Does this cap make my butt look big? For reals though does this look a little bulgy

[Locrian99]

Alright so I am just looking for some
Ither opions these are in a 30 year old amp. Does the first one look like it might be bad to you.

Here is the same value cap doing the same thing in the b channel.



thanks

 

[cooder]

I think it's hard to tell in this shot, it could be just the plastic foil around it "overshooting" the end.
Can you take a photo from an angle showing the positive side if theres really a bulge coming out or any goopy filling?

 

[Locrian99]

Here we go

 

[Locrian99]

In case you are wondering.

 

[cooder]

From photo, which is limited diagnosis, I would say it doesn't look obviously bulged or leaking.
Does the amp give you issues, noise?
I do think many amp techs would say: for a 30 year old amp if you would pull the pcb and all out it would be good idea to consider swapping all the electrolytics while you're at it.

 

[Locrian99]

From photo, which is limited diagnosis, I would say it doesn't look obviously bulged or leaking.
Does the amp give you issues, noise?
I do think many amp techs would say: for a 30 year old amp if you would pull the pcb and all out it would be good idea to consider swapping all the electrolytics while you're at it.

So the symptoms of the amp are a channel gives noise. A channels reverb control does nothing. A channels volume control does nothing. However the pre amp gain works as volume and gets loud very quick. B channel everything works great. Reverb etc. this uses a fet system to switch between channels which is a switchable op amp. This electro is in the feedback loop of the op amp for the reverb recovery stage connecting it to ground. In the a channel where the noise is and where the reverb and volume control do not work. The volume control is in the stage following this and the 12ax7.

 

[cooder]

I'm not familiar enough with the JCM900 topology and the channels witching there to give any recommendations based on the symptons, you describe there, sorry. Maybe someone else can chime in who has worked on those.

 

[HamishR]

I often say that the main problems you will encounter with tube amps is tubes. That may well be the case here but it could be a pot too. At least the 900s look to have used full size pots. However they are mounted directly to the board, and can cause problems if the pot takes any impact. Not only can the pot be damaged but the PCB might be damaged from any impact on a pot. Look for cracking, broken traces or cold joints on the pots. If the volume pot was damaged it might be kinda frozen at one setting which would explain why the gain acts as a volume - in a non-master volume amp it is the volume.

That cap doesn't look bad but that doesn't mean it isn't. However it's more commonly the filter caps which are the issue with bulging because they do a lot of work.

 

[Locrian99]

I often say that the main problems you will encounter with tube amps is tubes. That may well be the case here but it could be a pot too. At least the 900s look to have used full size pots. However they are mounted directly to the board, and can cause problems if the pot takes any impact. Not only can the pot be damaged but the PCB might be damaged from any impact on a pot. Look for cracking, broken traces or cold joints on the pots. If the volume pot was damaged it might be kinda frozen at one setting which would explain why the gain acts as a volume - in a non-master volume amp it is the volume.

That cap doesn't look bad but that doesn't mean it isn't. However it's more commonly the filter caps which are the issue with bulging because they do a lot of work.

Hey thanks. There’s also a fair amount of noise on the a channel. None on b channel. You can hear the noise with the gain at 0. I’ll look for some damage around the volume pot but does it make sense the reverb would be out as well? I guess they are right next to each other.

 

[Locrian99]

Hey thanks. There’s also a fair amount of noise on the a channel. None on b channel. You can hear the noise with the gain at 0. I’ll look for some damage around the volume pot but does it make sense the reverb would be out as well? I guess they are right next to each other.

I should note as well we swapped the preamp tubes and the problem was still there. I would think if it was a tube issue both channels would be effected ?

 

[HamishR]

Well it's probably not tube related in this case! One simple technique you can try is the chopstick test. With the amp open, plugged in and turned on (connected to a speaker of course) you can gently tap various parts to see if one of them makes a noise when tapped. Of course this is potentially a very dangerous thing to do so be careful and use only one hand.

Have you checked the filter caps? Measured the pot resistance?

 

[Locrian99]

Well it's probably not tube related in this case! One simple technique you can try is the chopstick test. With the amp open, plugged in and turned on (connected to a speaker of course) you can gently tap various parts to see if one of them makes a noise when tapped. Of course this is potentially a very dangerous thing to do so be careful and use only one hand.

Have you checked the filter caps? Measured the pot resistance?

So I meansured between pins 1 and 3 of the volume pot for a channel and no matter what it always reads 435k. But does that
Explain the reverb.

I have a cheap oscilloscope I’m not great with using. But I can see the noise as a wavy line. I do not see it before the reverb tank at the tl071 output. The op amp right after the reverb pot the noise is there. I can’t poke around too much with the scope because I have it hooked to the speakers and there’s a god awful whine when I touch the cal there.

 

[Locrian99]

Here’s the schem

https://www.drtube.com/schematics/marshall/cd0189-iss7.pdf

 

[Locrian99]

Well it's probably not tube related in this case! One simple technique you can try is the chopstick test. With the amp open, plugged in and turned on (connected to a speaker of course) you can gently tap various parts to see if one of them makes a noise when tapped. Of course this is potentially a very dangerous thing to do so be careful and use only one hand.

Have you checked the filter caps? Measured the pot resistance?

it was both the reverb and volume pots. Works great now I tried a couple solder lug replacements. Got the right pin type on the way.

 

[HamishR]

Hey that's great! And I almost helped! Must feel good to have worked it out.