.
And last would be injecting a RF signal at the antenna input to confirm the RF, mixer and oscillator are working correctly.
You're going to see that you will need a signal generator for generating RF, IF and audio signals with a variable output. You already have the scope for looking at AC signals and a meter for DC but if you are going to get anywhere working on receivers you going to need a good signal generator.
At first it may all be overwhelming trying to separate a large complex system like the sixty but with time you will learn how the stages work and that many superheterodynes share the same structure.
Sometimes its best once you get your signal generator to take something that already works first and learn by trying injecting signals at different points in it and seeing what type of levels to use and what results to expect. After you master a basic tube type AM radio you can then progress to the more advanced sets.

[W1DAN]

Hi W1TTL:

If I had your hum symptoms and levels, I would not worry about it. I have a number of HRO's, NC183, 183D, etc, and all have some hum. Most of the hum is below the noise level. Often the reason of the hum in tube receivers is that the audio power amp sources it's HV from the power supply first stage that has less filtering than the second stage.

My National receivers also have a non-linear RF gain controls where most of the gain occurs in the last third of the pot's travel-this is normal.

Hans's post is interesting and he may be on to something. Worth trying the swap and see what a scope reveals.

I wonder if the power supply grounding is low resistance? Maybe the supply ground is floating a little due to a loose chassis connection?

If you scope the 6SN7 grid, then the 6V6 grids in differential mode, do you see the hum there?

Once I am happy with the radio's performance, I do two things to my HRO-60s: The first is to add a 1/4A or better a 200ma fuse from the power transformer HV center tap to ground. This should save the (now rare) power transformer in case of an HV short. The second thing I do sometimes is to remove the output 6V6 tubes and clip lead a wire from the volume pot center tap to exit the rear cooling slots of the radio. I then put a 0.1uF mylar cap and 100k resistor in series to an external amp and speaker (or nice powered speaker such as a Fostex 6301B). Removing the tubes reduces the power load quite a bit and in my opinion the external audio sounds better.

If I am solid-stating the power supply, I add a CL90A (or two in parallel) inrush limiter at the line fuse socket, and reduce the fuse to 1A.

Please report any progress!

Dan

[Tom WA3KLR]

Others have mentioned heater-cathode leakage as a source.

One residual source of hum is magnetic coupling from the power transformer to the audio output transformer. Looking at the manual, I see that National has the power transformer at the right rear and the output transformer is near the left rear on the underside of the chassis. So this source should be zero, good design there.

Both filaments in the rectifier tube should be lit. I assume by now you have confirmed good B+ filtering. The filter choke is marked as 0.7 Henry which isn't a whole lot but ideally you should have no more than about 30 mV peak-to-peak on the B+.

The next source of residual hum is due to the current loop from the ground return side of the secondary winding of the output transformer. For the lowest hum on headphones and speaker, the ground connection at the rear audio output terminal E-3 should not be connected to chassis there but be lifted and isolated wiring connects the audio secondary return wire and this COM terminal to chassis ground only at the front headphone jack ground; this completely eliminates any chassis currents from the filaments and pilot lamps from superimposing on the speaker and phones.

(You can't just throw the power transformer filament winding return, output secondary wire, and speaker ground connection anywhere and then expect no hum. The chassis is not zero impedance. I don't know how the HRO-60 is wired here.)

[W1TTL]

Gentlemen,

Thank you for all the replies.  It is much appreciated.

OK.  Back to basics:

1. I pulled all the tubes and tested them.   They all appeared to test good.

2. When in phono mode, there is no audible hum.  If I plug an audio source into the phono RCA jack in the back, I can hear the music I am playing just fine.  So, I am going to assume that the audio amplification section is working OK.

3. I put in coil pack "D".  I tried injecting a 455Khz signal at the stator of C-5C (which is the mixer variable capacitor).  I also tried injecting a 455KHz signal at pin 4 (the grid) of V6 (the 1st IF amp) as KA3EKH suggested.  In CW mode, I do not hear a tone at all.  I tried adjusting L-16, which is the CW oscillator (V11) variable inductor to try to hear the beat note in CW mode, and heard nothing.  The interesting thing is that when I move my signal generator up to ~480KHz and tune up the band a little bit, I can faintly hear a CW note that I can vary with the CWO knob.  When I modulate the signal, I hear the note generated by the signal generator loud and clear.

As a side note, if I inject 3.5MHz with a modulated signal at the antenna jack, I can tune up the band and hear it loud and clear on both CW and AM modes.  However, I do not hear the CW oscillator when I turn off modulation on the signal generator.  That got me thinking... maybe I have a problem with my CW oscillator.  I scoped pin 4 on V11 and I could clearly see what appears to be the oscillation (see attached).  However, when I scope the other side of R69, a 220k resistor (see attached photo and schematic) that connects to pin4 on V11 and goes to the control switch and then on to other areas (including the screens of the two 6V6 audio output tubes) there's a flat line on my o-scope.  I measured out R69 and it is indeed ~220k.  Am I chasing a ghost?  If I turn up the RF gain and the AF gain I very faintly hear the CW oscillator working and can adjust it with the CW oscillator knob as well as L16.

