[sdiy] EMS Synthi A

gavin elmystico at earthlink.net
Thu Aug 15 23:37:33 CEST 2002

From: "gavin" <elmystico at earthlink.net>
To: steve thomas <s.thomas at qmul.ac.uk>
Subject: Re: [sdiy] EMS Synthi A
Date: Thu, Aug 15, 2002, 5:36 PM

>                as regards the synthi A psu.  I believe there were different
> psu designs used
> in  synthis depending on the period. The schematic i have for a synthi A
> shows the transformer to have TWIN 15 volt
> windings which are connected to 4 rectifying diodes in the usual bridge
> arrangment for split power
> supplies. 

Do you have a scanned or web version of those schematics?

certainly 52 Volts that your friends is putting out  is way off!
> I am not sure the solution to
> use a 317 to drop the voltage to 18 V is a good one... can 317's take 52V
> input??!

This was my concern too, usually the voltage rating is just a couple volts
above what you want to get out of the reg.  My first question was, "Doesn't
the 317 get hot?" Also one of the big ceramic resistors that I believe are
used to drop the voltage for supply (pardon my ignorance here I'm not too
familiar with the Synthi's psu type)  The big grey resistors apparently
"Gets so hot it burned the board and melted the traces off".  Whoa!!

> If this was an original transformer ..and its putting out
> 52V that sounds like the transformer is dead. Better to replace it with a
> similar spec type.

This sounds like the key to me, no way that thing could have been designed
to convert 35-52V AC to +12 and -9V.

> It seems to me the weird problems he is having is due to a problem with the
> psu.
> Did he check the +12/-9V rails for stability during operation?
> If there arebig  fluctuations on these rails..they will cause large
> in the
> frequencies of the oscillators etc. This could be happening   due to
> of a defunct  psu when
> he manually triggers the env.
> Also I guess he was careful in checking the orientation of the electrolytic
> caps when he changed them?

My first question when he told me he had recapped it, "Did you make sure all
the polarities were right on the electrolytics?"  Here's my theory.  The
thing was acting unstable b/c the transformer was maybe wearing out.  At
least one electrolytic was in backward and killed the Xformer.  The weird
phenomena are an interaction of the dead Xformer and the still backwards
electrolytics.  So maybe he'll let me work on it for a while instead. 
Disconnect the transformer from everything, test the voltage coming through,
systematically check all the electrolytics, check the spot where the traces
"burned off"  see where I'm at then.

Thanks for the help,
> hope this helps
> cheers
> steve Thomas
>> Synth-DIY pages:-
>> http://monopole.ph.qmw.ac.uk/~thomas/synthdiy/index.htm
>> ----- Original Message -----
>> From: gavin <elmystico at earthlink.net>
>> To: synth <synth-diy at dropmix.xs4all.nl>
>> Sent: Thursday, August 15, 2002 5:31 PM
>> Subject: [sdiy] EMS Synthi A
>> > Hey all,
>> > got two here, first are there any EMS SYNTHI wizards out there.  A
>> > has done some work on stabilizing the power supply on one.  Apparently
>> > takes in +18V DC and divides it down to +12 and -9 rails.  The
>> > was putting out up to 52V!  His solution was to put in a 314 (317?) reg
>> > drop the voltage to +18V.  He also recapped the entire thing.  Now there
>> is
>> > a strange bleed through of the Gate and Envelope into the CV's for all
>> > oscillators and the filter.  When the envelope is manually triggered,
>> > oscillator pitch or the filter cutoff rises with the envelope shape.  As
>> if
>> > they were patched together, but there not.  Here's the real weird thing,
>> > when the envelope is set to 0 it's the gate that bleeds through, you
>> a
>> > very clear 5V jump in the pitch or cutoff and then back down when the
>> > trigger's released.  Also, it seems that the waveforms on the
>> > are not quite stable when the trigger's not pressed, then they stabilize
>> > when the trigger's pressed and they jump pitch.  Whe the trigger's not
>> > pressed they sound "weak" and the waveforms on the scope are shaky.  Any
>> > takers?
>> > Now over here I've been looking through the Old Crow's Yamaha CS-80
>> > resource.  I have a CS-60 that has a massive pitch stability problem.  I
>> > plan to do the temperature compensation diode repositioning mod, and
>> it
>> > a tune but it's REAL wacked out.  The pitch response on any key varies
>> > constantly and sometimes appears to be modulated in an LFO vibrato type
>> way.
>> >  This seems akin to some of the pitch stability problems he cured by
>> > replacing the 4000 series CMOS chips.  Anyone worked on these?  and by
>> > way does anyone know any method of giving them a MIDI retrofit?
>> > Thanks to all for any help you can throw this way,
>> > Gavin
>> >

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