Isolated the hum

[jpatti75]

Thanks, Gadget! I installed all new JJ tubes in the amp when I rehabbed it last year. The unit still had all original tubes in it when I bought it, but a number were microphonic, so I replaced all of them. I’ll have to dig up the receipt, as I am pretty sure I bought the JJs in matched sets, but maybe that was only the 12AX7s and not the 6BQ5 output tubes? I still have the original tubes and can try some swapping.

(As an aside, I recently picked up a Sencore MU-150 Gm tube tester. Shipped it out for calibration and it seemingly didn’t survive transport and couldn’t be powered on. Tech said the motherboard fried and was too costly to repair. Still bummed. Would have come in handy for this.)

 

[TriodeLuvr]

The hum disappears when you remove the 12AX7, correct? And you're replaced the 12AX7, but that didn't help. This implies that either A) one of the components around the 12AX7 has drifted or failed, and that's causing the two sections of the tube to be out of balance, or B) the hum is in the signal coming in at the grid of the 12AX7. One way to test for this would be to temporarily connect the first grid to ground by putting a jumper across C25. If the hum disappears, the problem is in the signal path ahead of the tube.

Jack

 

[jpatti75]

The hum disappears when you remove the 12AX7, correct? And you're replaced the 12AX7, but that didn't help. This implies that either A) one of the components around the 12AX7 has drifted or failed, and that's causing the two sections of the tube to be out of balance, or B) the hum is in the signal coming in at the grid of the 12AX7. One way to test for this would be to temporarily connect the first grid to ground by putting a jumper across C25. If the hum disappears, the problem is in the signal path ahead of the tube.

Jack

Jack- thanks! Yes, those are the symptoms. What is the safe way to set up the jumper across C25?

 

[TriodeLuvr]

It concerns me that you had to ask this question. I always worry about advising folks who may not have the experience or skills to do this sort of thing safely. That aside, you should turn off the amplifier and connect a clip lead across the capacitor. This won't hurt the amp, it's just a test to isolate the problem. I'll probably back out of this thread after this. Hopefully, I've been able to help.

 

[jpatti75]

It concerns me that you had to ask this question. I always worry about advising folks who may not have the experience or skills to do this sort of thing safely. That aside, you should turn off the amplifier and connect a clip lead across the capacitor. This won't hurt the amp, it's just a test to isolate the problem. I'll probably back out of this thread after this. Hopefully, I've been able to help.

Jack- First off, thank you very much for your assistance ... I truly appreciate it. While I won't pretend to be a vintage audio technician by any stretch, I have been working with various electrical applications for decades and have what I believe is thorough respect and understanding of safety practices when dealing with high-voltage situations. As an example, my primary hobby is restoring antique outboard motors, and as part of that hobby I have troubleshot, rebuilt, and replaced numerous ignition systems that generate between 400-500 volts. My question on jumping the capacitor was with respect to protecting the amp components and whether I can accomplish the jumping through use of clips/wire or if something else is advisable? I certainly intend to power down the amp and check residual voltages with my meter before making contact with any components, as I have done throughout this project. Anyway, if you are not comfortable providing additional guidance, I understand and thoroughly appreciate all of the help you and so many of the members of this forum have provided. (Sometimes I feel like I am a nuisance, but I truly desire to learn how to restore these systems safely and correctly and value the guidance I receive here.) That said, I wanted to at least give you some appreciation that I am not inexperienced with safety in high-voltage electrical applications, but just want to make sure that anything I am doing with vintage audio isn't going to fry something that is already rather difficult to get parts for.

 

[jpatti75]

Well, I jumped the C25 cap, and that did seem to cut the hum, so I will test the components ahead in the signal path.

Thanks,
JP

 

[jpatti75]

For anyone still following this thread, I had the chance to go through all the ceramic capacitors and resistors in the amp today — those that I didn’t touch when I first rehabbed this unit (I only tested/replaced paper and electrolytic capacitors). While most of them tested in spec, I had a decent amount that had drifted beyond acceptable tolerances … and of those that had drifted the most, almost all of them are associated with the center channel where I am getting the most hum. Here are those I believe need replacing:

Caps
C22 - tested @ .714nf - spec is .800nf
C25 - tested @ .374nf - spec is .470nf
C26 - tested @ .00213uf - spec is .0015uf

Resistors
R38 - tested @ 273.7 kohm - spec is 560 kohm
R39 - tested @ 209.5 kohm - spec is 270 kohm
R43 - tested @ 301 kohm - spec is 680 kohm
R44 - tested @ 219 kohm - spec is 270 kohm
R42 - tested @ 208.8 kohm - spec is 390 kohm

I’m guessing that any/all of these could be contributing to the hum issue. I also plan on going through the pre-amp and checking all of the ceramics caps and resistors there to determine if any have drifted significantly. I do hear a slight hum in the right channel, so I might as well do this right … and get all the soldering done at once.

