On 2016-Nov-16, at 11:34 PM, Michael Newton wrote:
That's right, there is a -5v test point that reads
zero.
From the previous discussion, presumably you mean -12V.
Any guidance? Like if I need to pull parts off and
test them, which ones
might I go for? I'm a caveman with electronics.
Looking at:
7595-7596_HardwareSupportManual_07595-90025_168pages_Feb90.pdf
from
hpmuseum.net via the link you supplied, the
document seems to be truncated.
The document simply ends part way through the
schematics pages, and of course the power supply schematic is missing.
(Aside, where did you find that document link? I didn't find it ref'd on the
hpmuseum pages.)
There is a minimal diagram for the power supply on page 5-4 (pdf.30), which shows the -12V
supply as an independent (not part of the control loop) secondary out of the switching
supply. That's good as it limits the likely problem region. There will be more
components involved than shown there, but the diode seen there on the -12V supply will
lead to a filter cap and possibly a 3-terminal linear regulator such as a 7912 or
LM320-12, or even a zener
regulator. There may be current limiting or overvoltage circuitry between there and the
actual -12 output of the supply.
- identify the -12V componentry in the power supply.
- if there is a 3-terminal -12V regulator check for input to the reg vs output.
- confirm that it's not the load side of the -12 causing the problem.
- pic(s) of the power supply board might help us identify the area or get a better idea
of what we're dealing with.