Help for a Mesa Exporess 5 50 + problem

boz_h

My Mesa Express 5 50 + has developed a fault where it is very quiet and horribly distorted on all channels.  I have taken it too Anthony at Tube Distinctions and he says there is some kind of voltage drain in the preamp between valve two and four.  Unfortunately he is struggling to find the cause.  He has contacted Mesa and they have made a few suggestions, none of them have worked.  Has anyone here come across something similar and/or can make a suggestion that I can pass on to Anthoney.  Sorry for my first post to be a bit of a beggin one, and thanks in advance.

uncledick

Bit of a long shot but I had similar loss of output on a smaller Boogie caused by a faulty op-amp which was pulling one of the supply rails down.  The resistance across its Vcc and gnd pins had dropped to about 150 Ohms iirc.  A new one was nearer 10k.  Rather than desolder some of the components I isolated them by cutting the pcb tracks in strategic places.  Easy enough to solder a bridge wire back on when you've finished.

Failing that, you could just buy my Mk V in the classifieds

boz_h

Thanks for the reply I shall pass it on.  As far as the MKV, I believe death or to be more accurate the silent treatment shall come on swift wings.  Very tempting though. 

ICBM

Unfortunately I don't have a schematic for these amps, but a fault I've come across in a couple of more modern Mesas is a failed JFET transistor which is part of the muting circuit that prevents pops from the channel switching relays. These are briefly turned on during the channel change operation and short the signal to ground to mute the pop. If one develops a short and effectively gets stuck on it will do what you're experiencing. I'm pretty sure the Express does use these.

boz_h

Thanks you  I shall pass this on as well.  I am always impressed by the depth of knowledge and helpfulness of the people on this forum, as well as the ability to tempt me into getting into real trouble with she who must be obeyed.