Mesa Express noise

[Randall]

Mesa Express 5:25 in for a static noise at idle, sort of comes and goes a bit. Sounds like a component breaking down. I pulled tubes V1 and V2, no effect. Pulled V3 and it stops. Controls have no effect on the noise. I have isolated it down to the Contour/Loop area on page five of this schematic.

Grounding pin 7 of V3 stops it, as does lifting 1uF C28. Note: I pulled the J175 at the loop jacks for now because it fooled me for a while in thinking a dummy plug in the Return jack had no effect. I think it is an normally on bypass? Now the Return jack interrupts the signal.

I also lifted the .01 cap after RY1/L on the page above it, which had no effect, so I think the problem is between Q1 and C28. Am I correct in thinking the Contour filters are not likely the culprit? The real problem is, these things are just a beast to get under the board, or even identify components. Q 1-4 are all jammed in together in a tight spot .

If anyone could think of an idea for this I would appreciate the help.

http://el34world.com/charts/Schemati...ss_525-550.pdf

[Pedro Vecino]

I have found a very similar problem. It may or may not be the one you expose.
The solution that I applied was to replace the marked capacitor (probably C24 0.01uF but I can not guarantee it because I do not see the reference under it) by a high value capacitor (0.33uF). Try it with same or similar high value.
Please confirm me if that capacitor corresponds to that reference.

[Randall]

Pedro,

Yes that cap is C24, but as I said I lifted it and it had no effect, so I don't think that is my problem.

[Pedro Vecino]

I also lifted it along with other things to isolate the origin. It was when I used a new one, raising its value very much when the problem disappeared.
That's why I remember it. Because of the oddness of the solution

[Randall]

Changing that cap made no difference. Gotta get some freeze spray.

[Randall]

I sure could use some help with this beast, because I am at wit's end here. Enzo won't like this but after replacing Q 1 - 4 with no results, ended up replacing every component in the section from Q1 over to, but not including the .1 uF C28. I lifted .01 C24 and the JFET below it in the master volume section on the page above it, and AFAIK there is nothing else connected to the base of Q1. It measures a short from the cap hole at RY1/L to the base of Q1.

Here is where I get tossed. Grounding the base of Q1 stops the noise, yet I don't see the noise anywhere on Q1. I do however see it from the base of Q2 forward. Grounding the base of Q2 also stops it. So, as I wondered before, could it be coming from the passive contour control section below it? I don't see how, but what else can it be? Contour controls switch on and off, and the noise is equal in both channels. I wish I could see a way to disconnect the contour section to see what does or does not happen.

ARGH! I have so much time into this thing already, but I want to get it fixed. I hate Mesas.

[bob p]

Gotta get some freeze spray.

Don't go to AutoZone.

[bob p]

I seem to remember seeing a similar schematic in a YouTube video where a guy was complaining that Mesa deliberately introduces errors in their schematics to confound the process of troubleshooting by people who don't contact mesa for schematics. More likely is that he was misreading the schematic or that he had the wrong schematic for the amp in question. Maybe it would be worth giving Mesa a call on this one, to confirm that you have the right schematic for your amp. I'd also trace the circuit to confirm there's nothing wrong with the schematic.

edit: that "contour" circuit is the same basic graphic EQ circuit that's used in other Mesa amps like the Bass 400.

[The Dude]

If grounding the base of Q1 stops the noise, it must be before that. If you can't see the noise on Q1, it's likely just that low level. I'd work backwards from there. If removing C24 did not cure the noise, I'd be looking at that J175 between C24 and Q1.

[Enzo]

Agree^^^. If grounding base of Q1 stops the noise, then that is where to look back from.

If lifting C24 has no effect, then we look between there and Q1, and yes, that J175 becomes a suspect, lift it.

Since C24 has no effect, can I assume the two master volume controls do not affect it either?

[Randall]

As stated above that JFET, a J175 IS lifted, so as near as I can tell there is nothing on the grid of Q1. I did that way back. I DID find one component on the board that is not on the schematic, a third SA22 diode connected to R90 near the Send/Return connection. I lifted it, no change.

So this is exactly what is vexing me. If those two components are lifted, and grounding the grid at Q1 stops the noise, WTF is going on?

[The Dude]

How about C25- the 15UF? It might be arcing.

[Randall]

Nope, it's lifted, too. Forgot to mention that.

[Enzo]

It measures a short from the cap hole at RY1/L to the base of Q1.

This concerns me. If C24 is in place, then this seems to show it is shorted. If C24 is still lifted, then where is the circuit for continuity between relay and Q1? C24 should open that path.

Grounding Q1 base still silences the hum? power off, what resistance to ground from base Q1? From base to collector, do you see 473k?

C25 is lifted? Try tacking something in its place, value not important. Got a 20uf/25v cathode cap for a Fender hanging around? That is there to clean up the bias to the base of Q1.

[Randall]

C24 is not shorted. It is lifted not on the relay side, but on the "To Contour Filters" side, I am measuring a short from that hole to the base of Q1. So for those who say look backwards from Q1, what is there to look at in these conditions?

I tried Pedro's suggestion to try a larger cap, nothing.

From base of Q1 to ground I see 325K. From base to collector I see 325K (C25 still lifted). From Collector to ground I see 5 ohms. This sounds like it is a problem? I see nothing on this drawing that would make that possible. Powered up, I get a clean looking 12v at the collector.

Tacking in the cap you suggested at C25 has no change.