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HP 9872T Trouble Shooting
06-09-2018, 07:19 PM
Post: #1
HP 9872T Trouble Shooting
Last week I obtained a HP-9872T plotter in very good condition. In my dreams it will be controlled by a HP-41 (Plot Module, HP IL module, HP IL/HP IB interface) -- some day.

Unfortunately the HP-9872T does not initialize correctly when powered on (see short video http://www.huswelt.de/MoHPC/HP9872T_PowerOn.mp4).

Does someone know how to fix this, or knows someone who knows… Any hint is highly appreciated.

Thanks in advance.

Wolfgang
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06-09-2018, 08:56 PM
Post: #2
RE: HP 9872T Trouble Shooting
There is a service guide for the 9872C/T at hpmuseum.net along with user manuals.
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06-09-2018, 09:56 PM
Post: #3
RE: HP 9872T Trouble Shooting
Thanks for your reply.

(06-09-2018 08:56 PM)Paul Berger (Canada) Wrote:  There is a service guide for the 9872C/T at hpmuseum.net along with user manuals.

I have found no mention of my plotters behaviour neither in the “Operating and Programming Manual” nor in the “Service Manual”.

The “Confidence Test” cannot be started: the plotter simply ignores flipping the test switch on the rear panel to 1.

The internal red push button (reset) sometimes stops the noise (at least as long as the red button is pushed) but the self test cannot be continued because the green push button (continue) is ignored.

Help! ;-)

Wolfgang
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06-09-2018, 11:48 PM
Post: #4
RE: HP 9872T Trouble Shooting
I have two 9872C plotters, which are very similar, as you probably know. One of mine was stored where one or more mice took up residence, chewing one of the multi-conductor cables which carried the signals from the limit switches. On power up, the initialization sequence would cause the carriage to run to the extreme +Y direction and made a horrible noise until powered off.

If you haven't opened it up to check, you should do so to verify lack of cable damage, that there are no obviously bad capacitors. If you have the service manual, verify all supply voltages are correct.
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06-10-2018, 01:31 PM
Post: #5
RE: HP 9872T Trouble Shooting
Thanks for your reply.

(06-09-2018 11:48 PM)cruff Wrote:  … verify lack of cable damage, that there are no obviously bad capacitors. If you have the service manual, verify all supply voltages are correct.

I looked for cable damage, looked for leaking or swelling capacitors, checked every fuse and verified every supply voltage. Everything seems to be ok, no mice ;-)

BTW: The noise comes from the y-axis motor: if I turn off the y-axis motor (switch “S2 Y MUTE”) the noise vanishes. Unfortunately the plotter shows the same behaviour as before: front panel LEDs “ENTER”, “OUT OF LIMIT”, “ERROR”, and “CHART LOAD” are on and no self test can be started.

Wolfgang
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06-10-2018, 02:11 PM
Post: #6
RE: HP 9872T Trouble Shooting
Try joining then posting questions on the the Vintage HP Computer Systems group, found here: https://groups.io/g/VintHPcom.

The group is fairly active, and likely your best chance of finding someone with experience with one of these plotters.

Good Luck.

--Bob Prosperi
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06-10-2018, 04:29 PM
Post: #7
RE: HP 9872T Trouble Shooting
(06-10-2018 02:11 PM)rprosperi Wrote:  Try joining then posting questions on the the Vintage HP Computer Systems group, found here: https://groups.io/g/VintHPcom.

The group is fairly active, and likely your best chance of finding someone with experience with one of these plotters.

I'm a member of that group also, I'll try to help out either place.

What happens when you follow the self test steps in the service manual section 2-23? Note that the self test switch in this section is internal, not the one on the back panel.
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06-10-2018, 06:47 PM
Post: #8
RE: HP 9872T Trouble Shooting
(06-10-2018 02:11 PM)rprosperi Wrote:  Try joining then posting questions on the the Vintage HP Computer Systems group, found here: https://groups.io/g/VintHPcom.

Thanks for that hint. My posting is waiting to be approved by the moderator of VintHPcom.

Wolfgang
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06-10-2018, 08:05 PM
Post: #9
RE: HP 9872T Trouble Shooting
(06-10-2018 04:29 PM)cruff Wrote:  
(06-10-2018 02:11 PM)rprosperi Wrote:  The group [VintHPcom] is fairly active, and likely your best chance of finding someone with experience with one of these plotters.

