HP Draftmaster RX pen plotter needs love

[Brent Hilpert]

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.