[sdiy] Non-DIY, but looking for repair advice...

[John Ames]

On Mon, 13 May 2024 16:34:24 +0100
S Ridley <spridley1 at gmail.com> wrote:

> There is a test point on the VCO board output and the service manual
> shows the signal levels, so if you have a scope it should be easy to
> check.

Gah, life catches up with you and before you know it it's six months
later and you've almost forgotten what you were even working on...but I
got a hankering today and poked at this some more after digging through
the service manual a bit...and I *thiiink* I mighta found the problem.
VR8 is, if I'm reading the schematic correctly, supposed to be a 100K
trimmer - but measuring resistance between the wiper and the other two
legs of the pot, I'm only adding up to a total of about 12-15K ohms. It
looks (if I'm following things) like this controls a negative offset
going into the VCA's expo-CV input; it seems possible that it's just
getting negative-CVed into oblivion.

(Two of the other trimmers on the board are similarly wacky - VR7
doesn't even appear to pass a continuity check.)

Luckily, due to a mis-filled order, I happen to have some 100K trimpots
of about the right footprint on hand - but before I go replacing things
willy-nilly, I just want to sanity-check my methodology. The trimmer
legs are arranged triangle-fashion; AFAIK the "solo" leg at the tip of
the triangle is the wiper, and the others are the ends of the resistive
element.

With the trimmer in its default position, I measured between the wiper
and one leg, then the wiper and the other leg, and added the two
measurements together for an estimate of the total resistance - which
was about 12-15K for VR8 and 30-50K for one of VR5/6 (can't remember
which.) VR7, as mentioned, didn't appear to pass continuity at all. I
tweaked VR8 a bit, and the total seemed to be about the same.

The other of VR5/6 was around 110K total, which is within tolerance for
a 10% pot, and I tried it on one of the brand-new trimpots and totaled
100K at either extreme, so I think I'm doing this right; I just figure
I should ask before charging in blindly with a soldering iron... ;)