Vintage Synth Repair

Alternative 4, is my guess.
The micromoog (and the multimoog) needs very specially selected CA3080
chips in various places.. I have run into this problem, and it was
only when I put in an old stock chip (and not a new one) that the
function came back to life.




tw.




Den 14. feb.. 2017 kl. 06.30 skrev dolguin@...
[vintagesynthrepair]:


>
>
> This is my first post to this group.
>
> I'm having a couple of problems with my MicroMoog that may, or may
> not be related. I'm a competent tech, however, I mostly work on pro
> audio equipment, so I'm afraid I may not speak synthtalian well
> enough to diagnose the problem, or properly identify it in the
> service manual.
>
> Problem #1:
> Using the switch/knob positions from the 'Sample and Hold' template
> in the manual, I'm not getting the cool randomization in the S&H
> AUTO position on the mod source switch. I DO get the 'choppy' sound
> that varies with the RATE knob, but the mod wheel just acts like a
> pitch bender.
>
> Problem #2:
> When the mod source knob is in the S&H KEYBOARD position, pressing
> any key does not even give me the 'choppy' sound that I'm getting in
> the S&H AUTO position... it's just a continuous note. The mod wheel
> also acts like a pitch bend wheel, just like the first problem.
>
> I AM getting the +/-14V at the junction of R802 and C909. My scope
> IS showing some kid of noise source on pin 3 of IC902, but I'm not
> sure if it's white, pink red or purple, nor what level it's supposed
> to be at. I'm also not sure what I'm supposed to be seeing at the
> junction of IC902 pin 6 and the gate of Q901. When probing the
> 'source' leg of Q901, I'm NOT getting the random +/-5V waveform
> that's shown on the schematic. I've swapped out IC902, Q901, C909
> and C908. For the sake of full disclosure, I replaced Q901with a
> NTE467 instead of the stock E112. I have lifted and checked every
> component in the Sample & Hold section that's shown on the schematic
> around IC902 and Q901. I have also done continuity checks between
> all of the components, on the component legs (as opposed to just
> probing the traces), and all is well there. I've also done
> continuity checks 'through' the switches, and they are working as
> expected.
>
> I'm spinning my wheels, so I'm afraid that I might be
> troubleshooting the wrong section of the synth, or, as the manual
> suggests that IC902 may need to be specially selected to work
> properly in this circuit.
>
> My best guesses at what may be causing the problems are:
> The noise source at pin 3 of IC902 may not be at the correct level
> or color.
> The pulse feeding the control input (pin 5) of IC902 may not be at
> the correct level.
> Q901 may need to be a 'true' E112 (instead of a NTE467) to function
> properly.
> IC902 may need to be specially 'selected' to function properly.
>
> Any insight or technical expertise on these problems would be
> greatly appreciated.

>
>


Terje Winther
terje.winther@...
http://wintherstormer.no/