Mark III output section intermittent cutout

[noumena]

Mark III Simul-Class combo, late 1980s. (Furthr details available).
Getting a reliably decent tone from this amp is a struggle, because it is
so inconsistent.

Symptoms:
Sometimes it sounds OK, at other times not. The clean channel ALWAYS sounds dull and
clunky; lead channel sounds 'hairy', as if there is a fuzz plugged in, or as if
there is really bad string buzz (which I know there isn't - I can't hear it on the clean channel).
With treble boost and other distortion pedals, the amp over-distorts and goes all 'farty' on all
channels. This is true whatever the guitar, ranging from terrible on single
coils (American Deluxe Strat), to only slightly better with humbuckers (PRS Custom 22).

Also, high settings of Master no longer cause the bulb and fan to work
on a THD HotPlate. I've been in touch with a tech at THD, who thinks there's
something wrong with the output of the amp, not a problem with the HotPlate. I'm
beginning to believe that.

Also, I'm convinced it sounds different (slightly better?) when the chassis is
out of the cab...

But here's the thing I've recently noticed: You are supposed to be able to plug
into the Effects Return jack and 'produce sound' (says the manual). If not, there
is something wrong with V5 and the output tubes. In my case, there is sound, but
it eventually fades and cuts out to silence after a few seconds. To get the
sound back, I have to flip the standby switch off and then back on. This
suggests there is something wrong with V2 or the output tubes. A similar thing
happens when I plug any pedals into the effects loop, and sometimes no sound is
produced at all.

Anybody seen this before and got any clues as to what's going on? What could be causing the
intermittent cutout in the output section of the amp, and is that what's
causing the inconsistency and bad tone?

 

[>Photi G<]

This could mean a few things. First, a full retube. Second, possibly a full electrolytic cap job, and third, the C30 mod to calm the buzz on the lead channel.

 

[noumena]

OK, many thanks. I'll look into the C30 mod.

 

[Neptical]

I think you need a complete (tubes,caps, the works!) overhaul by the man himself, Mike Bendelli. This amp should be sparkling whether using single coils or humbuckers. To take off the 'buzz', I'd keep the presence off and the 6600hz slider a hair below the mid line, and keep the rest of the GEQ in 'sort of' the V fashion..or,maybe in the softer 'smiley face' setting. Might as well get hooked up with the R2 mod while your at it to balance the volume of the channels.

~Nep~

 

[noumena]

Thank you Neptical. I will try/consider all those options. Although having th amp overhauled by MB is out of the question - I'm in London, UK. But I know a good amp tech...