MG-1 won't play B, C and C#

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rich
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Joined: Tue Jan 12, 2010 11:00 am

MG-1 won't play B, C and C#

Post by rich » Tue Jan 12, 2010 11:13 am

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?

Just Me
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Post by Just Me » Tue Jan 12, 2010 2:44 pm

Sounds like a gate or trigger problem on those keys. Check those circuits first.
"Music expresses that which can not be said and on which it is impossible to be silent."

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latigid on
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Joined: Mon Jan 30, 2006 3:47 pm
Location: Auckland, New Zealand

Post by latigid on » Tue Jan 12, 2010 2:47 pm

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.

funkatron5
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Joined: Fri Mar 23, 2018 4:05 pm

Re: MG-1 won't play B, C and C#

Post by funkatron5 » Fri Mar 23, 2018 5:52 pm

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?

zaiusz
Posts: 47
Joined: Mon Mar 05, 2012 10:58 pm
Location: Torontoish

Re: MG-1 won't play B, C and C#

Post by zaiusz » Sun Mar 25, 2018 2:34 am

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.

funkatron5
Posts: 3
Joined: Fri Mar 23, 2018 4:05 pm

Re: MG-1 won't play B, C and C#

Post by funkatron5 » Mon Mar 26, 2018 10:48 am

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!

funkatron5
Posts: 3
Joined: Fri Mar 23, 2018 4:05 pm

Re: MG-1 won't play B, C and C#

Post by funkatron5 » Fri Apr 06, 2018 12:14 pm

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 :)

zaiusz
Posts: 47
Joined: Mon Mar 05, 2012 10:58 pm
Location: Torontoish

Re: MG-1 won't play B, C and C#

Post by zaiusz » Mon Apr 30, 2018 12:33 pm

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?

electrojoe
Posts: 38
Joined: Sun Feb 04, 2007 10:15 pm

Re: MG-1 won't play B, C and C#

Post by electrojoe » Sat May 05, 2018 11:38 pm

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

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