On Thu, Nov 17, 2016 at 12:10 AM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
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.
Yes -12v, sorry
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.)
It's linked from
http://www.hpmuseum.net/collection_document.php - search
the page for "DraftMaster"
The document does appear to be missing the full schematic of the analog
board. I haven't been able to find them anywhere.
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.
I took photos:
https://goo.gl/photos/tRWV3ATTqx2R3eDz6