Vintage Synth Repair

At 19:32 Uhr -0400 04.05.2008, Roy J. Tellason wrote:

> It took a little digging since I don't seem to have my ARP stuff handy here,
> but I found my electronic copies of the schematic (which are nowhere near as
> handy for flipping back and forth quick between multiple diagrams...)
>
> If you look at say the Upper Voicing Board where those 22uF caps are used,
> you'll see a point that's marked 0V for short release, and -15V for long
> release. This is incorrect! That same error comes up in two different
> places, I forget where the other one is.
>
> Following that from the source, which is the upper right corner of the Lower
> Voicing Board schematic, you can see that point near the sustain footswitch
> jack and some transistors and whatnot quite plainly marked "+15V". Since the
> negative end of all those tantalum caps is tied to -15V, then what appears
> at the top end of them is going to depend on where you have the release
> slider set -- have it set for the longest release time and you'll have 30V
> across all those 25V tantalum caps.


Hmmmm.

I'm not quite sure that this is 100% how it works.

The +15V are the "Sustain Reference"; this is not what is going directly to
the sustain buss. This is (as far as I can see) the voltage passed to the
string control board; and the release slider then sends only part of it
back.

Indeed the 0V for short release is an error that is also to be found on the
lower voicing board. However the schematic for the string control board
gives those figures: long release -15V, short release +8V.
This is also the way it is printed on the schematic for the voice boards of
the Omni-2.

So the voltage difference would be 23V max which is pretty close to the 25V
rating of the caps but nominally still within tolerance

If I'm making any mistakes here please point them out; as I said I'm still
learning this stuff.

-- 
Malte Rogacki gacki@...
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"Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason why you
 get no sound out of it." (ARP 2600 Owner's Manual)
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