Vintage Synth Repair

> none of the modulation to the filter works and the filter is really
> scratchy.

OK, first of all you identify on which board the filter is on and look at
the appropriate schematic section.
In this particular case it's board C (or C-I in the schematics).

Now, this whole problem looks pretty straightforward to me. Let's see if we
can identify possible culprits.

When looking at the schematic we can see the various modulations sources
for the filter all on the left side. They pass through the slider
potentiometers P10, P11 and P12 and then pass through A1 (an LM301 opamp).
So first I would measure (with an oscilloscope) if something comes into the
opamp (for example try the LFO); and then if it again leaves the opamp.

That's pretty much the modulation path before it reaches the filter module;
if a modulation signal comes out of A1 we have to look elsewhere. But this
would be my first "suspect".

-- 
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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