Moog System 35 from Hell!!!!

In a Moog Mood? Here's a forum for discussion of general Moog topics.
Post Reply
MikeBorish
Posts: 7
Joined: Mon Jan 21, 2013 4:59 pm
Location: Chicago, IL
Contact:

Moog System 35 from Hell!!!!

Post by MikeBorish » Sun Jan 29, 2017 3:45 pm

I'm in the middle of fixing a really fun one here. This System 35 thinks it's a Theremin! As you can see in the video below, 921B oscillator frequency changes based on proximity to the module and cabling. Some 921B's are more prominent than others. Ironically, the 921 a few spaces over is dead on and maintains rock solid pitch. My supply rails are spot on, and I've replaced all of the electrolytics in this thing.

I've got a gut feeling that there is some sort of impedance or resistance in the current return path of the oscillators or oscillator controllers, but that's about it at this point. It's located in an old building and there could be some issues with ground integrity.

I've got a few different theories of why it is doing this and I'm going to work on some solutions this week. What do you guys think? Has anyone here ever seen this kind of a problem before? I've never seen anything like it in 20 years I've been doing this.

https://youtu.be/wnFMfGenjAc
-Mike Borish

http://www.borishelectronics.com
Chicago, IL USA

User avatar
thealien666
Posts: 2791
Joined: Mon Nov 14, 2011 8:42 pm
Location: Quebec, Canada

Re: Moog System 35 from Hell!!!!

Post by thealien666 » Sun Jan 29, 2017 5:11 pm

It looks like a grounding issue, maybe ?
Moog Minimoog D (1975)
DSI OB6
DSI Prophet REV2
Oberheim Matrix-6
Ensoniq SQ-80
Korg DW8000
Behringer DeepMind 12
Alesis Ion

User avatar
CZ Rider
Posts: 586
Joined: Thu Jul 14, 2005 12:13 am
Location: Southeastern, PA

Re: Moog System 35 from Hell!!!!

Post by CZ Rider » Mon Jan 30, 2017 8:30 pm

Do the symptoms go away when the CV1 buss is disconnected? The lighted switch for the CV1 buss is lit on both 921A controllers and there is no connection from this buss to the large 921. Might be a bad connection to the rear from the multi pin connector that connects CV1 from a keyboard (951) or ribbon (956) depending on what is connected. Would disconnect the multi pin connector from any controller if this is the problem and diagnose the controllers.
Sorry, but no help if flipping those switches does not make the wobbly CV go away. :?:
1P Modular,Minimoog,VoyagerRME,CustomMinimoog,Prodigy,MG-1 TaurusII,Opus3,Rogue,Source,Liberation,Micromoog,1125S&H,
1130Perc.x2,1150Ribbonx2,Custom1150,1120Pedal,Songprod,CP-251,VX-351
VX-352,Etherwave,Synampx2,Lil'Phatty,Sonic Six

MikeBorish
Posts: 7
Joined: Mon Jan 21, 2013 4:59 pm
Location: Chicago, IL
Contact:

Re: Moog System 35 from Hell!!!!

Post by MikeBorish » Tue Jan 31, 2017 12:28 pm

The symptoms are still there when the CV1 buss is disconnected. I'm going to try and disconnect the sync pins on the edge cards and see what happens next. This is a really strange problem. Everything appears correct and ground appears to be correct. I must have missed something I guess.
-Mike Borish

http://www.borishelectronics.com
Chicago, IL USA

Stevie Ray
Posts: 254
Joined: Sun Apr 17, 2016 2:25 pm

Re: Moog System 35 from Hell!!!!

Post by Stevie Ray » Mon Feb 06, 2017 6:17 pm

Sounds like a front panel grounding issue but surely there's nothing in the 921B that should be that sensitive to stray capacitance effects etc.

Maybe some kind of high-gain amplification effect happening unintentionally (floating/disconnected transistor lead?).

Does it play and track properly when your hand is away from the panel?

electrojoe
Posts: 38
Joined: Sun Feb 04, 2007 10:15 pm

Re: Moog System 35 from Hell!!!!

Post by electrojoe » Wed Feb 08, 2017 5:16 pm

Hello,
I would try to isolate the issue first by removing the 921A and 921B (the most troublesome one) from the cabinet and putting it on your bench with another power supply and connectors.

Based on the schematics, it only requires +12 -6 Gnd and then the summing node, reference ground and the PW input from the 921A.

That should allow you to eliminate the rest of the system and help pinpoint the issue.

Hope that helps

GD

Stevie Ray
Posts: 254
Joined: Sun Apr 17, 2016 2:25 pm

Re: Moog System 35 from Hell!!!!

Post by Stevie Ray » Mon Feb 13, 2017 6:08 pm

electrojoe wrote:Hello,
I would try to isolate the issue first by removing the 921A and 921B (the most troublesome one) from the cabinet and putting it on your bench with another power supply and connectors.

Based on the schematics, it only requires +12 -6 Gnd and then the summing node, reference ground and the PW input from the 921A.

That should allow you to eliminate the rest of the system and help pinpoint the issue.

Hope that helps

GD


Why is the 921B more troublesome than the 921A?

Post Reply