Moog Music Forum

hi, i'm new here, i love vintage synths but can't afford a moog so i'm into SH and CS

i'm fixing a MG-1 for a friend and i'm almost done (opened all sliders and switches and cleaned them well (after removing the tar pit), changed the output jacks for 1/4"... so everything works perfectly except it won't play B, C and C# on all octaves, if a lower note is held and the problem notes are played, they will play but not by themselves so a key contact problem isn't likely, they will also play if "contour" is set on 'continuous'...
the problem notes won't play either in polyphony mode... our first thought where the famously problematic octave divider chips MM5823 but i changed all three for new replacements without any success, i also replaced the trigger extractor chips without any change...

anyone had a similar problem?

Sounds like a gate or trigger problem on those keys. Check those circuits first.

I don't have any experience with the MG-1, but it could be something to do with the top octave generator.

All the best with your repair.

i'm having this issue with mine. Just the Note A in all three octaves isn't working. I've replaced the TOG (Top OCtave generator), all 3 octave divider chips MM5823 and the IC next to them the 4031 iC. And its still occurring.

Any ideas?

funkatron5 wrote:i'm having this issue with mine. Just the Note A in all three octaves isn't working. I've replaced the TOG (Top OCtave generator), all 3 octave divider chips MM5823 and the IC next to them the 4031 iC. And its still occurring.

Any ideas?

Well, according to the service manual if you are having a problem like notes will play only when others are held down, check U7 (4558) and U8(741) on the same board the dividers are on. Check for dc offset measuring between the inverting inputs. According the SM >10mv difference =bad chip.

zaiusz wrote:

funkatron5 wrote:i'm having this issue with mine. Just the Note A in all three octaves isn't working. I've replaced the TOG (Top OCtave generator), all 3 octave divider chips MM5823 and the IC next to them the 4031 iC. And its still occurring.

Any ideas?

Well, according to the service manual if you are having a problem like notes will play only when others are held down, check U7 (4558) and U8(741) on the same board the dividers are on. Check for dc offset measuring between the inverting inputs. According the SM >10mv difference =bad chip.

thanks i will do so and report back!

replaced the 4558 and de-soldered and re-soldered all the new MM5823 and still no High A, the middle and low A are warble/fuzz notes in Polyphony, and in Mono mode i get that notes only play when others are held down.

time to try the U8 741 and time to buy a multimeter

funkatron5 wrote:replaced the 4558 and de-soldered and re-soldered all the new MM5823 and still no High A, the middle and low A are warble/fuzz notes in Polyphony, and in Mono mode i get that notes only play when others are held down.

time to try the U8 741 and time to buy a multimeter

Did you check the traces on the key bus that go to the digital ICs?

Have you scoped out the top active divider yet?
The trigger for the mono section is generated from the poly section.
So if you have notes out on the TOC, the mono notes will not trigger.

Oscilloscope is your best friend for testing those.

GD