HP-85 power supply problem


Just when I thought that I finally managed to repair the tape drive in an HP-85, suddenly its power supply died on me. At least that's my diagnosis given that not even the power LED lights up anymore.

I don't know about others, but I really hate poking around in an unknown power supply that may have dangerous DC voltages. Besides, just taking out the PCB that contains the power supply in the HP-85 looks like an ugly chore.

For this reason, I thought I'd ask first: does anybody have an HP-85 Service Manual? Unfortunately, it doesn't appear to be on the Museum CDs.




I don't have an answer, but I had to quote: "It just goes to show you: If it's not one thing, it's another!!"
<grin> Murphy was an optimist!



I can't help you with a service manual, but I seem to remember (from last time I was inside an 85) that the PSU in the HP85
starts by transforming the mains down to a relatively low AC voltage (20-30V or so), rectifies/smooths that and then produces all the output voltages from that. There are no particularly nasty DC (or AC) voltages on the PSU board. The only dangerous voltage in the PSU is the mains on the primary side of the transformer.



You remember correctly, line power is indeed transformed down to about 36VAC. But 36VAC can easily mean 70-100VDC after two-way rectification (and there ARE some real nasty-looking capacitors in there right after the diode bridge.)

Also don't forget that this unit has an integrated CRT (and it's not clear from simply looking at it which, if any, components of the CRT circuit are also on the same PCB.) A CRT circuit definitely has dangerous voltages (in fact, there's a label on one of the PCBs to this effect). So I rather be cautious and wait until I have the necessary information rather than risk frying this poor machine or electrocuting myself. Call me a coward :-)




I have a service manual on the 85, may take a day to find it but you are welcome to any info it may provide.



I thought it was 36V centre-tapped to earth (so 18V AC on each side or so). In which case you're not likely to have voltages higher than +/-25V on the PSU board. OK, 50V can still be nasty, but it's a lot safer than mains (or worse still rectified mains like you find in normal SMPSUs).
The CRT monitor seems to be a separate module IIRC. It runs off one of the DC voltage lines (+12V, I think) and generates the high voltages from the flyback transformer in the traditional way. So don't worry about finding that sort of voltage on the PSU board.
I seem to remember you can remove the PSU board, pull the printer chip (which is on the PSU board) for safety's sake (of the chip) and then reconnect the transformer wires for testing. This protects the rest of the machine if you do something stupid and get one of the PSU outputs to go high, and it also prevents the CRT from powering up.
FWIW, the only time I've had problmes with an HP85 PSU, the cause was a shorted electrolytic capacitor. Worth checking anyway.



The 85 power supply can be isolated from the rest of the machine by removing the ribbon cables J1 and J2, however the +5vdc line must be loaded, otherwise the main fuse will blow. To load the +5vdc line connect an 22 ohm (2 watt) resistor accross the anode side (+5vdc) of diode CR10 and the cathode side (GND) of diode VR2. This loads the supply with about 300mA(s)

The voltages should be as below:

12 measured at J1 16/17 11.4 to 12.6
6 measured at J2-6 5.7 to 6.3
5 measured accross external load 5.2 to 5.6 (adjustable with R23)
-12 measured at J2-8 -10.8 to -13.2
-5 measured at J2-4 -4.75 to -5.25

As Tony mentioned it is good ideal to remove the printer control IC U5. Hope this helps





Thank you very much, this is exactly the kind of information I was looking for. By the way, any chance you might contribute a scan of the HP-85 service manual to a future museum CD?




You're right about the center-tapped transformer but even so... here I am, trying to lead by example, doing all the right things you're supposed to do when repairing an unknown power supply in order to avoid being electrocuted, and you're advising me to be reckless? Shame on you! :-)


PS: I do thank you for the advice, by the way. Especially the bit about removing the printer controller chip.


Hi Viktor:

This may be a simplistic view of things, but I've had luck just checking the "pico fuses" in a couple of seemingly dead HP-85's. These are the little things that look like resistors (without any color bands) but are really fuses. HP seems to have had a preference for using these things, like on the HP-9114B battery charger PCB, and in these HP-85's.

Look around for one or two of these in the power supply, and check for continuity. It worked for me, but was probably dumb luck.

Dan M



Embarrassing it is to admit but... YES, that was the problem!

I did, as a matter of fact, look for on board fuses before, but I didn't see any. So I assumed that there aren't any. Since you indicated that there ARE, I looked. And looked again. And looked for a third time, and then I found it. And the fuse, of course, was blown. Thank you very much for this suggestion!

All fixed, everything works like a charm, including my freshly repaired tape drive. Now I found out that my new-old-stock tapes are all faulty. They were probably in storage for a very long time, and the oxide material came off the transparent tape at a spot, and the drive interprets this as an end-of-tape. Darn!



You might try a black marker or some other similar item to black out the spot where the oxide is missing... assuming they use an opto sensor looking for clear leader. Note that some black markers are TOTALLY tranparent to infrared light and may not be suitible.


I tried the marker, but it's not good enough; the damaged part of the tape is actually an area that would already be used for data. So I can erase and rewind the tape, but I cannot write anything onto it that's longer than a single (short) line of code.

So I did something every teenager learned to do in the 70s before the CD era when cassettes were still the rage: I spliced the tape, cutting out the damaged part. I now have a working tape!



Viktor >All fixed, everything works like a charm, including my freshly repaired tape drive.

I don't know if you mentioned this in another thread, but did your drive suffer the dreaded deteriorated capstan syndrome?

If so, can I ask how you repaired it (the common method around here seems to be heat-shrink tubing)?




Yes, I did have a deteriorated capstan, and I repaired it using a bit of model airplane fuel hose. This was originally obtained to fix HP-67 card readers, but this particular material wasn't working very well for that purpose. However, it is extremely flexible, so I was able to place it over the (much wider in diameter) capstan wheel and it is working fine.

Unfortunately now I have other problems with this tape drive... <sigh>


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