I'm a member of that group also, I'll try to help out either place.

What happens when you follow the self test steps in the service manual section 2-23? Note that the self test switch in this section is internal, not the one on the back panel.

Thanks a lot for your offer. Here we go:

    [2.23.a] Remove power from the plotter. ✔︎
    [2.23.b] Remove all interface cables. ✔︎
    [2.23.c] Open the plotter. ✔︎
    [2.23.d] Apply power to the plotter. The plotter powers up with front panel LEDs “ENTER”, “OUT OF LIMIT”, “ERROR”, and “CHART LOAD” on and a y-axis motor buzzing very loudly (see http://www.huswelt.de/MoHPC/HP9872T_PowerOn.mp4).
    [2.23.e] Set the self test switch S4-2 to the on position. I assume the on position is towards the front panel away from the fan. Setting switch S4-2 to the on position has no effect on front panel LEDs and y-axis motor.
    [2.23.f] Press the Reset switch S5 pushbutton (red). While the reset button is pressed the y-axis motor stops buzzing but the front panel LEDs stay on. Releasing the reset button has no effect on the front panel LEDs but in most cases the y-axis motor starts buzzing again. Whether the y-axis motor is buzzing or not has no effect on the following results.
    [2.23.g] The Self Test LEDs will flash sequentially as a lamp test. Total failure here: each and every self test LED stays off.
    [2.23.h] Press the Continue switch S3 pushbutton (green). ✔︎
    [2.23.i] The test will run … Even after waiting more than two hours the front panel LEDs stay on, the y-axis motor continues buzzing loudly (if not silenced in 2.23.f), and all self test LEDs stay off.


Thanks for your efforts.

Wolfgang
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06-10-2018, 09:37 PM
Post: #10
RE: HP 9872T Trouble Shooting
(06-10-2018 08:05 PM)husmann Wrote:  [2.23.g] The Self Test LEDs will flash sequentially as a lamp test. Total failure here: each and every self test LED stays off.

This means the BPC microprocessor is not executing instructions properly. Do you have an oscilloscope or logic analyzer available to help with the diagnosis? Fortunately the logic levels should all be 5 V.

If ROMs U55 and U56 are socketed, try reseating the ROM chips. Perhaps some pin/socket contact oxidation is interfering.

Can you observe the clock signal at pins 5 and 6 of clock divider U60? They should be 5 MHz with alternating levels.
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06-10-2018, 10:49 PM
Post: #11
RE: HP 9872T Trouble Shooting
(06-10-2018 09:37 PM)cruff Wrote:  This means …

Thanks a lot for all your hints. I’ll need some time to check them all. As soon as I have completed them I’ll post my results here.

Wolfgang
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06-13-2018, 11:38 PM
Post: #12
RE: HP 9872T Trouble Shooting
(06-10-2018 10:49 PM)husmann Wrote:  Thanks a lot for all your hints. I’ll need some time to check them all. As soon as I have completed them I’ll post my results here.

Finally I managed to have a few minutes to return to my sick mechanical pet. My results:

• reseating ROMs U55 and U56 had no effect

• photos of the clock signal at pins 5 and 6 of clock divider U60 can be downloaded at http://huswelt.de/HP-9872T/HP-9872T.zip - the photos are taken from a borrowed simple oscilloscope

Thanks, Wolfgang
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06-14-2018, 09:41 PM
Post: #13
RE: HP 9872T Trouble Shooting
This thread is continued in the Vintage HP Computer Systems group (https://groups.io/g/VintHPcom).

Thanks, Wolfgang
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06-16-2018, 10:10 PM
Post: #14
RE: HP 9872T Trouble Shooting
I’m giving up. After having spent (again) quite a few hours analyzing the defective electronics of my HP-9872T I come to the conclusion that it is time to withdrawal. I do not have neither the equipment nor the time budget to continue this quest. Thanks to all who helped me.

If someone has interest in this HP-9872T: I give it away for free – complete with one spare paper roll, the service manual, the operating and programming manual, and the quick reference guide (no cables, no pens, 240 V AC).
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