[sdiy] EMS Synthi A

[steve thomas]

Hi Gavin..
                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. 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??!
 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.

 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
changes
 in the
 frequencies of the oscillators etc. This could be happening   due to
loading
 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?

 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
friend
> > has done some work on stabilizing the power supply on one.  Apparently
it
> > takes in +18V DC and divides it down to +12 and -9 rails.  The
transformer
> > was putting out up to 52V!  His solution was to put in a 314 (317?) reg
to
> > 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,
the
> > 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
hear
> 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
oscillators
> > 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
give
> 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
the
> > way does anyone know any method of giving them a MIDI retrofit?
> > Thanks to all for any help you can throw this way,
> > Gavin
> >
>