more midi2cv...

[Andrew Schrock]

On Tue, 1 Jun 1999 WeAreAs1 at aol.com wrote:
> <<Any ideas? Replace the opamp buffers? Power supply? Help...? >>
> 
> I don't have a midi2cv schematic, so I don't know the exact circuitry 
> involved, but there's probably a 4051 analog mux (or two) in there somewhere 
> (right before the output buffer amps).  I would try replacing it with a new 
> one.  I have seen the same problem you are describing (different analog 
> channels interfering with/beeding into others) in other units that turned out 
> to be caused by bad 4051's.  If that doesn't fix it, try replacing the output 
> buffer opamps (it has LM324 quad amps, right?).

On the nose, it definitely has a 4051. I didn't have a spare, even tho I
was sure I had a few around from replacing the bad ones on my octave cat!
I'll pick some up tonight, keep your fingers crossed. 

If it did indeed go, I'm not sure how the 4051 went bad, but perhaps last
time it was in for service it slipped by unnoticed. It strikes me as
pretty easy to miss the fact the output drops by .02 volts when the key is
depressed, especially when it was just given the once-over by Paia in 20
minutes. 

It has an LM324 and 2 TL084s I think. I'll probably replace the latter
with TL074's just to be safe, even tho they're far less likely to go than
the 4051. 

Thanks... 
Andrew

-| Andrew Schrock | aschrock at cs.brandeis.edu |-