Vintage Synth Repair

Many thanks for your reply Scott. I think I might have *stumbled* on root of the problem. By no means sorted - because it is a real funny one.

Here goes...

As you know I replaced the TOS chip with a 'retro' the KS-196

Now this retro is a little cct board with three chips on it and long legs that plug into where the original AY chip was.

One of the chips on this cct board is a 74HC4046 (phase locked loop with VCO) I found that if I touched the chip then the pitch drifted. I could almost 'tune' it by pressing harder, wetting my finger etc.

So, investigating further I found that if I only touch pin 4 (VCO out) of this IC (using a pin or small screwdriver) - everything is spot on! It just seems to jump to how it all should be. Solid. Now obviously I need to replace me touching it with a better solution, only what!?

I tried grounding that pin - no. I tried grounding it through a capacitor - no. I am at a loss as, now having found the answer I really don't want to blow anything up with trial and error.

Could anyone here maybe look at the data sheet for that IC and fathom out why it should behave like this and what needs doing with pin 4?

I could email the pdf of the chip data or upload intoour files section it here. Please let me know if anyone can help.

Thanks again,
TOM




--- In vintagesynthrepair@yahoogroups.com, "Scott" <painintheamp@...> wrote:
>
> 
> Hi tom,
> 
> If there are any ckts using a bi-polar supply scheme( and part of the ckt you're working on), check to see if theyre where they need to be 
> 
> ie; +/- 12 or +/- 15 volts
>