Moog Music Forum

Rebuilding the power supply got my Sonic Six up and running, but there's always been a low level high pitched noise on the output. It's not effected by the filter or amplifier and it's steady in pitch but the amplitude varies some.

This video shows the same problem on someone else's Sonic V:

http://www.youtube.com/watch?v=ZEAt7eO_ ... r_embedded

Page 54 shows a schematic of the region in suspect (Thanks to Kevin for these):

http://www.synthfool.com/docs/Moog/Moog ... Manual.pdf

I'm a novice when it comes to diagnosing these things, but two CA3080 OTA chips are the last amplifiers in the audio path without an attenuator. Grasping at straws I think I start by replacing these with NTE996's. This is my first post here and any advice is much appreciated!

If you have a high freq pitch audible and it's not adjustable anywhere, it's likely from the 8000 hz waveform used in the keyboard circuitry.

It's a one bussbar keyboard instrument and in order to obtain both gate and CV, they impart this 8KC signal (AC) on top of the DC current used to obtain CV.
They use it to sense a key closure which forms a gate, it's then filtered off and what remains is DC for keyboard CV.

I don't have the schemos in front of me, but many Moogs used this scheme.
The difference is that the Sonic Six uses a pretty low freq (8KC) as compared to later ones which use around 35khz.

Also, this service manual indicates to use Cramolin to clean buss bars, if I recall.
Moog later discovered this wasn't good advice and changed their later service manuals to indicate *not* using it.
Caig's deoxit is today's modern-day equivalent, but I don't recommend using any chemicals like this at all on the buss bars.

I'd offer that the instrument could use a good cleaning and recapping.
If it was mine, I'd completely clean the boards, the keyboard and replace all the electrolytic caps. Likely also several other caps Moog used in this era which now crack easily.
On the last S6 I serviced, many of the resistors were out of spec too.
Many parts needed replacement.
I should also add that back then, they didn't always use plated through holes for their components.
You may have to solder parts on both sides of a board.

I don't think the problem has anything to do with the OTAs at the end, but it's up to you to try and see what works and what doesn't.

You might even check to see if both the instrument and amp system are properly grounded.

Good luck.
Sorry that I can't delve into troubleshooting synths deeply on forums.

Kevin, would the Hammond bus bar lube be good on these old keyboards?
I know it works wonders on the 54' of bus bar in an a-100. (But they also used Paladium contact wires.)

Thanks Kevin,

8000 Hz: spot on! I'll have a look at that keyboard circuit and share whatever I learn. I used isopropyl alcohol to clean the bus bar a while back and haven't had any triggering issues, deoxit is the plague imho.