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Model: MM-4
Fault: intermittant total overdrive hum output, peak LED's lit
Date: 02/02/00
I learned early not to get too deep into pre-servicing things.
“Oh doctor, I have a terrible pain in all the diodes down my left side, what do you think it could be?”
Doctors complain about people trying to get free advice at parties, and only a minority of people are sick.
But everybody has something electronic that doesn't work.
You have to ask the client what the symptoms are, but mostly what you get told is immaterial and often quite misleading, so I try to take it all in but reserve forming opinions until after I have seen the actual animal.
You can expend a lot of mental energy chasing shadows if you are not careful.
John dropped in the Leem mini-mixer which had started playing up at the gig the night before.
Apart from getting on it right away, he had made a detailed examination of the symptoms to report to me.
From this I guessed that the main power supply filter cap had come adrift somehow.
From the bench notes...
Noted something rattling loose inside the case.
Open case, clean and inspect. Loose screw found inside.
All peak LEDs lighting means that it isn't just an open screen on one input source, but is something common to the whole mixer.
Being a loud hum, we are looking for a ground off somewhere.
At first glance it seems like a reasonable design, but the quality of construction is a bit 'ordinary' - a trifle rough.
First stop, the power supply and on closer inspection... bingo!
The ground end of the main filter capacitor for one of the two supply rails (+ & - 15 volts) is clearly an intermittant 'dry' solder joint, and prodding it produced the symptoms described - exactly.
Why? Well these capacitors are bulky and are normally glued down to the board to stop them flapping about with vibration and impacts.
Space has been allowed for this on the board, but these ones simply aren't, and this is obviously the basic cause of the failure.
Major disassembly of 'innards'.
This is one of those delightfully “economic” designs where everything is mounted on a single printed circuit board mounted parallel to, and just behind, the front panel using the controls for mounting.
This looks reasonable until you have to get to the other side to solder something, then every single knob, pot, and socket has to be removed/unbolted from the front panel, roughly 30 items in a four channel mixer.
The result is that one dry joint (due to slack manufacture anyway) takes three hours to fix, even when we know exactly what it is before we start.
Repair joint(s), glue caps down, extensive clean including all sockets (not pots as all seem okay on test and it's better not to disturb).
Exposed mains points sleeved for safety.
Massive reassemble and test. Now seems all okay.
Oh yes, the loose screw.
Potentially quite serious, but not the cause of anything except some intermittant hum and/or noise at a low level since it was only one of several grounding screws, nothing like this fault. Replaced.
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<<<OzValveAmps
http://www.ozvalveamps.org/leemmixer.html | Last update: 22:36 14/01/06 |