HP Draftmaster RX pen plotter needs love

David Collins davidkcollins2 at gmail.com
Wed Nov 30 20:06:00 CST 2016

If you have an oscilloscope or logic analyser see if you have a clock signal at the main CPU.  If you do, see if your EPROMs are being accessed. If not you may have a clock issue, a bad CPU or the CPU reset stuck low. 

David Collins

(Sent from out of office)

> On 1 Dec. 2016, at 12:48 pm, Michael Newton <michael.newton at gmail.com> wrote:
> You were correct, I was looking for the -12v in the wrong place. I did find
> -12v on the anode of CR2. So all voltages are correct at the test points.
> So the LCD shows nothing and the device does not seem to respond to keypad
> presses.
> The other symptom is that I can hear some feeble and irregular clicky
> noises from the paper drive motor. So it isn't purely just that the display
> failed.
> thanks
> M.
>> On Sun, Nov 27, 2016 at 1:02 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
>> (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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