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.
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
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
the
boards,
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?
"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
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
If the buzz occurs with no jack plugged into the input and varies with the volume level, it`s the contacts on the input jack not shorting it when there`s no plug in
into
being
no
pins
the
on,
and
TDA7293.
happen
o/p
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.
d
th
ck.
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ink
e s r ane
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ude
Have you tried cooling it for 24 hours in a refrigerator and then testing it, or is it too big for that?
with
back.
the
the
the
switch
heatsink
line
was
amplitude
Have you tried cooling it for 24 hours in a refrigerator and then testing it, or is it too big for that?
*********8The 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
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