Mesa Triple Rectifier Signal Cutoff

[misterc57]

Odd issue.

The clean (green) channel signal volume pot cuts the signal when the dial is set past "3". Short story... Cleaned the pot. Replaced the pot. Swapped first two preanp tubes. Still have the issue.

I am trying to find a schematic to signal trace the problem.

Anyone have a schematic or thoughts on this? Thank you

Mark

[nevetslab]

I have a couple files here, that of the Owner's manual for the triple Rectifier, and the schematics for the Dual Rectifier which should have enough info to assist in your pursuit.

Dual & Triple Rectifier Solo OM.pdf

dual_rectifier.pdf

[g1]

Odd issue.

The clean (green) channel signal volume pot cuts the signal when the dial is set past "3".

HF (beyond hearing) oscillation can do this.

[The Dude]

Is there any DC voltage on that pot?

[misterc57]

Is there any DC voltage on that pot?

Yes, around 10 VDC. The other channels do not have DC on the volume pots.

So it appears there is no triple rect. schematic to be had. I am looking at the dual schematic but I cannot seem to locate that pot for the green/clean channel.

And for both dual and triple rectifier amps. V1A process all channels and V1B is clean channel only. Cannot find V1B on the schematic either.

mesa boogie dual rectifier.pdf

[GoldtoneWorkshop]

Is there any DC voltage on that pot?

Came here to say this. Check the coupling cap leading to the pot, it sounds like it is leaking

[The Dude]

Came here to say this. Check the coupling cap leading to the pot, it sounds like it is leaking

Yes, that's why I asked. I also suspect a leaky coupling cap.

[The Dude]

Yes, around 10 VDC. The other channels do not have DC on the volume pots.

So it appears there is no triple rect. schematic to be had. I am looking at the dual schematic but I cannot seem to locate that pot for the green/clean channel.

And for both dual and triple rectifier amps. V1A process all channels and V1B is clean channel only. Cannot find V1B on the schematic either.

mesa boogie dual rectifier.pdf

I can't tell you where exactly to look without a proper schematic, but you shouldn't have DC on that pot. You'll have to trace out the circuit . You could have a leaky cap from the previous stage plate circuit, or a leaky cap in a tone stack. Look for caps that have your 10VDC on one side and check them. Also, have you asked Mesa for a schematic?

[Jazz P Bass]

Here is what I have for the dual/ triple rectifier.

[The Dude]

If that's indeed the correct schematic, I'd suspect one of the tone stack caps has high leakage.

[misterc57]

Dude. New information has come to light.

Thank you

dual_rectifier.pdf

[misterc57]

I told my bud I did not want to work on this amp but he told me it needed a pot which was easy to get to. I dread having to pull the board up.

I'm thinking the leak will show up on all 4 caps. If I can get to the legs and check resistance would I see a low resistance across the culprit?

Thank you

[misterc57]

No leaks on the tone stack which was tied to the gain pot. I found that it was the 47pF that was leaking with around 170 VDC on one side and around 11 VDC on the leaking side to the pot.

I installed a new 47pF / 500V and within a short period of time it started to leak the full 170 VDC to the pot.

Blue cap is the old one, brown cap is the new one.

[Helmholtz]

If you see DCV at the 47p cap, the 0.047µ coupling cap is leaky. Read the schematic.
Were you speaking of the CH1 master pot all the time?

[misterc57]

If you see DCV at the 47p cap, the 0.047µ coupling cap is leaky. Read the schematic.
Were you speaking of the CH1 master pot all the time?

Yes CH1 master volume pot. And yes that made more sense and fixed the issue. Thank you Helmholtz