Autumnal musty fruity smell

[N_Cook]

Inside a Marshall MG50DFX. Someone must have thrown a pint of cider into the
fan vent, inwards blowing fan. Heatsink protected the TDA7293 from being
spattered but 6 faults found (so far), dotted around the amp. Washed boards,
redid solder points where there was sediment etc. So far working with no
reaction to twizzle- sticking, I wonder if it will come bouncing back.

 

[N_Cook]

MG50 eh? You might as well change all the switched jack sockets on the
back panel while you have it in bits, they always give trouble.

Ron

one fault but by different method. The effects loop yes , problem in the
ribbon.
Not crimping , assumed corrosion in the recess of a socket, cleaned the pins
, put alternating set on the pins , both ends, and swapped end for end the
connectors.

The second time I've come across one of these with a loud buzz at switch on,
sometimes !!. I suspected a leaky path via the supposed isolted heatsink and
have added extra isolation and its not returned. One time I monitored
the -38V rail , when powering up and it dropped to -28V until lightly
tapping and went to -38V and no buzz. -rail is on the tab of the TDA7293.
I've powered up numerous times since , and no return, but I had that happen
before - is this a known problem? Problem with the electros around the o/p
board or internal problem in the TDA ? How to force the problem if its
hiding for the moment?

 

[N_Cook]

"but I had that happen before"
on another MG50DFX that is, thought I'd cured the problem, tested soak, but
not repeated on/off and that problem returned intermittently, owner did not
return so real problem not diagnosed

 

[N_Cook]

The vast majority of problems I`ve encountered with these amps has been
switched socket related. the effects loop is the obvious one, but the
foot switch socket gives trouble, and the headphone socket. It`s quicker
to replace them all, adding a drop of electrolube to the contact
surfaces after soldering them in.

You might check for cracks in the main pcb usually eminating from the
mounting pillar in the centre - caused by rough handling. I`ve changed a
couple of TDA`s probably caused by the user adding extra speakers and
thrashing the amp, they tend to be abused by 'young musicians'

Smoothing caps tend to drop off the board.

Ron

so far switched on/off with up to hours between and no return.

Previous time I convinced myself it was a problem on the muting signal line
as there was oscillation on it. I thought it was at the preamp end there was
a problem but it was probably a problem associated with the TDA feeding
forward. It was only for 2 or 3 seconds at start up but very large amplitude
buzz, independent of vol/gain settings , ie setting all at zero made no
difference.

 

[N_Cook]

with back. the switch heatsink





so far switched on/off with up to hours between and no return.

Previous time I convinced myself it was a problem on the muting signal line
as there was oscillation on it. I thought it was at the preamp end there was
a problem but it was probably a problem associated with the TDA feeding
forward. It was only for 2 or 3 seconds at start up but very large amplitude
buzz, independent of vol/gain settings , ie setting all at zero made no
difference.- Hide quoted text -

- Show quoted text -

Have you tried cooling it for 24 hours in a refrigerator and then
testing it, or is it too big for that?

*********8

The output section of these is demountable and could be placed in a fridge
quite easily but I'm not sure it would tell me much as this fault is not
consistent, does not always happen at switch on.

I only have a 7294 sitting around as a spare rather than 7293 , will have
to order one for next time. Must have switched on 20 to 30 mtimes by now and
the problem not returned once so will return it to owner and order a 7293
for if it should bounce back. After that then caps problem is the next
recourse