Vintage Synth Repair

On Monday 05 May 2008 07:30, Malte Rogacki wrote:
> 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.

I could be mistaken about that -- my recollection is a bit fuzzy,  and I don't 
have my printed schematics at hand,  the electronic versions I was looking at 
onscreen being a less-than-optimum substitute (as in better than not having 
them).

I heard about this from a gentleman at the ARP factory whose name I 
unfortunately can't remember,  when I was there last -- in 1977!  :-)

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin