gripes and more on cheap sampling

[Stopp,Gene]

Wow - sounds like you're in analog hell :) I remember being there....

Do you have a scope? It's really really really really useful for
situations like this. If not, do try to get one, borrow, trade, buy,
steal. Okay don't steal. Or if you do, don't say I told you to.

The cap thing sounds weird. Could have been bad caps, I guess, any
chance they were in backwards? Tantalums *hate* that.

A big warning about 3140's - I know it's in the ASM documentation, but
it bears repeating: they blow up real easy! They are easily the most
static-sensitive parts I've ever dealt with. Use excessive ESD
precautions and keep the cats away. It's the price we pay for that teeny
input current. It's not much of a stretch to believe that they're also
kind of intolerant to abuse when installed, as well.

The 30 pf cap being needed depends on the dual op-amp that you use in
the VCA. A 1458 wouldn't need it, for example, but the TL082 does. It
squelches a supersonic oscillation (another place where a scope would
lead to the solution quickly).

As for the failed circuits, without a scope, your only recourse is to
swap. "Shot-gunning", we used to call it on the manufacturing line....

Good luck!

 - Gene

 ----------
From: Entropy Struct
To: synth-diy at mailhost.bpa.nl
Subject: gripes and more on cheap sampling
Date: Wednesday, January 28, 1998 1:56AM

(will this go through? I hope so... It's been a few months since I've
been
on the list. There's a lot of griping/troubleshooting in this mail with
a
little bit of intelligent discussion at the bottom..)

I finally got my asm-1 working to some degree. (2 asm boards, 2xVCF,
3xVCA, 3xVCO, 2xLFO, 1xnoise, 4xADSR, 1xglide) I had awful problems with
the LFO apparently bleeding into the outputs (most notably VCF and noise
generator), but after I disconnected the two 10uF tantalum conditioning
caps it went away like a charm, so i guess it was coming from the noise
source, not the LFO. Bad caps?

Basically the only problem I have left is that the VCO's are still dead.
I
checked the placement of the JFET and 3904's.. no go. I have the
FET properly placed and the 3904's right with the flat sides facing the
MAT-2 square.. There's a bit of low current running through the outputs,
but nothing more. Adjusting the trimpots doesn't help. 1 bad VCO I can
understand, but *3*? I have to be doing something fundementally wrong
here. It's the only "mystery" about this synth that's still bugging
me...
and also, well, keeping it from being a usable synthesizer. If anybody
has ideas about what could be wrong I would *really* appreciate it.

I know this has been mentioned, but I would also like to say that the
30pF
cap across the VCA outs really should be required. It sounds so much
better, and doesn't have that "buzz" at a certain point in the envelope.

I also shorted out something when I accidentally slipped my trimpot
screwdriver down inbetween the 3140 by the Q out and the offset
trimpot..
I thought I nailed either the closest 3140 or the 3080 but I replaced
those with no improvement... no mystery here though, I'll probably end
up
replacing the chips one by one until something clicks. <sigh> I seem to
be
pretty jinxed; my paia midi->cv died on me, too. The only thing I can
get
out of the thing in trigger mode is a ground hum when I hook a proper
ground up to it... this never used to happen. It responds to midi data,
so
something on the analog end must have gotten jostled in the move.
<grumble> It's too damn small, what could have possibly gotten fried?