Any suggestions?  Shouldn't I see oscillation on the screens of the 6V6 tubes or am I misunderstanding something?  I am guessing that R69 attenuates the oscillation from V11 to the screens of the 6V6's but I assume it shouldn't be attenuating the oscillator THAT much.  I am guessing that until I get the CW oscillator working, I am not going to hear the 455Khz tone when injecting it into the IF section.

Thanks and 73,
Tony W1TTL

[W4AMV]

However, when I scope the other side of R69, a 220k resistor (see attached photo and schematic) that connects to pin4 on V11 and goes to the control switch and then on to other areas (including the screens of the two 6V6 audio output tubes) there's a flat line on my o-scope.

BFO Injection is via C125, to the 2nd detector. You should see clean beat multiplication between the IF and BFO if all properly adjusted at the 2nd detector output (pin 4) off to the audio stage. Nothing to see via the 220 k as this is the plate supply V for the BFO.

[KA3EKH]

The output of the BFO oscillator is coupled to the detector by that little 3pf capacitor, it don't take much to make the CW side tone work. Fairly straight forward Hartley. L16 sets the course frequency and the trimmer (BFO control) makes small changes in frequency don't know about the sixty but generally the way i would set the frequency is do a IF Alignment at 455 kHz and peak for max, then set the BFO control dead center and adjust the coil so it is a zero beat. Also be warned that when you connect you scope or meter to anything around the BFO or HFO oscillators that will often cause them to go off frequency. you can reduce that effect by putting the scope probe in X10 but connecting will still affect frequency.
Other then aligning L16 so its zero beat at 455 if its working on air would not worry beyond that.
Not getting any Hum in the phono position tells you that you power supply and  audio stages are good, maybe not perfect but so what. Certain some will tell you you need to sweep the audio stage and work with a distortion analyzer and all that sort but sounds like you are getting it working and that's the fun part.
Remember that's a high gain receiver and often when everything is  cranked all the way up they get noisy.
Keep up the good work, that's a nice clean radio you have there.

[W1TTL]

Guys,

Thank you for your suggestions.  Now that I understand the BFO circuit a bit better, I probed around it and it appears to be working alright... I am just still having trouble getting an audio sidetone when I inject a carrier at the antenna.

I think I am hopefully getting closer to solving this:

I injected 455KHz at the grid (p4) at the the 1st IF amp (V6).  Funny thing, while I had the scope probe on pin5 of the 2nd detector (V9) I could find the sidetone by adjusting L16 although it was very faint with the RF gain down and the AF gain up.  Removing the probe and the faint sidetone disappeared.

So... I probed the anode (p8) of the 1st IF amp (V6) with my scope and I could see the 455KHz signal getting amplified.  HOWEVER, when I probe pin4 (grid) or pin8 (anode) of the 2nd IF amp (V7), I see nothing on my scope!  

Shouldn't I see something on the grid or anode of the 2nd IF amp?  I checked the cathode, too, of the 2nd IF amp and saw nothing on my scope.  Is it possible that my 455KHz signal isn't making it from the 1st IF amp to the 2nd IF amp?

P.S. Thanks for the compliments on the rig. It was a gift and it's going to be my daily driver.  So I am very motivated to get it running again!

73,
Tony

[W4AMV]

Check all grid and plate voltages on V7. Assume no one messed with tuning of the IF transformers (alignment), check DC continuity of all the INDUCTORS making up the IF section T6, T7. Like L3,L4, L7,L8 within T6, T7 which have no DC voltages applied.

[W1TTL]

Alan,

Thanks for the tips.  I am happy to report things are looking better now.

I started by injecting a 455KHz unmodulated signal at pin 4 of V6.  Then, I put my scope's probe where L3 meets L14 (the first inductor coupling) and then adjusted L2 and L3 for maximum amplitude on the scope.  I would then move my scope probe to the end of L4 and L5 (pin 4 on V7) and then adjust L4 and L5 for maximum amplitude.  I had to do this all the way through to L11, going section by section in the IF, tuning for max amplitude.  All I have to say is WOW.  All the inductors were tuned WAY off.  I went back through and did it a second time, peaking everything again.  

To make sure L12 and L13 were peaked, I injected the 455KHz signal at pin 7 of the mixer (V3) and peaked L12/L13 while monitoring pin 5 of the 2nd detector (V9).  Similar to the other inductors, L12 and L13 were way off.  This all makes me wonder if someone had been monkeying with the inductors in the past.  

Finally, when injecting the 455KHz signal at the stator of the mixer tuning capacitor (C-5C), I could hear the note on the speaker.  Success!

I was able to begin the IF amplifier alignment as in the manual, which requires a 455KHz signal injected at the stator of C-5C.  I'm going to try that next.  Thank you all for your help. I couldn't have done it without you.

