[sdiy] sh09 cv out drops again

Roman Sowa modular at go2.pl
Wed Jan 14 11:16:52 CET 2015


By flux I mean this (usually) brown stuff that makes soldering easier, 
and stays on the board around solder. It collects humidity and may 
become leaking electrically over long period of time.

Have you checked the gate voltage of the FET when no key is pressed? It 
should be close to negative supply.

One semitone per 15s not that bad, although my old synths do not drop 
that fast either I think. You may try also the dirty fix without finding 
real cause, just make the holding cap considerably bigger, like 4 times 
or so. Slight glide may appear as side effect, but drop rate should be 
acceptable then, so it will not be noticeable even at long release times.

Roman

W dniu 2015-01-14 o 11:04, Carlos Portela pisze:
> The psu was checked, no leakage on the keyboard contact. Dont know
> what you mean with flux
> It has no guard ring on the pcb around LF13741, but datasheet talks
> about that. It has a cardboard with metal foil connected to ground
> above this area.
> The hold is not holding (a semitone down every 15 sec). When the key
> is pressed no pitch drop, only when it is on VCA HOLD
> I didnt replaced all components components at same time, otherwise I
> could not track the problem.
> I had the opportunity to clean the the board area, all contacts in the
> area were re-soldered.
>
>
>
> 2015-01-14 8:57 GMT+01:00 Roman Sowa <modular at go2.pl>:
>> Just an idea: clean the board in area of FET-CAP-OPAMP
>> Soldering might have evaporated all water from the flux/dust/chewinggum that
>> is there, and later air humidity helped the dust conducting.
>>
>> Roman
>>
>> W dniu 2015-01-13 o 20:01, Carlos Portela pisze:
>>>
>>> Hi all,
>>>
>>> After 2 months I turn the sh09 on and the cv out/pitch drops when it
>>> is on VCA hold.
>>>
>>> Replaced IC2 lf13741 by CA3140, the fat capacitor, and IC1 4558 with
>>> TL072. the problem persist.
>>>
>>> Again removed the 2x 2sk30A GR to test them. After re-solder them the
>>> pitch was stable.
>>> I found that's (maybe) because of the heat from the iron.
>>> 10 min later was drooping again.
>>>
>>> Also checked the sample and hold from lfo section (last time I
>>> switched the sample n hold FET with the keyboard tracking FET) and
>>> there was a kind of portmento between the played notes...
>>>
>>> Any ideas what might be causing this? Bad Fets?
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