Mesa Boogie TA15 intermittent noise on the top boost channel only, Known issue

[Arbutt]

Hello
I have a Mesa Boogie TA15 with a intermittent noise , "like a noisy tube" this only effects the Top Boost channel
posted a audio clip in 2nd post
Does not effect the Tweed channel.
I think this a known issue, as i have found a mesa boogie service bullitin on this issue. Attached
I have done this , and i still get it the problem
The amp has had all new tubes from the customer and the fault is still here
I know there is a mention of serial no on the service bullitin, I'll have to check what mine is

With V1 and V2 out i still get this problem when on Top boost..
Changed R13 and C1 and C2 , and did a tempary connection to a new vol pot
remove v3 no noise issues
when the noise starts , if you switch to tweed channel and back its back to normal
I did remove the 1uf 35v and add a 10uf 35v instead, just as a shot in the dark

Has anyone had this issue and found a cure ?

output.pdf schematic diagram just the 1st stage, not the output

[Arbutt]

2025_03_07_12_00_46.mp3 audio clip of noise from top boost channel

[g1]

The service bulletin description is about the sound dropping out completely, not a 'noisy tube' sound. I think you can forget about the bulletin and just start troubleshooting as a regular 'top boost channel noisy' issue. If switching back and forth gets rid of it, it could be the relay itself or any solder connection in that channel's circuitry. To start, you might try hardwiring the relay connection.

[Arbutt]

Thanks g1. I was beginning to think that this was a different fault. I hate working on boogies

[g1]

It's hard to tell from the sound file, but it could be the thing where the body of a resistor (or other component) is arcing to a trace underneath it. It's a problem that seems to be cropping up in modern boogie stuff. Looking at the board in the dark will sometimes help to find it.

[Arbutt]

The amp is now done
I did the dark room test, Nothing came up.
With V1 and V2 missing the fault was still there. With 3 out it wasn't
When i grounded Pin 7 on V3 it didn't stop it. But then i grounded Pin 3 of V3 it did
I found a 56PF( C3) silver mica cap just before the tone control for Top boost channel
Changed that, so far its been on for 5 hrs + without any issues
thanks again for all your help
Mr A

[Greg Robinson]

I found a 56PF( C3) silver mica cap just before the tone control for Top boost channel
Changed that, so far its been on for 5 hrs + without any issues

FWIW I abandoned using silver mica caps about 10 years ago as I was finding them to be a frequent failure point, often causing noise, presumably due to intermittent leakage. They're also much more expensive than equivalent film or C0G/NP0 ceramics which I've never known to fail in the same manner. I've heard reports that the silver mica caps made by CDE do not fail in the same way, but they're still overpriced, so I haven't investigaged myself.