 

[Mr Boochie]

Did you measure those resistors while they were in the circuit? Or did you pull one leg, first? Resistors often read way off spec, when measured in the circuit.

 

[gadget73]

resistors often check funny in circuit. They almost always fail high, but in circuit where something is across they will read low. I wouldn't worry too much about those.

Ceramics going low in value isn't hugely surprising.

 

[jpatti75]

Thanks, gents. I didn’t pull a leg up just because getting at some of them are a real pain. Most of them read within 10% of their value even connected to the circuit. Just for reference, the large cement resistors seem to read in spec whether connected or disconnected from the circuit (those I had actually tested previously with a leg off, as I replaced a couple last year). Is reading the same whether connected or disconnected unique to the cement variety resistors?

I’ll concentrate primarily on the capacitors, but for a couple of those resistors that read significantly off (38, 42, 43) , I’ll pull a leg up to see if I get an acceptable reading.

 

[jpatti75]

Follow-up question on the ceramic capacitors. If a voltage rating isn’t specified for a specific cap in the Sams sheet, what is the rule of thumb to follow?

Thanks,
JP

 

[Mr Boochie]

Follow-up question on the ceramic capacitors. If a voltage rating isn’t specified for a specific cap in the Sams sheet, what is the rule of thumb to follow?

Thanks,
JP

Use at least 600VDC rated ceramic disc caps. I use 1KV ones, as I have a whole bunch of Sprague NOS I bought eons ago.

 

[jpatti75]

Thanks so much, Mr. Boochie!

 

[jpatti75]

Went through the ceramic capacitors in the pre-amp today, and all but two of them were notably out of tolerance. So, I have an order in with Digi-Key for the caps for both the amp and the pre-amp and hope to have them later this week (Mr. B- went with 1KV rated). Keeping fingers crossed that replacing these caps will solve this nagging hum issue.

Appreciate all of the help!
JP

 

[TriodeLuvr]

I've never seen a ceramic cap cause hum. This is almost certainly a bad electrolytic in the power supply, a bad ground, a disconnected/broken cable shield, a missing tube shield or an inappropriate DIY connection to whatever is feeding the grid of the tube that you grounded. It might be something else, but those issues probably constitute 95% of most likely causes.

 

[gadget73]

agree they aren't likely to cause hum, but they could certainly cause issues with frequency response, especially if they are in the tone control circuits.

also depending where they are, the caps may not be getting any appreciable voltage across them. Hard to say without knowing which particular ones you're talking about.

 

[jpatti75]

Well, this one has been a stumper for sure. While I fully agree that the electrolytics are the likely culprits in these scenarios -- and the only thing I changed out initially -- I have been over them so many times, checking values and solder connections, and I can't find anything faulty with them. I even replaced the main filter can in the amp a second time (thinking perhaps I had a bad can despite getting accurate values on the individual caps), but got the exact same results. I have grounded everything as original. As I go through replacing these additional caps, I will continue to look for any bad solder connections, as I have had original/brittle connections weaken/break as I have replaced components previously. There is nothing neat about Motorola's wiring, but I suppose that is somewhat expected since this unit is pre-circuit boards. The only ceramic disc capacitors in the pre-amp that tested in spec were the two associated with the treble pot, all others were outside of spec -- some significantly. I'll keep you guys posted. Two of the three that tested outside of tolerances in the amp are associated with the center channel, where the hum is most noticeable.

 

[gadget73]

can you post some pics of the underside? Maybe something visual will ring a bell here.


also does it keep humming with the inputs shorted or no? If its humming with the inputs to the power amp shorted to ground, the problem isn't in the preamp.

 

[Mr Boochie]

can you post some pics of the underside? Maybe something visual will ring a bell here.


also does it keep humming with the inputs shorted or no? If its humming with the inputs to the power amp shorted to ground, the problem isn't in the preamp.

Close-up photos of the power supply wiring, output stage, and input/gain stages would be nice!

 

[jpatti75]

Will do, gents! I'll take some high res photos when I get home tonight and will post them.

Thanks much!