Korg Mono/Poly

It is possible for a bad chip to cause stability or tuning problems,  however, it's far more likely that the external heating resistor is bad (or the transistor controlling it is),  perhaps a bad opamp in the control voltage circuitry,  or even just old electrolytic caps.   The ssm2033 is a really robust chip. 

I would make the assumption that it isn't the ssm2033,  and try to disprove your assumption by eliminating other possibilities. 

Am 04.10.2015 um 18:47 schrieb Steve Wilson
lighterthanairflight@gmail.com [korg_mono-poly]:

> however, it's far more likely that the external heating resistor is
> bad (or the transistor controlling it is), perhaps a bad opamp in the
> control voltage circuitry, or even just old electrolytic caps.

To be honest: the most likely failure are the trimmers, they collect
dirt and the center trace can corrode - even the closed models.

If I am in restoring any old synth, then the first action is replacing
the trimmers with modern sealed cermet trimmers, if possible 25-turn
trimmers.


Florian

(and please everyone: forget the electrolytic caps fairy tale - drying
elco caps will happen under thermal stress. This is reality in a
professional mixing consol from the seventies, which wasn't switched off
while the last 40 years. Those consoles get quite hot (we measured in a
SSL G4000 an average temperature of 55 degrees celsius!), but in a
normal synth such temperature never will happen - even in a CS80.

If recapping is required then at old mylar caps or tantalum capacitors.
And if those go bad, you will hear and smell it.

I will agree with you that trim pots are a good idea,  but I will respectfully disagree about the electrolytic capacitors.   I have seen electrolytics spilling their guts in polysixs monopolys and plenty of other synths,  and they do cause problems.   I've certainly seen more problems due to electrolytics in mono/polys and polysixes than I have from trim pots being dirty.

Also,  the ssm2033 doesn't have a thermistor on it in the MP4,  those are just metal film resistors.  What I would suggest if you really want to move the chip is to unsolder the resistor and move it with the chip without removing the resistor from the chip.   The thermal conformal coating that Korg used is pretty nice,  but once you break the bond, the resistor won't help the chip heat up as well...  You can always use thermal compound to help transfer the heat,  but i have found that the heating rates are different So you'll have one chip that lags behind the others.