Yes, it could be a failed span trimmer. Is there some reason you suspect it?
On Sun, Aug 16, 2015 at 3:31 PM, Stephen Richards okayjustalittle@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:Voltage on Pin 7 of that TL072 is fluctuating in time with the pitch. CA3140 has 15V on Pin 7 and -15V on Pin 4.When i feed Pin 7 of that TL072 to the good VCO , it warbles. Could this be a failed 10K SPAN Trimmer ?
From: "Ken Stone otherunicorn@... [cgs_synth]" <cgs_synth@yahoogroups.com>
To: cgs_synth@yahoogroups.com
Sent: Sunday, August 16, 2015 2:50 PM
Subject: Re: [cgs_synth] CGS48 VCO problem
Check you are not picking up interference on the sync line. Perhaps swap the CA3140, which is the most vulnerable chip in the VCO.the first thing is to work out where it is occurring.Take the voltage from pin 7 of the TL072 that is connected to the 56k feeding the LM394. Monitor it on a scope, or feed it into the CV input of the other VCO. If it warbles, you problem is likely to be in the mixer stage. If the voltage on pin 7 is steady, it's the core having issues.
Clean off any flux that may be left on the rear of the board.
Check the supply voltage AT THE CHIPS.On Sun, Aug 16, 2015 at 2:14 PM, Stephen Richards okayjustalittle@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:Hi Ken and all,One of my CGS48 VCO's has suddenly started completely loosing control of pitch.Every output is displaying the same behaviour. The pitch is wavering like a mosquito and occasionally jumps or drops dramatically in pitch.Supply voltage is fine , chips seem fine. I swapped the LM394s from my working VCO , no change.FM, PWM and SINE TO SAW shaper all working.Any ideas where i could start testing ?
--
--