Mesa Express noise

[Enzo]

Given the 5 ohms through the power supply, those 300k numbers look OK.

The 5 ohms I will assume is the heaters of the tubes, pull them and the 5 ohms goes up to maybe 60 ohms?

[Randall]

With all tubes pulled, it goes up to 116 ohms.

[Enzo]

Good enough. The things I was considering are covered.

[Randall]

Would anyone care to weigh in on whether the contour filter circuit could be a factor here? I wish I could find a way to disconnect it to verify. I really don't know what to do at this point.

[bob p]

The graphic EQ could give you noise at Q2.

I've been holding back on mentioning what I think is sort of a red herring problem until the obvious problems were looked at first.

Check J175C1 and J175C2.
If that doesn't work, check C44-C47.
Here's the red herring: if the insulation has broken down on L1-L4 they could cause popping noises.

[Randall]

"The graphic EQ could give you noise at Q2."

The thing is, there is no graphic EQ, even though the flow chart says there is. It's a fixed contour circuit. Plus, how does it reconcile with the fact that grounding the base of Q1 stops the noise?

"Check J175C1 and J175C2. "

Grounding R87 at the contour level section has no effect, so wouldn't that preclude J175C1 and J175C2? The noise is in both channels also.

"If that doesn't work, check C44-C47."

Ugh. That means pulling this board, which is no small thing. But I will do what I must.

"Here's the red herring if the insulation has broken down on L1-L4 they could cause popping noises"

I wouldn't describe the noises as popping as much as a radio tuned in between stations with the treble rolled off a bit, with an occasional spike. Anyway. how do we determine if an inductor has said condition?

[nsubulysses]

I wouldn't describe the noises as popping as much as a radio tuned in between stations with the treble rolled off a bit, with an occasional spike. Anyway. how do we determine if an inductor has said condition?

I have battled some insane noises in mesas and have sometimes won and sometimes lost. What you describe makes me shudder just reading it. I feel for you and hope you are paying yourself adequately. You really deserve it

[Randall]

If I don't figure this out, all this will be for no pay, as I don't charge for something I don't fix.

[Pedro Vecino]

"If that doesn't work, check C44-C47."

Ugh. That means pulling this board, which is no small thing.

You dŽont need pull out the board. These capacitors must be easily accessible. Possibly the mustard tantalums at the right of the inductors.

[diydidi]

Randall. I have exactly the same issue you described. Did you fix it? What was the solution?