power supply problem

[Kenneth Martinez]

Hi...I'm pretty new to this list.  I plan to embark on some DIY projects
this winter, but I thought I'd get started by trying to diagnose some
odd behavior of my Xpander (plus, I haven't found any Oberheim techs
around Albuquerque, and don't want to ship it out of state for service).

I'm looking at the power supply...it has several outputs, and all work
correctly except one: the -12V DC output is putting out -9.4V, so it's
way below spec.  Now I'm trying to determine which component is causing
this problem, and I could really use some advice.  Here's the schematic:

http://www.flash.net/~klmart/xpsvc07.jpg

I checked a few spots with my DVM, and here's what I see:

  junction of T2 & D4 reads 17VAC
  junction of D4 & C5 reads 15.7VDC
  junction of U2 & C4 reads 12.2VDC
  junction of T2 & D6 reads 18.9VAC
  junction of D6 & C8 reads -11.8VDC
  junction of U1 & C2 reads -9.4VDC

(I'm obviously not an electronics tech...if there's a better way to
describe locations in the circuit, just tell me.)

I look at the +12V supply and see the voltage difference across diode D4
is 1.3V, while on the -12V portion there's a 7.1V difference across D6;
but I'm not sure whether this indicates a problem with D6, or U1, or C8,
or ...?  I suppose I could unsolder each component in the -12V circuit
and test each separately, or perhaps unhook/replace a component at a
time to see the effect of that, but is there any way to tell which is
bad by checking them while they're all still in the circuit?

Also, are there any books I should read that might help me with this
type of troubleshooting?  I'm sure I'll need to do in-circuit (is that
the term?) troubleshooting as I work on my DIY projects later.

thanks for your help