Thanks all and 73,
Tony W1TTL

[KA3EKH]

The IF stage is the high gain stage of the radio, in a superhetrodynd that makes or breaks the quality of the receiver. proper combination of gain and low noise. IF Alignment never changes on its own, least that I have seen, but people who don't know better always think that's were the trouble is when a radio is not preforming well and will crank on all the adjustments.
on that receiver you can use the S meter to peak the IF alignment so you don't need to be probing around the stages. just keep the radio in manual gain control, AVC off and when the meter gets too high back down the RF gain or crank down the signal generator. Remember that attaching a scope or meter to the grids of tubes will often change its capacitance and result in improper alignment.
I always use a 1 KC modulated AM signal to do the alignment and go back afterwords and align the BFO for center pot using a unmodulated carrier.
If you want to get"Big Time" with it you are going to want to sweep the IF or at least check at a couple different frequencies because if its cranked down too tight it becomes narrow band and will affect the audio response. The pass band or band with of the IF stage has a huge effect on the radios ability to reproduce high quality sound. Lot of the band with function of the receiver is determined by the Q of the IF transformers and has little variation but on some receivers they will have some stagger tuning to allow better IF response and that produces better fidelity. 
I don't know if the sixty was considered a good audio fidelity receiver within the context of AM, almost every superhetrodyne is more then good enough for SSB and CW, for CW receivers you want narrow band with IF amplifiers to reduce co channel noise. But with High Fidelity AM a lot of time you want filters and IF amplifiers that are as many as 10 to 15 KHz wide. A ten KHz wide IF would have the same response (gain) from 450 to 460 KHz and drop off from there. 
Before this is all over you will be ready to tackle all sorts of receivers, maybe I can send you a couple of mine to fix.

[W4AMV]

Excellent. The comment on "no one messed with tuning" was tongue in cheek. The golden screwdriver can cause significant pain. That said, I restored a few years ago a HRO5TA and the receiver was a pleasure to work on despite some crazy forks in the road.

[W1TTL]

Well, I finished the IF alignment per the manual.  When I got to the alignment procedure for the 2nd conversion section, I noticed that L17 had been turned so much that the threaded tuner had been screwed flush.  Not only that, but whoever did it had bent the tab that provides friction on the threads inside the can.  Fantastic.  Luckily I have a parts HRO-60 and swapped in a new L17.  I was then able to align the 2nd IF conversion section just fine.  Tuning around, I can now hear some signals on the air!

I have to address the mechanics of the PW gear drive.  It's all jammed up and the eccentric is having troubles turning.  I'll clean and lube that up in the next couple of days.

KA3EKH -- I've learned so much with this.  I've repaired a Ranger and a Thunderbolt before but this was my first receiver.  Totally different beast and a fun challenge.  Thanks for the offer to tackle yours but I'll wait!  Haha.

W4AMV -- oh yes.  I caught the tongue in cheek!  I was definitely thinking about your comment as I went through EVERY variable inductor and thought the same thing: "Who used their golden screwdriver in this thing?!"

For anyone wondering what became of the hum issue, I decided it was just the nature of the thing.  Once I heard static from turning up the RF gain, I couldn't hear the hum any more.  The hum was below the noise level.  I did try swapping the heater pins of V17 per Hans's suggestion but that didn't change anything with the hum.  

73,
Tony

[KA3EKH]

Enormous amounts of knowledge can be gained by picking up junk at Hamfest and from estates and reworking it. Old Hallicrafters, Heathkits and my favorite military surplus can teach you everything you need to know about obsolete electronics. The military surplus stuff like BC-348 or 342 receivers are often hacked and non working and you can learn a lot from putting them back into functional order. My preference on lots of those receivers is to build up something that's working , with an AC power supply and that would have been popular in the fifties and sixties. Military transmitters like the ART-13 or BC-191 the next step. Most people start with the command sets like the ARC-5 stuff and that's a option too but not known for its performance.
After becoming proficient that you can start down the road of restoration and that becomes a way of life for some people.
 

[W1TTL]

I completely agree.  At the high school ham club that I advise (W1HLO), we are currently restoring a Hammarlund HQ-129X receiver and Harvey Wells TBS-90D Bandmaster Deluxe.  (Both are what the club used to use in the 50's and 60's.)  So far, we have been re-capping the receiver and the students love it.  They are learning how to solder and what capacitors and inductors do.  When I let them cut apart an old wax paper cap, you should've seen the astonishment when they saw it was just a roll of foil and paper!

I needed to learn how to align a receiver and better understand its sections before I tackle the HQ-129X with them.  So, restoring this HRO-60 first was perfect.  I learned a lot thanks to you guys and now I can pass the knowledge onto my students.

The HRO-60 now has its four coil packs aligned (A through D) thanks to Steve Reed KW4H's article in Electric Radio Issue #391 (May/June 2022).  Here's a video after I finally got the C pack aligned:

https://youtu.be/sYwYq1JjLHE

I am listening to it now on 40m and wow... it is stable, sounds good, and is pulling in the signals nice and strong.  I am happy.

73,
Tony W1TTL