Ok its an amp question… jc 120 noise

[Locrian99]

Hey Ive got this jc 120 and its has a noise issue on channel 2.   Im fairly certain its a bad electro but its somewhat intermittent and ive had a hard time getting it scoped.   It seems to happen after the amp has "warmed" up.   With the volume up a bit, then will continue.   It is not effected by the volume
Control.   It also goes away as soon as vibrato or chorus are turned on. 

https://youtube.com/shorts/ri2WF2u8ovg?si=wRcQAr5JPfIDyhrt

Theres a video.




Im kind of leaning towards replacing c46 and seeing if thats it.   Any educated guesses are helpful going to try to get it to do it, but the only real time i have to scope it is late and dont have a dummy load set up at the moment so i can run it at a high enough volune.   

Thx

[Rob Strand]

The way I understand the chorus works is it disconnects the dry signal on channel 2 and passes only chorus signal to the channel 2 power amp.

So that and the fact the volume control has no effect means the problem is between the output of the volume control and the output of the stage Q16/Q15(? hard to read).   In fact I'd narrow that down to the Q16/Q15 amplifier stage alone.

The stage has gain so it's possible it could be amplifying existing noise.    If we eliminate that then perhaps C53 starts off open then gradually warms up and starts working.  That process would increase the gain of the stage and give the impression of increased noise.  However, the warmed-up state is likely to be the working state so that would mean the noise is always there (basically the gain of the stage boosting noise).

I suppose you could try to debug the stage further.  Perhaps even consider noise on the PSU getting in from bad PSU caps.

[Locrian99]

So Ive managed to get the noise to continue and ive got it on the bench.   The first place i confidently see the noise is r95.   Could it be being introduced around q13/14?
And not be affecting the chorus?

[Rob Strand]

So Ive managed to get the noise to continue and ive got it on the bench.   The first place i confidently see the noise is r95.   Could it be being introduced around q13/14?
And not be affecting the chorus?

R95 is the feedback amplifier resistor for the power amp.  If there was noise generated at that point I wouldn't expect there to be an *audible* change when the chorus is switched in.  Probing the base side of R95 can cause the power amplifier to oscillate - so be careful.

Any noise you from the preamp will pass through the power amplifier.   If there is noise present at the input of the power amplifier then it will appear on the base of Q13, and, due to the feedback, the noise would also appear on the base of Q14.   It's not just noise but any signal appearing on base of Q13, the input of the power amplifier, would follow that logic.

So you really need to make sure there is no noise on base of Q13 or from any preceding circuits before you should consider Q13, Q14.   If Q13 and/or Q14 are damaged it is possible for the power amplifier itself be the source of the noise *but* the noise would be there (heard in the speaker) all the time and would not be expected to be switched out by the chorus/vibrato.

So I'd start but hunting down noise before the input of the power amp.

[Locrian99]

So Ive managed to get the noise to continue and ive got it on the bench.   The first place i confidently see the noise is r95.   Could it be being introduced around q13/14?
And not be affecting the chorus?

R95 is the feedback amplifier resistor for the power amp.  If there was noise generated at that point I wouldn't expect there to be an *audible* change when the chorus is switched in.  Probing the base side of R95 can cause the power amplifier to oscillate - so be careful.

Any noise you from the preamp will pass through the power amplifier.   If there is noise present at the input of the power amplifier then it will appear on the base of Q13, and, due to the feedback, the noise would also appear on the base of Q14.   It's not just noise but any signal appearing on base of Q13, the input of the power amplifier, would follow that logic.

So you really need to make sure there is no noise on base of Q13 or from any preceding circuits before you should consider Q13, Q14.   If Q13 and/or Q14 are damaged it is possible for the power amplifier itself be the source of the noise *but* the noise would be there (heard in the speaker) all the time and would not be expected to be switched out by the chorus/vibrato.

So I'd start but hunting down noise before the input of the power amp.

Not the best excercise in practicing the art of trouble shooting.  But it seems most likely the problem is coming from an electrolytic cap.   The electros are 40 yeFs old.   Wouldnt take that long to just replace all the electros on in channel 2 and the power supply caps, and wouldnt be terribly costly for parts.... Just a thought... ill have to see what i can find tomorrow...

Thanks as always

[Rob Strand]

Not the best excercise in practicing the art of trouble shooting.  But it seems most likely the problem is coming from an electrolytic cap.   The electros are 40 yeFs old.   Wouldnt take that long to just replace all the electros on in channel 2 and the power supply caps, and wouldnt be terribly costly for parts.... Just a thought... ill have to see what i can find tomorrow...

It's certainly possible it's an electrolytic.  The fact it occurs a warm-up is a sign.    Semi's can go noisy when zapped with ESD or reversed voltaged but they normally start off noisy.

