After a few weeks, I claim Success!! All twelve voices work and my Matrix-12 passes all tuning tests.

In the end, I needed to replace two Curtis Electromusic 3372 VCF chips. Evidently, they can fail in different ways, since both my problem voices were due to 3372 failures, My original theory that a dead voice was due to a 3374 VCO failure was dead wrong.

I was able to locate both the 3372 VCF and 3374 VCO chips from a European source. Once they arrived, I discovered that my 3374 was fine after all, so I ended up with an extra CEM 3374 chip. Given how scarce these are, I will keep it as a spare. Replacing the two 3372s brought the two broken voices back to life and now all twelve of the Matrix-12 voices are fully functional.

Lessons learned: 1. I should have done more chip swapping in the beginning to isolate the fault. 2. A bad 3372 can completely kill a voice.

Finally, to the engineers and designers at Oberheim: I thank you! Thank you very much for including chip sockets in your board designs. If I ever meet you in person, you are entitled to a few intoxicating (or not) drinks of your choice on me.

From the by-the-way department: Upon searching for the Curtis Electromusic chips, I came upon some electronic parts vendors who would claim to have dozens in inventory. These are electronics parts distributors that sell business-to-business, have minimum orders, etc etc. so I did not pursue them. Just curious, but are they for real Has anyone obtained CEM chips this way Is it possible there are large numbers of these chips still in distributors warehouses

PS: Another btw: The top voice board in the Matrix-12 is voices 7 through 12, the bottom board is voices 1 through 6.

--- In oberheim@yahoogroups.com , "Joigaloid" <andre_sand@...> wrote:
>
> Hello! I have an Oberheim Matrix-12 that I would like to restore. It spent a lot of time in storage (which caused a few problems) but there are a couple of functional problems I'd like to fix before attacking the cosmetic problems.
>
> The first issue is that I've noticed some notes were being dropped while playing. After checking a few things with the tuning page I noted the following observations:
>
> VCO Tuning: Voice #6 fails
> PW Tuning: Voice #6 fails
> Res & VCF Tuning: Voices #6 and #12 fail
> VCA Tuning: Voice #6 fails
>
> After tuning, the Matrix turns off the two failed voices. I can always turn on voice #12 manually and get sound from it. If I manually turn on voice #6, I get no sound from it. All other voices are fine.
>
> My guess on the above is that I've lost the Curtis VCO chip for voice #6. The failure of voice #12 is more subtle, but it seems likely
> I've lost the CEM3372 which performs all the filtering options. I am faced with the frightening prospect of trying to locate a replacement
> CEM3374 and likely a CEM3372 as well.
>
> I will swap out chips between voices, but would prefer to keep swaps to a minimum. The Matrix-12 has two identical voice boards, I'll have to guess which one is voices 1 through 6 and which one is voices 7 through 12. Any suggestions
>
>
> I will be posting my experiences on this project both on the vintagesynthrepair and on the oberheim yahoo groups. Hopefully, others may benefit from them, and of course, I am open to any tips or suggestions from the group.
>
> -as
>