Cruma Trilogy oscillators problem

Synth Repair
1

Hello!

I’m troubleshooting a Crumar Trilogy that shows a problem related to the oscillators board.
Here’s the link to download the schematics, the oscillators board is at page 5, DWG1.

The circuit is fairly simple, two master oscillators (IC3, IC2) drive two top octave generators (IC11, IC10).
You can individually tune the oscillators and when you slightly detune one of the two you have a pleasant phasing effect.
The problem occurs when you synchronize the oscillators, manually with the tune knobs or automatically pushing the “Sync” button and only “master oscillator 1” drives both TOGs
When the “Sync” is pushed the waves from IC10 and IC11 stuck out of phase by a certain degree, it depends to the moment you sync the oscillators.
Here’s a small video of the oscilloscope showing this.

[media]https://youtube.com/shorts/1BZVdHs-VU8?si=YJUH2fHYJBOYMBcZ[/media]

The oscilloscope and audio signals are taken from the synth’s “Organ” output.
The problem is related to all the keyboard keys, but they are affected differently depending to the moment you activate the “sync”.
I’ve monitored the master clock at pin 2 of IC10 and IC11 with a dual trace oscilloscope and they are of course perfectly in-phase when the sync is activated.

So, what’s going on here, in your opinion?

2

Its a good thing you have an oscilloscope, because you will have to check every logic chip between the “sync” button and IC10-11, then check any timing caps for time expired operation.

if there are any Tantalum capacitors in the circuit, you should know to pull them, and replace them.

I assume you have checked the power supplies and the caps in that section are replaced also?

Time expired parts is most of the reasons for oscillators to fail or not sync.