OBERHEIM SYNTH

One thing that I have seen happen before is a CMOS latch-up that pulls
down the +5.6 voltage which effectively destroys any voice control.
The CEM outputs seem to look good, though low, on a scope but you get
no audio output.  Then, when you test the +V of all the control CMOS
it is lower than +5.  Those chips latch up if they receive an input
voltage greater than the supply, which as it turns out, is how one of
the mux chips is wired.  The latch up sinks current and drags the
voltage down.  The unused input of that chip is tied to +5, and since
the 5.6 is derived from divider, the +5 gets there before the 5.6
rises fully.  I usually tie that input to ground and never have any
problems again.  You may need to replace the chip though if this is
the case as it may be damaged.

http://synthahol.tumblr.com/post/76933893079/oberheim-matrix-6-cmos-latch-up

On 5/7/14, Narfman96 - Narfland Studio <narfman96@...> wrote:
> I'm not sure where TP12 is on the schematic. Check D703 and the two
> capacitors near it. This circuit generates the 5.6v. It is shown on page
> 12 of the schematic I have. Right in the center of the page.
>
> It could be that one of the 4051 chips is dragging down the 5.6v but more
> likely the problem is somewhere around the diode. The 5.6 volts is required
> for the CV to work properly. You are very close to finding the problem.
> On Tuesday, May 6, 2014 8:01 PM, "zc40@hotmail.com" <zc40@...>
> wrote:
>
>
>>Hello, now I have another matrix unit that is correct, I've tried. or
>> 801/3/4/9/10/11/12 and all is well ... The cpu also think it's good.
>>
>>
>>I see a strange thing. TP 12. voltage is 4.5 V ... 5.6 should not be?
>>
>>
>>
>>
>>Thanks :)
>>
>>
>>


-- 
Patrick
Synth Tech @
http://www.switchedonaustin.com