Rebuilding a 921 VCO

System 55 • System 35 • Model 15
Post Reply
ProspectHillMusic
Posts: 12
Joined: Mon Jan 09, 2023 4:47 pm

Rebuilding a 921 VCO

Post by ProspectHillMusic » Tue Apr 04, 2023 8:51 pm

A number of years ago I bought a large batch of Moog related parts and pieces from an Ebay seller. I got several boxes of transistors, wirewound trim pots, capacitors, jacks, C-frames, some blank panels, etc. I also received parts for a 921 VCO consisting of a front panel and two circuit boards, and nothing else. The VCO parts sat in a box on a shelf for well over a decade, I knew putting it back together would be a lot of work and would be time consuming so I kept putting it off.
IMG_8693.JPG
So, a few weeks ago I decided to just get it done. The first thing I did was put the front panel back together. That was fairly straightforward, I had everything on hand except for two rotary switches, I was able to buy two similar model switches from the same manufacturer that made the switches that Moog used.

Both circuit boards looked like they were in good shape despite being nearly 50 years old. They needed some major cleaning though.
IMG_8839.JPG
The 91-123 board had all of the wiring chopped off with only about 2" - 3" remaining which was way too short to reuse. The 91-125B board had all wiring removed and was also missing an E112 FET. The first step was to replace all of the wiring on both boards and replace the missing FET. This required running about 50 new color coded wires between each board and from each board to the front panel and took about two full days of work. There was additional wiring between various jacks, pots, and switches on the front panel itself. I was able to duplicate the original color coding with only 4 substitutions for wire color coding that I didn't have. I didn't have another 921 to just copy so I had to rely on the schematic and a few photos found on the internet to make all fo the connections.

After completing all of the work I powered it up and .... it worked! All of the front panel controls do what they are supposed to do and all of the waveforms are being produced. I was actually a little surprised, after making more than 100 connections to two untested circuit boards I expected a need to do troubleshooting but it didn't turn out that way. All that is left is a full calibration.

Everyone knows that the vintage Moog modular equipment and the recent reissues are expensive and after partially rebuilding a Moog module it is more than apparent to me why that is. That type of assembly is very time consuming and is a far more expensive way of manufacturing compared to a circuit board full of surface mounted parts like in the products that Behringer is putting out.

I've attached a few photos of the work in progress and also the final result.
Attachments
IMG_8841.JPG
Last edited by ProspectHillMusic on Wed Apr 05, 2023 8:34 pm, edited 4 times in total.

ProspectHillMusic
Posts: 12
Joined: Mon Jan 09, 2023 4:47 pm

Re: Rebuilding a 921 VCO

Post by ProspectHillMusic » Tue Apr 04, 2023 8:53 pm

Additional photos, this is after completion of the work.
Attachments
IMG_8855.JPG
IMG_8854.JPG
IMG_8853.JPG

User avatar
analogmonster
Posts: 266
Joined: Wed Nov 24, 2010 9:50 am
Location: Germany
Contact:

Re: Rebuilding a 921 VCO

Post by analogmonster » Wed Apr 05, 2023 3:20 am

Congrats - That's so cool 8) A very nice project. Once I tried to clone the 921 - and I failed. A friend tried the same and succeeded - so it is possible, definitely, but I made a mistake somewhere obviously :(
But your success encourages me to try it again :)

User avatar
ummagumma
Posts: 827
Joined: Sun Aug 21, 2016 8:37 pm
Location: Canada

Re: Rebuilding a 921 VCO

Post by ummagumma » Thu Apr 13, 2023 4:12 am

Prospecthill that looks sweet!

Congratulations on bringing it back to working condition

synthguy
Posts: 142
Joined: Wed Sep 17, 2014 7:37 pm

Re: Rebuilding a 921 VCO

Post by synthguy » Tue Apr 18, 2023 3:17 am

Nice job! That looks really nice.

The 921 is a great oscillator, one of my favorites.
I always wish I had more than one in my system!
:D
Visit my synthblog...Moogs and more!
Www.synthguy.com

Post Reply