Vintage Synth Repair

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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In a message dated 8/9/05 1:27:08 AM, Bakis Sirros 
<synth_freak_2000@...> writes:

<<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. >>

You probably do not have a bad chip.  Your problem is probably a result of 
incorrect wiring of the modification.  Your tech needs to check his work again, 
and very carefully.  There is, however, one chip that can cause this kind of 
LFO modulation problem -- It is A23 on the main board.  It is an LM3080A IC.  
Be careful with this chip.  They are very hard to find these days.  Again, the 
chip is probably OK, and your problem is more likely due to incorrect wiring.

Michael Bacich

hello,
many thanks for your helpful reply!
btw, the patchbay/modifications have been checked and
are totally ok, the problem lies somewhere in the PSU
board, or the vco boards.
anyway, i hope that, soon, my modularized SEM will be
fine!
thank you,
best regards,
bakis.





--- WeAreAs1@... wrote:

> 
> In a message dated 8/9/05 1:27:08 AM, Bakis Sirros 
> <synth_freak_2000@...> writes:
> 
> <<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. >>
> 
> You probably do not have a bad chip.  Your problem
> is probably a result of 
> incorrect wiring of the modification.  Your tech
> needs to check his work again, 
> and very carefully.  There is, however, one chip
> that can cause this kind of 
> LFO modulation problem -- It is A23 on the main
> board.  It is an LM3080A IC.  
> Be careful with this chip.  They are very hard to
> find these days.  Again, the 
> chip is probably OK, and your problem is more likely
> due to incorrect wiring.
> 
> Michael Bacich
> 
> 


Bakis Siros
Parallel Worlds / Polariton
Athens-Greece
[Doepfer_a100] group owner
http://www.shimarecords.co.uk
http://www.ward12.com


		
____________________________________________________
Start your day with Yahoo! - make it your home page 
http://www.yahoo.com/r/hs

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
> 
> __________________________________________________
> Do You Yahoo!?
> Tired of spam?  Yahoo! Mail has the best spam protection around 
> http://mail.yahoo.com

In a message dated 8/15/05 1:05:33 AM, "mantridcat" <mantridcat@...> 
writes:

<< 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. >>

Hello Mandricat,

Thanks for your help and experience on the subject.  Nevertheless, I can't 
readily see how a failing A24 could cause full modulation.  It could definitely 
cause no modulation, or could cause a huge static pitch offset if its output 
went high or low, but how could it increase modulation, if there is no 
modulation signal placed at its input?  Have you really seen this before as a failure 
mode?  If so, what am I missing?

Mike B.

hello,
many thanks for your help!
i'll let you know if the SEM repair will not happen
soon.
best regards,
bakis.





--- mantridcat <mantridcat@...> wrote:

> 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 \ufffd1.00
> 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
> > 
> > __________________________________________________
> > Do You Yahoo!?
> > Tired of spam?  Yahoo! Mail has the best spam
> protection around 
> > http://mail.yahoo.com
> 
> 
> 


Bakis Siros
Parallel Worlds / Polariton
Athens-Greece
[Doepfer_a100] group owner
http://www.shimarecords.co.uk
http://www.ward12.com


		
____________________________________________________
Start your day with Yahoo! - make it your home page 
http://www.yahoo.com/r/hs

Sorry it took so long to get back on this, I was gigging & ingnored my mail for a 
week or so. Ok, here goes...I checked back on the SEMs  (about 6) I've fixed 
that had this problem-sure enough, one required both A23 & A24 replaced for 
NO modulation.The other SEM had 2/3 to near-full  modulation and looking 
closely at my notes for the client I saw I had indicated A24 as the cause.
of that issue due to "liquid ingress"-The client had spilt a pint of best 
Guinness**  as I recall, so the A24 issue does indeed look like a bit of a red 
herring for  our friend Bakis !  Ahh... must update my memory-chip!!
Thanks Mike!

Regards,

Mantridcat



**-Not the strangest by any means....I've had a Farfisa Compact organ that 
kept detuning intermittantly-the cause was a live caterpillar pupating between 
the pillars of the main tuning-choke!!
The 're-housing' of a small family of mice I found living happily in a Hammond 
A101 is a story for another day......



--- In vintagesynthrepair@yahoogroups.com, WeAreAs1@a... wrote:
> 
> In a message dated 8/15/05 1:05:33 AM, "mantridcat" <mantridcat@y...> 
> writes:
> 
> << 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. >>
> 
> Hello Mandricat,
> 
> Thanks for your help and experience on the subject.  Nevertheless, I can't 
> readily see how a failing A24 could cause full modulation.  It could definitely 
> cause no modulation, or could cause a huge static pitch offset if its output 
> went high or low, but how could it increase modulation, if there is no 
> modulation signal placed at its input?  Have you really seen this before as a 
failure 

> mode?  If so, what am I missing?
> 
> Mike B.

hello,
many thanks for the helpful email.
i have forwarded your email to my tech.
i hope that soon my SEM will be in fine condition
again.
best regards,
bakis.





--- mantridcat <mantridcat@...> wrote:

> Sorry it took so long to get back on this, I was
> gigging & ingnored my mail for a 
> week or so. Ok, here goes...I checked back on the
> SEMs  (about 6) I've fixed 
> that had this problem-sure enough, one required both
> A23 & A24 replaced for 
> NO modulation.The other SEM had 2/3 to near-full 
> modulation and looking 
> closely at my notes for the client I saw I had
> indicated A24 as the cause.
> of that issue due to "liquid ingress"-The client had
> spilt a pint of best 
> Guinness**  as I recall, so the A24 issue does
> indeed look like a bit of a red 
> herring for  our friend Bakis !  Ahh... must update
> my memory-chip!!
> Thanks Mike!
> 
> Regards,
> 
> Mantridcat
> 
> 
> 
> **-Not the strangest by any means....I've had a
> Farfisa Compact organ that 
> kept detuning intermittantly-the cause was a live
> caterpillar pupating between 
> the pillars of the main tuning-choke!!
> The 're-housing' of a small family of mice I found
> living happily in a Hammond 
> A101 is a story for another day......
> 
> 
> 
> --- In vintagesynthrepair@yahoogroups.com,
> WeAreAs1@a... wrote:
> > 
> > In a message dated 8/15/05 1:05:33 AM,
> "mantridcat" <mantridcat@y...> 
> > writes:
> > 
> > << 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. >>
> > 
> > Hello Mandricat,
> > 
> > Thanks for your help and experience on the
> subject.  Nevertheless, I can't 
> > readily see how a failing A24 could cause full
> modulation.  It could definitely 
> > cause no modulation, or could cause a huge static
> pitch offset if its output 
> > went high or low, but how could it increase
> modulation, if there is no 
> > modulation signal placed at its input?  Have you
> really seen this before as a 
> failure 
> > mode?  If so, what am I missing?
> > 
> > Mike B.
> 
> 
> 


Bakis Siros
Parallel Worlds / Polariton
Athens-Greece
[Doepfer_a100] group owner
http://www.shimarecords.co.uk
http://www.ward12.com


	
		
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