Vintage Synth Repair

Hi Bakis!

I am a synth engineer/performer who bought an Oberheim 2-Voice, also from 
Richard @ RL Music & (thankfully) have had no problems with it so far.
I concur with Michael's diagnosis on the SEM being A23 (an LM3080 Transc. 
op amp) being the potential culprit. Another likely cause is A24 -This is an
LM741 common or garden op-amp and I have had this cause no or full 
modulation when it goes belly-up.You should be able to get both from George 
Tait electronic suppliers in Glasgow for about £1.00 or less- I get a lot of chips 
there that are not readily or cheaply available elsewhere that I need for fixing 
& restoring synths at Singing Cat Services (plug!). 
http://www.tait-components.com.
I also have a full set of SEM 1a diagrams in jpg format which I can post you if 
you need it giving the schematic, jumper & Etc. locations layout and the wiring 
for all the modifications. If all else fails, contact me at Singing Cat (see Links 
page) and we'll try & work something out!

Best Regards,

Mantridcat


--- In vintagesynthrepair@yahoogroups.com, Bakis Sirros <
synth_freak_2000@y...> wrote:
> hello list,
> i had given my standalone SEM module(bought in great
> condition from RLMUSIC - UK, last year) to my tech ot
> transform it and perform all the mods so it will
> become totaly patchable.
> he made all the mods but obviously he made a mistake(
> a short sircuit occured somewhere during the
> modification works probably) and one(or more?) of the
> chips of the SEM board has been broken. the result?
> now, with no patchings at all in the SEM's newly added
> patchboard, the lfo constantly modulates the pitch of
> both vco's.(without the frontpanel switches to be set
> that way of course). normally, the lfo only modulates
> something if it is choosen via the frontpanel switches
> to do so, or if it is patched via the newly added
> patchpanel. but now the lfo pitch modulation of both
> vco's is constant and noticable all the time.
> my tech said that, in order for him to find the
> malfunctioning chip(s), he will have to replace many
> chips and that will take very long time.
>  
> is there any chance that you have any idea which of
> the chip(s) could be broken(from the shortsircuit) so
> they create that constant vco pitch modulation from
> the lfo?
>  
> the power supply unit is checked and working fine!
> 
> my tech's email, in case any of you want to give him
> some technical details/help, is available of course.
> just contact me off-list.
> many thanks for your help in advance!
> bakis.
> 
> 
> 
> 
> 
> 
> 
> Bakis Siros
> Parallel Worlds / Polariton
> Athens-Greece
> [Doepfer_a100] group owner
> http://www.shimarecords.co.uk
> http://www.ward12.com
> 
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