17 Sep
2008
17 Sep
'08
1:31 p.m.
This is a
relatively unconventional and repairer-friendly SMPSU! The chopper
control circuitry is all on the output side of the isoaltion barrier and
the startup supply comes from a little mains transformer and linear regulator.
Start by checking for the +8V supply from U1 on the PSU mainboard. This, I
suspect is present, it's the 8V you've seen on the op-amp.
Yep, 8V from U1. (well, 7.92V actually, I assume that's within
acceptable range?) Now check for the 5V reference voltage from pin
14 of U4 (the TL494).
Hmm, I get 6.75V (measuring from Pin 7 (gnd) to 14.) Now, I think
that TL494 should be oscillating, at least at startup. See if
you get anything on pin 8. A TL494 datasheet would be a useful thing to have!
7.91V on pin 8. (I have a datasheet at http://yahozna.dyndns.org/scratch/tl494.pdf). I
scoped it out, and pin
5 is oscillating, a nice sawtooth.
OK, the oscillator is running, but it's not driving the output stage.
What voltage
do you see on pin 3 (Comp) of this chip? That's the output of the
error amplifier IIRC. It appears there's an overcurrent shutdown circuit
connected there, maybe that's shutting it down.
0.06V on pin 3.