Randall V2 power amp problem

[bsco]

Hi Mike. Bernie here. How did you make out with the Randall amp after??? Just curious.....

[mikeydee77]

Hi Mike. Bernie here. How did you make out with the Randall amp after??? Just curious.....

Hi Bernie,
Yes all sorted now.
These amps are a total pain to work on in terms of access but I have to say they do sound very powerful once back in order. Sounds like I had the same issue with one of the 70v power supply rails having very little smoothing giving the symptom of humming regardless of where the volume and tone controls were positioned. Replacing these caps did the trick but I did learn a lesson on this amp that is worth sharing and that is regarding the use of a dim light bulb. In this case with the dim light bulb in circuit very little of the amp would function at all and it took me a while to think of that.

It's worth noting that this amp has lots of PCB fuses protecting various parts of the circuit. I had not seen them before but they look like little rectangular capacitors either brick red or black on this amp. Worth checking these out.
Mike

[bsco]

You are correct....These amps are a real pain in the ass to take apart and put back together...I double checked my schematics after and I never did get the one for this...I have others but not this one....Glad you got it working...

[Ampsandstuff]

Hi, I have an issue as well with the Randall V2, so just like you I got the two big capacitors replaced and the hum is gone BUT Channel 3 (the one that uses tube distortion) is DEAD, no sound at all when switched to channel 3?! Also channel 2 works fine except the Level (volume) knob of the channel 2 is not responsive at all, only the amp master volume is. What's wrong? Something that was not re-connected properly?

[nevetslab]

Also....if the insulator is not greased Mica, but instead one of the rubber0zed silicon-imbedded insulators, those are NOT intended for re-use, as the insulation peels off from long-term thermoset, and the replacement xstr can now break down. If greased mica, you're probably good to go...or, use a fresh mica insulator and apply the silicon grease to both sides for proper thermal conductivity.

[Sebastian S.]

Hi I have the same problem with my V2. Today I opened the Amp and l saw these two capacitors are those the right ones you talk about? They are on the Mainboard of the Amp and not inside the power supply cube which is mounted upside the Amp, is this correct?

[bsco]

Hi I have the same problem with my V2. Today I opened the Amp and l saw these two capacitors are those the right ones you talk about? They are on the Mainboard of the Amp and not inside the power supply cube which is mounted upside the Amp, is this correct?

The ones that I replaced was in the power supply....8200uf @ 80V...but while you are in there, see if you can check the other cps for high ESR.....
Cheers
By the way, Happy New Year!!

[Sebastian S.]

The ones that I replaced was in the power supply....8200uf @ 80V...but while you are in there, see if you can check the other cps for high ESR.....
Cheers
By the way, Happy New Year!!

Thanks for quick response and a happy new year to you!
I'm still not sure as the both big cps I saw at the Mainboard, had the same value (8200uf and 80V. So what you had done was to open the cube that is mounted on top of the Amp, wich is next to the tubes on the right side?

[Sebastian S.]

https://i.imgur.com/XSwgE4g.jpg

[bsco]

Thanks for quick response and a happy new year to you!
I'm still not sure as the both big cps I saw at the Mainboard, had the same value (8200uf and 80V. So what you had done was to open the cube that is mounted on top of the Amp, wich is next to the tubes on the right side?

When I took out that main board in order to replace those 8200uf caps, I had to label every single harness, plug and any individual wires that had individual connectors so that when I had to hook everything back up I was able to make sure I didn't get anything mixed up.....you solved your hum problem but you might have something hooked up wrong...maybe not.....did you take any pics before you started and to be quite honest there are so many harnesses involved I didn't trust that so I labeled all of mine......it has been a few years since I have had another one of these to repair....I only had that one......maybe somebody here might have a bunch of pics of the guts.....
Cheers

[Sebastian S.]

I did not disconnect anything till yet as I first wanted to take a look where the cps are. So after taking the photo I took everything back to place. So the both big Cps on the photo are the correct one?

[bsco]

I did not disconnect anything till yet as I first wanted to take a look where the cps are. So after taking the photo I took everything back to place. So the both big Cps on the photo are the correct one?

OK.Sorry but I am a bit confused.....I was looking at the post #19 by mistake...that is why I had thought you had already replaced them.....yes..if you are having a hum issue that was like mine, the master volume would control the volume of the hum, then my issue was with one of the filter caps....they are the biggest caps there and they are marked.........I am not saying that is your problem but it was mine........can you measure the pos and neg voltage rails to see which one is out of wack???.... you have to disconnect ALL the wiring in order to get the board out.....so replace both of them.....and make sure that you know exactly where every single connector goes when re-installing the board......Let us know how you made out with it......and there are lots of other techs here that are way more experienced than me but I did have this problem with one of these amps a few years back and the filter caps were the issue....
Cheers