Crumar Musical Instruments User Group.

Yeehaw!

The CEM3328 Bit is now fully repaired.

This one had some bad CEM3328 filters, bad TL082 op-amps, and then was
mostly functional aside from being out of tune and having one voice
still out.  I decided that the problem was probably not from the lower
filter board, since switching things around down there didn't change
the bad voice, so I looked at the upper board.  There I found another
row of TL082 op-amps, so I socketed these and began moving the chips
around.  The bad voice followed a chip, so I replaced it and presto!

It is worth mentioning that some of the TL082s were running pretty hot
- about 110-120 degrees f.  One of them was over 220 degrees!  I
swapped these out until I got a full set that were 100 degrees or
below.  It's 82 here, so that seemed acceptable ;)

I tuned the wacky voice by adjusting one of the potentiometers on the
left side of the upper board, with the detune slider in the middle. 
Seems okay now, but I'm not sure why it went so far out of tune(?)

Here are some of the things that I learned while repairing the Bits:

1) Your finger is not a good diagnostic instrument.  I found this out
by trying to check the temperature of an IC that was running at around
220 degrees.  Bad idea, get a temperature probe for your meter
instead.  I got a meter with the temperature probe for $30 on ebay.  I
would guess that you can get a probe that plugs into any meter for
less.  If you really have money, Fluke makes a nifty device that uses
a laser and infrared, so you point a laser pointer at the chip, and it
tells you how hot it is!  I didn't spring for this, but may in the
future.  In any case, finding chips that are running inexplicably hot
is very helpful.

2) Print out the datasheets for ICs that you plan to test.  It
definitely helps to know where to look for supply voltage, ground,
output voltage, etc... Everyone's help from here in the forum was
invaluable in getting me started.

3) Check the ribbon cables.  Some of the cables do not have connectors
on both ends, but instead are soldered to the board.  Many of these
failed while disassembling and assembling the keyboard.  They are more
than 20 years old, so I can't complain - but if you take the keyboard
apart, then put it back together and things get worse, be sure to
check these since the little exposed wire likes to break.

4) If you don't know what to test (like me) socket the board ;)  I
didn't know how to read the schematics well enough to clearly trace
the circuit path for each voice, so I desoldered all of the ICs on the
board, put in sockets, then started to switch the ICs around and/or
leave some out (with the power turned off!).  I could then test each
voice individually and figure out which chips controlled which voices.
 That made it possible to narrow down the problems, and to find the
bad parts.

Thanks to everyone for all the help!