I've got a few pieces of equipment which are getting old and haven't been used for a while and I'm dreading turning them on and frying something.  All very good candidates for bad electrolytic caps.   Just the other day I pulled out one of my old Lab power supplies which I haven't used for years.   I didn't bother powering it up. I opened it up and found the main filter cap had started to leak.  Just caught it in time.   

In my earlier post I was trying to use the symptoms and a bit of logic to narrow down where the issue is.

Something else to check would be the DC bias voltages when cold and when warm.   In some cases cap leakage will cause the DC shift but it could be something else like a resistor with band internal connection.

[Locrian99]

Now it seems to have the noise whether warm or cold... still cant seem find the noise until after q13/q14.   Tried using an audio probe similar findings.      Ill get some transistor voltage info next.   

[Rob Strand]

Now it seems to have the noise whether warm or cold... still cant seem find the noise until after q13/q14.   Tried using an audio probe similar findings.      Ill get some transistor voltage info next

Could be something to do with an electrolytic cap on the edge and because the unit has been switched on more during the debug process.  Or the fact the unit doesn't have a chance to cool-off 100%.

Something you could try is get a 6.5mm jack wired as a short and plug it into the Main In connector for channel 2.  That will break the connection between the Preamp and Power amp.  If you still see and hear noise from channel 2 then the noise could be from the power amp.

Something else to try would be to wire preamp 1 to power amp 2 and preamp 2 to power amp 1 and see if the noise shifts speakers or not.   I don't think you can do that with external cables alone.

[Locrian99]

Now it seems to have the noise whether warm or cold... still cant seem find the noise until after q13/q14.   Tried using an audio probe similar findings.      Ill get some transistor voltage info next

Could be something to do with an electrolytic cap on the edge and because the unit has been switched on more during the debug process.  Or the fact the unit doesn't have a chance to cool-off 100%.

Something you could try is get a 6.5mm jack wired as a short and plug it into the Main In connector for channel 2.  That will break the connection between the Preamp and Power amp.  If you still see and hear noise from channel 2 then the noise could be from the power amp.

Something else to try would be to wire preamp 1 to power amp 2 and preamp 2 to power amp 1 and see if the noise shifts speakers or not.   I don't think you can do that with external cables alone.

Great idea!!!! Ok so cable plugged in noise goes bye bye!

Voltages between the volume control and the power amp.

Q#-D-G-S
5-6.35-.23-.48
6-29.2-13.14-14.79
15-12.14-6.091-11.49
16-11.49-7.72-11.49


After several hours of sitting off it is powering back up with the noise.

With no signal going into the amp.   Should i see any AC in the signal path?  Showing around 1mv at the gate of q5...

[Rob Strand]

Q#-D-G-S
5-6.35-.23-.48
6-29.2-13.14-14.79
15-12.14-6.091-11.49
16-11.49-7.72-11.49

The voltages look good except the source of Q16, which I think it a typo
or you measured the same pin twice and should be around 9V or so.

The JC schematics have some duplicated part designations on the schematic
and the unclear text on the schematic doesn't help.

We have two possible theories:
- the noise is before Q15/Q16 and amplified by Q15/Q16 (gain of about 10)
- the gain is caused by the Q15/Q16 stage, possibly amplifying its own noise.

Here's some things to try:

1)
The idea here is to shunt the noise from before Q5 so any noise
you hear is likely to come from Q15/Q16.   It's not a fix
just a debug step.

For Q5, Q5 is the JFET with R80 12k as the drain resistor.

- Take a 100nF to 1uF NP cap and connect it from Q5's drain to ground
  Use short leads.
  (this is a debug step not a fix.)
- See if that reduces or eliminates the noise.

2)
- Tack a 4.7k resistor across 8.2k R94.
  (this is a debug step not a fix.)
- Can you hear an increase in the noise?

What this is doing is increasing the gain of the stage with the aim
to confirm noise is amplified by the Q15/Q16 stage.

(Try with and without the 100nF cap from test 1 present.)

From those two steps it should be possible to isolate the cause.

With no signal going into the amp.   Should i see any AC in the signal path?  Showing around 1mv at the gate of q5...

It's easy to pickup stray signals at at level.  It's more important to know
what the source/cause of that signal is than the fact it is there.
If you use a DMM on AC and  measure 1mV you've got not idea if it's hum, noise,
or a stray signal coupling into the DMM leads.

[Rob Strand]

I took another listen to your YT video, and based on the info in the thread I'd be thinking C30 (1u NP elec) or C53 (10uF elec).   A less probable fault would be C26 (82nF film).

[Locrian99]

I took another listen to your YT video, and based on the info in the thread I'd be thinking C30 (1u NP elec) or C53 (10uF elec).   A less probable fault would be C26 (82nF film).

I believe i already replaced c53 let me double check.   I have a 1u np electronon the way, for simply the purpose of checking would a film or mlcc be ok there?  Inalso have a 1u tant but thats polarized

[Locrian99]

Wooooooot!   C26 was it!!!!

