HP Draftmaster RX pen plotter needs love

Michael Newton michael.newton at gmail.com
Fri Nov 18 17:17:20 CST 2016

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

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