Vintage Synth Repair

Hey folks,

I have a nice little Micropreset that is getting fussy.

Here are the general symptoms:

On certain "patches", everything is fine. On others, some stops are missing, and the two "Synthe" buttons don't work at all. 


Here are the details:

Voice - All good. 
Wood - only 8 and 32 work.
String - all good.
Brass - all good.
Synthe 1 - none work.
Synthe 2 - none work.

I recently took it to a tech who decided that he wanted nothing to do with it, so I'm now on my own. I have a bit of electrical knowledge and soldering skill, so maybe with some help I can fix this neat old instrument.

Thanks everyone!

Hi;

I've fixed a Micro-Preset myself last year and made some notes that may be
helpful.

Do you have the schematic? Most of the copies I've seen so far are fairly
hard to read, though.

The good news is that it probably can be fixed; however finding the faults
can be a bit of a challenge.

A few thoughts to begin with:

Basically the Micro-Preset has three waveforms (saw, square and pulse) and
two noise types; it also has two filters.

Strings and Voice use saw and no filter
Wood 2, 4 and 16 use square and no filter
Wood 8 and 32 use pulse and no filter
Brass uses saw and the left filter in the schematic
Synthe1 uses pulse and the right filter
Synthe2 uses either saw or noise (depending on the octave switch) and the
right filter

So, what we can see here is the following:

- saw works
- pulse works
- left filter works

(Let's for the moment assume that triggering and envelopes also work.)

What you need to find out is:

1. Is square working at all?
2. Is any signal getting to the right filter (pin 3 of IC7)?

The first should be comparatively easy to check (with an oscilloscope) and
would definitely give us a clue what could be wrong here.

BTW, on mine some caps had leaked and partly destroyed traces.

Look for leaking capacitors and dead traces.
I repair mi korg a month ago.
Liquid of capacitors next to the filter, leaked and destroy some traces of the top side of the board.
If you see traces green colored check it, may be the oxide already destroy it totally.

Marcelo Malmierca

On 05/01/2014, at 10:26, Thomas Heckmann <troperecordings@...> wrote:

Hi,


it sounds like both Korg 35 Filters are gone (it has 2 installed)
They are very hard to find, but easy to build yourself as it's avery simple but effective filter.
Better to buy a working M500 for a cheap price and keep the other for parts.
Best,

Thomas

Thomas P. Heckmann
troperecordings - afu limited - afulab - trope mastering

> On Jan 3, 2014, at 10:08 PM, Jim Blair wrote:
>
>
> Hey folks,
>
> I have a nice little Micropreset that is getting fussy.
>
> Here are the general symptoms:
>
> On certain "patches", everything is fine. On others, some stops are missing, and the two "Synthe" buttons don't work at all.
>
> Here are the details:
>
> Voice - All good.
> Wood - only 8 and 32 work.
> String - all good.
> Brass - all good.
> Synthe 1 - none work.
> Synthe 2 - none work.
>
> I recently took it to a tech who decided that he wanted nothing to do with it, so I'm now on my own. I have a bit of electrical knowledge and soldering skill, so maybe with some help I can fix this neat old instrument.
>
> Thanks everyone!

So the presets that use the filters have a problem. However it seems
unlikely to me that both filter chips have failed (though one never
knows...). This in turn would make me search first for circuitry that is
common to both filters.


I take it you have already checked the switch Matrix? This makes many
connections that affect the general behaviour of the synth (like which
envelope is used, which waveform is routed to the Synthe filter and so on).


You mentioned that the envelope works on Synthe 1's volume; does it also
work on Synthe 2's volume? Both use different envelopes (Synthe 1 has no
Attack; Synthe 2 no Decay, just Attack and - essentially - Release even
though it is called Sustain here).


I'd probably start looking around IC5 (this is the final opamp between
filters and switch matrix). However IC5 is (I believe) also used for the
Wood presets.


The filter IC itself is the famous Korg 35 chip:


http://www.timstinchcombe.co.uk/index.php?pge=korg

Change the capacitors, check the traces on the top of the board.
Check the position of the filters chips. It goes on oposite way position between them.


By the way... Anyone have a readable schematics ?


Marcelo Malmierca


On 19/08/2014, at 18:20, "b3bilco@... [vintagesynthrepair]" <vintagesynthrepair@yahoogroups.com> wrote:


I have one of these on the bench now with similar issues. Brass and Synthe 1 and 2 are weak, and the traveler slider has little or no effect. This tells me there's a filter problem. The envelope works on synthe 1's volume, just no filter sweep. I cannot read the chip # for the filter ICs on the schematic-not much of anything can be easily read on it, actually. I have repaired some eroded traces, but some run under 14 pin DIP ICs that I am loth to pull for fear of wiping out a non-replaceable chip, so  I am unsure if I got the whole trace right or not. There is output from the synthe and brass sounds, but weak and no filter action apparently. Anybody have a legible PCB trace layout on this beast? IC number/parts list? ANY HELP GREATLY APPRECIATED!!!!