Oh man thank you so much!

[Rob Strand]

Wooooooot!   C26 was it!!!!

Oh man thank you so much

Awesome! Wow, faulty film cap.  That's pretty rare although I have had a few.

[Locrian99]

Wooooooot!   C26 was it!!!!

Oh man thank you so much

Awesome! Wow, faulty film cap.  That's pretty rare although I have had a few.

Am i good to leave the 1u film in for c30?  Or shoukd i put the np electro
Back in.

[Rob Strand]

Am i good to leave the 1u film in for c30?  Or shoukd i put the np electro
Back in

Technically the 1uF film should last out the amplifier.    If you are someone who likes to keep as many original parts as possible you could swap it back.   It's a bit of a judgement call.

The reality is these days if you want working vintage equipment at some point you have to put in new parts!

[Locrian99]

Am i good to leave the 1u film in for c30?  Or shoukd i put the np electro
Back in

Technically the 1uF film should last out the amplifier.    If you are someone who likes to keep as many original parts as possible you could swap it back.   It's a bit of a judgement call.

The reality is these days if you want working vintage equipment at some point you have to put in new parts!

Ill leave in in there i already replaced a couple other electros.   I have replacements coming for all the power supply filter caps too.   I ussually just hang on to whatever i replace in vintage stuff.

Pretty excited to have this going again, these are pretty amazing amps when they arent crackling.  Reverb on these old ones seems better than the new ones.

[Locrian99]

I took another listen to your YT video, and based on the info in the thread I'd be thinking C30 (1u NP elec) or C53 (10uF elec).   A less probable fault would be C26 (82nF film).

I believe i already replaced c53 let me double check.   I have a 1u np electronon the way, for simply the purpose of checking would a film or mlcc be ok there?  Inalso have a 1u tant but thats polarized

Well shit i lied its there again, of course after I put the damn thing back together.   Ill try the cap and resistor debugging you mentioned tomorrow.   Its less noisy it seems though.   There is a strange sound as well from the volume pot if channel 1 I hadnt noticed before.  Might not be anything...

https://youtube.com/shorts/_JgPhmGNcjk?si=IHEHUcNj3BqlkPe3 

Edit: It almost sounds like the reverb is affecting the noise...

[Rob Strand]

Well shit i lied its there again, of course after I put the damn thing back together.   Ill try the cap and resistor debugging you mentioned tomorrow.   Its less noisy it seems though.   There is a strange sound as well from the volume pot if channel 1 I hadnt noticed before.  Might not be anything...

https://youtube.com/shorts/_JgPhmGNcjk?si=IHEHUcNj3BqlkPe3 

Edit: It almost sounds like the reverb is affecting the noise..

Yep, the best thing you can do is narrow down the area causing the noise.   Then you can poke around in a smaller area.  It could even be a bad resistor.

If you audio probe through channel 2 you should be able to find where the noise appears.

The noisy pot just seems like a dirty pot.  Perhaps check the DC voltages around the pot, that would point to more bad caps.

More often than not I find the reverb responds to existing noises.   It is possible DC is getting through the reverb drive coil due to a bad cap.  If that's intermittent you can get noise on the power supply due to supply loading then that will appear as noise on the audio via the preamps.  I don't think that's the case here.

[Locrian99]

Well shit i lied its there again, of course after I put the damn thing back together.   Ill try the cap and resistor debugging you mentioned tomorrow.   Its less noisy it seems though.   There is a strange sound as well from the volume pot if channel 1 I hadnt noticed before.  Might not be anything...

https://youtube.com/shorts/_JgPhmGNcjk?si=IHEHUcNj3BqlkPe3 

Edit: It almost sounds like the reverb is affecting the noise..

Yep, the best thing you can do is narrow down the area causing the noise.   Then you can poke around in a smaller area.  It could even be a bad resistor.

If you audio probe through channel 2 you should be able to find where the noise appears.

The noisy pot just seems like a dirty pot.  Perhaps check the DC voltages around the pot, that would point to more bad caps.

More often than not I find the reverb responds to existing noises.   It is possible DC is getting through the reverb drive coil due to a bad cap.  If that's intermittent you can get noise on the power supply due to supply loading then that will appear as noise on the audio via the preamps.  I don't think that's the case here.

470n cap betweeen q5 drain and ground channel 2 is he same.  However its not passing guitar signal which im
assuming is expected.  didnt notice any difference of note in the noise.   With the 4k7 across r94.   I also tried with a decibel meter app and they were the sameish.   The noise while steady seems to have some variance.   


At this point i have replaced the folllowing:
C47
C16
C53
C26
C30

R58
R61
R55.

It doesnt make sense to me that it would be a filter cap but hopefully wednesday or thursday my order with caps for which includes all the supply caps will be here.