Vintage Synth Repair

Hi guys..bought a supposedly fully working 606 of off
eBay to find it has problems...

1. It wouldn't work with an AC adaptor, batteries
only. Anyway I fixed that, was some shorted caps and a
bad regulator. It had some battery damage too around
that area.

Next job..the Hi-hats and cymbal sounded really weird,
kind of flutey and going up and down very slowly in
pitch. Scoped it and found a bad 4558...fixed? Well
sort of, the cymbal sounds fine, the hi-hat sounds ok
except that the open hi-hat sounds identical to the
closed hi-hat i.e. not open with a long decay. 

All the transistors in the hi-hat circuit check out
fine. I\ufffdve changed the 2904 op amp, the oscillator IC
is fine (4584). The trigger is fine from the CPU...
The tempo clock signal is ok as well to the Hi-hats. I
thought maybe the closed hi hat trigger was triggering
the open too or something. 

So I moved to caps. I\ufffdve changed every single
Electrolytic marked in the hi-hats section. No change.
I notice one thing. If I change the value of cap C45 I
can make the open hi-hat decay longer but I\ufffdm moving
it well away from the stated 1uf it\ufffds supposed to be.

One other thing; when you start a pattern off the
first hi hat beat is raised in pitch also when you
switch between write mode or play mode, the pitch of
all the cymbals, hi-hats change. You get a higher
pitch in play mode.

So to recap I basically have an open hi-hat that
sounds identical to the closed hi-hat. And some weird
pitch change stuff going on.

Im looking at it again tonight so if anyone has any
ideas, shoot.

Thanks, Phil.



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