Mark III blue stripe strange noise. Help, please...

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Rudl

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Hi,
I own Mark III, blue stripe, 1992 year, and the problem is that when turned on, volume suddenly gets at least half lower, an strange crunchy sound is coming out of it. This hapens on all channels (most obvious is on clean). After few minutes i hear a quiet click and everything is normal again. I don't have possibillitie to take it to Mesa service, i had it at my local service for five times yet, they changed everything possible, from tubes, condensers, tube sockets, optocoupler and so on and on, but nothing helped. Any ideas, what is going on?
 
would get a look at bias current and if heater current is steady, heaters plays like switch on tube , any variation here could mess on output !
 
I had a similar problem with a 50 Cal. +. My local tech would clean all the contacts and could never duplicate the problems. As we looked through the schematic, he traced the signal going through the return Jack of the effects loop. So when the problem occurred again, I jumped the effects loop. The problem has never returned
 
It sounds like a heat or surge issue. Take a look at the MOV coming off the mains input. It's there to suppress surge and if it's going bad, it could cause similar issues.

Other than that, I would make absolutely certain all the power tube sockets are clean, and tight. And check for conductivity between the pins as well.
 

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