wrote in message
ups.com...
Maybe crackle is not a good term to use. I would say distorted
sounding. Almost like overdriven input to a soundboard or a blown
speaker (but its not that). Well I resoldered all joints on heatsink
and many of the others while I was there. No improvement or change at
all. Re-checked with headphones and left and right sound great! Does
that eliminate the circuitboard behind the front panel and focus us on
the board with the heatsinks? By the way what do the 2 big caps at the
back of the unit do?
Are they a filter of sorts? In my pinball repair they have used them to
stabilize voltages.
Wish I had some info the the test points TP1 and TP2. I wish I knew
what componets are used for the left channel, I could replace some and
hope for the best. Any other leads appreciated. Thanks Everyone Dave


The two big caps are the main smoothers ( filter caps ) for the positive and
negative supply rails to the output stages. You cannot repair DC coupled
amplifiers by guesswork and hopeful component replacement, I'm afraid. They
are a black art that can at times, drive even the most experienced of us to
distraction and tears ... If you have distorted sound, particularly if it is
harsh and raspy, as you say " like a blown speaker ", it is likely that one
half of the output stage is not being driven. This could be for many
reasons, but in your case, it may be for some very odd reason, as usually,
any inbalance in the drive conditions, will result in a DC offset at the
midpoint, which will be picked up by the protection circuitry, causing a
shutdown. The only way really to proceed with this type of fault, is to sit
down quietly with a schematic diagram, a good multimeter, and a 'scope. I
would have to say that a good degree of practical experience is also needed
to work on this type of amp with any liklihood of a degree of success.

As an aside, I have a Williams Cosmic Gunfight from 1982, which I spent many
happy hours restoring. Originally had faults on just about every board, and
some incorrect wiring on the playfield, where parts had been replaced. I
became quite adept at the 7 series CPU boards, and mended quite a few of
them for the dealer that I originally got the machine from for fifty quid.

Arfa