Vintage Synth Repair

Greetings to all, just joined the group a while back and have had a
ball reading the posts, I'd like to add a humble comment over this arp
2600 power supply problem, me I suggest to try to test the power
supply isolated from the rest of the electronics, that is; with the
synthesizer power cord detached from the wall, lift the +15 volts and
 -15 volt off the power supply board going the rest of the synth. then
attach a 15 ohm (15 to 20 watt) power resistor to the +15 v (and
ground)  and another to the -15 volt  and ground also. then measure
with a meter the voltage delivered by the regulator, this way you can
isolate  what side is failing but at this point it is clear the -15
volts is the culprit. I would replace all transistors and the ua748
opamp, the -15 volt is just an inverted 'copy' off the +15 v side, be
advised that the lm723 is known to fail, not frequently but it does
fail doing strange things. in a nut shell, the 'real' regulator is the
+15v supply, the -15V is a voltage inverted version of the other and
also incorporates current limiting via q3 and r15, now judging by the
vlue of this resistor (1ohm) the current gets hard limited to about
600 mA  at the -15V side, R6 does the same on the +15V side. The LM723
is known to have better transient response ( read better regulation)
than a monolithic regulator like the LM7815 for the same load needs.
But this chip is getting harder to get lately. FUTURLEC has them but I
have no idea how they operate their bussines ( read prompt or late
delivery, quality etc) see
http://www.futurlec.com/ICLinearRegulators.shtml.

 Anyhow, if by testing via the 'load resistors' indicates solid
regulator behaviour, then is time to look for the trouble deeper into
the synth.   
 last, if the uA/LM748 is nowhere to be found, try using a lm741
instead, there are some out there, (at least here in my parts bin)
other single 8 pin opamp's should work provided you desolder C5 just
in case.




--- In vintagesynthrepair@yahoogroups.com, "Roy J.
Tellason<rtellason@...> wrote:
>
> On Thursday 18 September 2008 11:00, r_j_d_2.phila wrote:
> > ok, new plan-i left the unit on for an hour, and the + rail seems
> > solid, but -15 drifted down to -12.5. both sides of x-former were in
> > the 23VAC range, so i have a new plan:
> >
> > 1-stick with old PSU
> > 2-replace + and - power transistors.
> > 3-socket and replace voltage regulators while im in there.
> 
> How about before you blame the PS you make sure that there isn't
something of 
> an excessive load pulling it down?  Could be that,  or the PS,  and
you're 
> assuming there without anything I'm seeing posted to indicate one
way or the 
> other...


   totally agree here... before I forget... check those 1ohmm
resistors, resolder and replace if ncecssary.

Mariano