[sdiy] Moog 909 power supply component ID question

[Gene Stopp]

Hi DIY,

Hey guys I fixed the 910 and it's back in the Moog. But thanks for all the
feedback! This happens to me a lot - I take something apart, scrutinize and
analyze and probe and jostle, and I don't see anything wrong, so I fire it
up again and it works. One thing I did do was splash some De-Ox-It in the
voltage adjust wirewound trimmers. I'll bet that was it.

Interesting bench trivia for the 910: I measured +12 to ground first, with a
24-ohm load, came up OK. Tried -6 with the same load, came up around -1.5
volts. Using my keen sense of "what the heck", I loaded the +12 with the -6
(skipping over ground) and got 18 volts. Aha! We need a minimum load on the
+12 to make the -6 regulate properly. Stuck load resistors on both supplies
to ground, everything was fine. Gotta move quick when doing this, to avoid
smoking the 1/2 watt resistors...

The -10 unreg output is more like -15, or -14 when installed and hooked to
the system, so always remember that when the schematic says "unregulated"
they mean it!

According to the 1965 spec sheet the power supply is good for 2 amps,
although the DC fuse on the raw DC is 1.5 amps.

The modules are fine. My last remaining problem is that the pilot lamp is
out now. Musta got whacked when the ps freaked out? The socket has 18 volts
DC on it. The bulb itself is a red dome on the front panel that unscrews -
the bulb is the dome. Darn it, what are the chances of finding a replacement
bulb for that? This is a mint System 10 so it's definitely got vintage
written all over it - sure would be nice to find NOS parts!

Eric!!! Help!!! Any ideas???

- Gene



-----Original Message-----
From: Turner Wallace [mailto:Wallace.Turner at uk.thalesgroup.com]
Sent: Thursday, December 05, 2002 12:19 AM
To: 'Gene Stopp'
Subject: RE: [sdiy] Moog 909 power supply component ID question


My mail doesn't appear to reach the list so here's a direct reply.

I have repaired a 910 supply which involved replacing all the transistors. I
couldn't source the exact output stage transistors so I thought I would
replace them with a modern darlington equivalent. 
Instant oscillation!
Traced an equivalent germanium transistor pair instead. Still oscillated!
(~2MHz).

The supply is difficult to debug due to the unsual design. A feedback within
feedback loop. This makes it quite complex to theoretically work out what is
going on. Poles and zeros on a root locus and control theory etc.

The solution to the oscillation problem was the inclusion of ferrite beads
on the wiring between the PCB and the output stage transistors on the
heatsink.

>From a modern perspective I wouldn't rate the design of the supply. The
stability of the oscillator modules rely on stable voltage rails. A
germanium zener reference doesn't cut it against a modern band gab based
voltage reference with respect to temperature variations.

The other question is do you want to hook up your expensive moog modules to
a supply that doesn't provide short circuit and crow bar protection?

Perhaps you might forward this to the list?