HP 7585B Pen Plotter Repair

This blog post is a repair thread from social media that began on February 24, 2018.

February 24, 2018

So, this happened. My latest retro delivery. Any guesses?

It is, yet, another plotter. An HP 7585B wide format plotter with a 36” wide paper path.

Unfortunately it failed the first power on test and now it smells bad. So it’s been taken to bits to find the culprit.

It’s an all metal case weighing 155lbs! It’s thoughtfully engineered inside too, different sections compartmentalized. There’s a card chassis and backplane for the logic section.

Here’s a better shot of the HP 7585B front panel. It reports E27 on the display.

I think this is where the magic smoke was released. U5 on the internal I/O card. It’s a ULN2068B quad darlington switch.

February 25, 2018

The HP 7585B power supply voltages (+5, +12, -12) are a bit wonky under load, but with no load are spot on.

February 27, 2018

This is the HP 7585B plotter motor power section that connects to the backplane via headers. I’m impressed by the modular engineering. Time to test these power transistors.

Here’s the back of the motor power card. I tested for continuity and both transistors marked in red are shorted together. That would be a problem. Q5: 2N5886 and Q6: 2N5884.

Removed the 2N5886 and 2N5884 and all three pins (the case is pin three) are shorted together. The rest of the card doesn’t read as shorted so maybe the rest are OK. On to find replacements…

I reinstalled the motor power card (sans the bad transistors) and retested. The X and Y axis move! It errors when it reaches the carousel but that’s because the stepper motor driver is missing too. But it’s progress. Enjoy a picture of the control panel lit.

March 3, 2018

The new ULN2068B has arrived. Time to install.

March 5, 2018

Replaced the two power transistors (2N5884/2N5886) at the bottom as well. Didn’t take pictures of the thin sliver of mica that’s used as an insulator underneath. Glad I didn’t crack those!

Ugh. The HP 7585B plotter still won’t blend. It still fails to complete the startup sequence and ends with an error. Must dig deeper.

Here’s that sliver of mica that insulates the transistor case from the heat sink. Thermal grease on both sides.

The LCR tester thing reports the right kind of part for the 2N6054. Testing this in circuit wasn’t working so I removed it.

March 8, 2018

Can anyone recommend a part to replace this HP transistor (voltage regulator?) HP part no. 1854-0892 “TRANSISTOR NPN DARL TO-220AB PD=50W” Q7 in this schematic.

Here’s the part. I think 4-892 is shorthand for 1854-0892

March 10, 2018

Today the plotter is giving error E06 which is a checksum error in the EAROM. There’s a test mode to read/write/program the EAROM but the format isn’t making sense. What is 484848?

Ok, 48 is ASCII “0”, etc. so that’s address 01 to 63 and data 000 to 255. How did that escape me.

Why does the read format specify data? Wouldn’t that be a value to be returned? Not getting any response from the plotter over RS-232.

I can reprogram the EAROM with the HP85 Service System and the EAROM Reprogramming Tape 5010-2503. Yep, I’ll never find that :-/

March 17, 2018

Transistors came in for the big plotter motor driver section. Good news is there was no smoke. Bad news is there’s no change in behavior.

X and Y axis travels in one direction successfully. The opposite direction causes an error. This led me to the amplifier section so I replaced three op amps. No change. At this point I’m making guesses and that’s not a good repair method :-/

Using a X/Y axis test mode and probing a test point while manually moving an axis by hand, I can see negative and positive voltages for left and right. If they’re supposed to be exact opposites of each other, positive seems low. Should it be closer to +9.5v?

That would be a blown fuse that I neglected to test. This is F2 on the +30v side which explains everything now. +30v still fed the +12v regulator at F4, which is where I tested. And remarkably, I have a replacement on hand. Thanks to @bwalter01 for helping!

It blends! After replacing the fuse, it shows signs of life. The pen carousel now turns properly and the X and Y axes move in both directions. Need to test further but it looks promising.

First plot on my (repaired!) HP 7585B plotter— the built in confidence test on 11×17 paper. This thing is so fast!

Do you have a HP 7585B pen plotter? Let me know in the comments!

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