[sdiy] Help!

[Jeff Farr]

As some of you already know I've been working on building my own
Tr-808 for some time now.  Things have been going quite well despite
there being alot more waiting involved than I would like.  Anyhoo,
I've now got all the analog circuits built and working except the
Cymbals, and thats my problem, I'm getting a perplexing output and I
really can't figure the circuit out well enough to find a culprit.
Here is what is happening:  at the output I get a constant 60hz-ish
buzz and some noisy crackle, if I trigger the sound I do in fact hear
a proper 808 cymbal except the decay has no effect and the tone
doesn't seem to have the right effect either.  The volume does work,
affecting the crackle, buzz and cymbal sound.  Now, when I put a scope
on the output what I see will depend on the volume, at full volume I
see a slightly filtered saw wave ~10vpp ~30KHZ (yes, 30K), and as I
drop the volume it appears like it is being filtered into a square
wave until the volume gets pretty low and then the signal goes flat.
Now, I think it is safe to assume that the saw/square wave is not the
proper thing to have in this circuit so I began tracing it back
through and it's there until the kathode of D11, at which point the
signal looks like the output of the schmitt trigger.  Now, remembering
that the tone control also didn't seem to have the proper effect on
the sound I started probing various places along the path that begins
with IC3a.  There the signal looks like it should (indeed quite
similar to tracing IC3b) until the kathode of D12, where as far as I
can tell it simply disappears.  Now, I've spent the last few days
checking and replacing various components and double checking against
a couple copies of the original schematics, and I am confident now
that I have the circuit built correctly.  However, this is as far as I
can get with the circuit analysis:

IC3a & b make up two frequency selective networks that process the
output of the scmitt trigger (which is some kind of high frequency
pseudo noise) and then sends their respective outputs through some
circuitry that shapes the amplitude and then buffers the two signals
so that VR4 can provide a crossfade between two different tones.  Q20
is what creates the amp envelope by buffering the accent voltage when
a trigger is received.  IC5 buffers the two tones from the output and
probably provides additional shaping of the tone.

And thats it.  At this moment I really don't know what Q16, 17 and 18
do and I am working of a set of assumptions to troubleshoot this
thing.  Here is what I assume to be correct:

--IC6 is working properly, as the volume control does what it is
supposed to, even though the signal passing through it isn't correct.
--The trigger and accent circuitry is working properly, remember that
I can hear the cymbal sound when I trigger it, but it is swamped by
the buzz and the noise.
--IC3a & b are doing what they are supposed to be doing.
--IC5a & b are doing what they are supposed to be doing.

Here is what I assume is not correct:
--Q17 and 18 are not working as they should.  They are both a part of
very similar circuit fragments and are being fed very similar signals,
but their outputs are totally different.  Q17 shows a high frequency
saw (kinda a toothy square, actually) Q18 appears to have no output.
--it is this fact that is why neither the tone or decay controls work
like they should.
--I may well be wrong in assuming that there are two different signals
that get mixed together at IC6

Schematics for this can be found here
http://xlargex.xl.funpic.de/projects/808/schema/808_CY300dpi.jpg