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

[David G Dixon]

Hey John, my advice would be to carefully remove the offending trimpot from
the board and then measure it.  Since it is connected to other things on the
PCB, you really don't know what the actual impedance between any two legs
should be, because the trimpot could be in parallel with other impedances on
the board.

If it still reads wacky after you have desoldered it, then you were right.
If it reads around 100k, then you still have some digging to do.


 

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of John
Ames via Synth-diy
Sent: Monday, October 28, 2024 7:25 PM
To: S Ridley; synth-diy at synth-diy.org
Subject: Re: [sdiy] Non-DIY, but looking for repair advice...

[CAUTION: Non-UBC Email]

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... ;)
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