HP Draftmaster RX pen plotter needs love

Brent Hilpert hilpert at cs.ubc.ca
Sun Nov 27 15:02:40 CST 2016

(Getting back to this after a couple days..)

The board photos help although not detailed enough for certainty in tracing.
I see the power supply test points on the photos and on the diagram 12-14 (page 12-28) of the manual.

Something to confirm: the -12V label is ambiguous in it's position on the board, do you know for sure you measured the right spot for -12V?
It looks like it would be the nice solder button to the left of the minus sign but from what I can discern it seems more likely to be the anode lead of the diode (CR2), right above the "2" in the "-12V" label.
The diagram 12-14 indicates the test point as the cathode (banded lead) of CR2, this is likely incorrect, if CR2 is the -12V rectifier the DC output would be the anode end. It looks likes the draftsman just copied the pattern for the +15V diode.

As you suggest, detailed assistance from a distance may be difficult, but you might check the above and see where things stand.
I'm not all that far away (Victoria area) but far enough to nonetheless be awkward for transport or trip.

On 2016-Nov-22, at 1:16 PM, Michael Newton wrote:
> I appreciate the guidance so far, thanks very much.
> I found another power supply/motor driver board ("analog board") of the same part number, hooked it up to the plotter and tested it. The -12v test point on the new board read zero, and the other voltages were present just like the original board.
> I don't suppose that proves anything, but at least now I have 2 power supplies.
> I don't have an electronics background other than hobby-level tinkering. I don't know how likely it is I'll be able to diagnose and fix this solely on remote guidance, and in any case that seems like too much for me to ask.
> If there is someone within driving distance of Seattle who could get this thing working I will pay them what their time and effort is worth (I'm trying to avoid shipping this monster or buying anyone plane tickets).
> thanks
> M.
> On Fri, Nov 18, 2016 at 3:17 PM, Michael Newton <michael.newton at gmail.com> wrote:
> 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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