hp85 trouble


My hp85 has a problem. It turns on, and it passes its self test. However, after ~1-2min it turns off and then back on again and hangs. If I tap on the corner (near the power switch) it will come back on. It of course sounds like some kind of intermitancy. I think I've traced the problem to a bad power switch. When I test the continuity across the switch in its "on" position, my meter reads closed for a while and then reads open.

Any thoughts? It seems like a high quality switch but I suppose it could certainly be to blame.



you could take the switch out of circuit and see if all is well.


According to the HP85 service manual, the power switch is a double-pole unit that breaks both the live and neutral mains wires. I assume you realised this, and are checking each section separately.
For testing you could try shorting out each section of the switch with a jumper wire (take care -- this is mains you're working with). If the machine then works (of course the power switch won't do anything if it's shorted out), then you know the switch is faulty.
Again, from what I can see in the service manual, each half of the switch has 2 wires of the same colour, so to short it out you just link those 2 wires of the same colour together, and keep the 2 groups separate.


If the switch is flakey, you can probably make it work again just by spraying it with contact cleaner. Most switches like this have sealed housings. I have successfully fixed many of them by drilling a small access hole in the side of the switch with a hand (pin vise) drill. I try to locate the hole away from probable internal workings like pivots or bearings. I drill in an upwards direction so that drill shavings do not fall into the switch.


Well, its not the power switch. I de-soldered both wires from the switch and soldered them directly together. After plugging it in, it came up just fine for about 2 minutes and 10 seconds. Then the power light goes off, the screen goes blank, and the cassette light comes on.

Then I can give the unit a nice solid tap on the corner just above the power switch and it resets itself and comes back on (picking up on that corner does the trick too). Then it returns to its locked-up state with the cassette light ~10-15 seconds later.

Should I check the big transformer next? It seems like a power supply problem to me.

Any thoughts?



Edited: 4 Sept 2003, 10:42 p.m.


I would try to start wiggling around wires and tapping around with a screwdriver handle to isolate where the intermittent connection or component is. These sort of things can be rather frustrating to find.


The fact that the cassette light turns on means it's not the transformer. In these machines, the transformer porvides only one voltage, which is then regulated on the PSU board (the one on the side of the paper roll holder) to give all the voltages the machine needs. The fact that the power LED goes out implies that you're losing the +6V line (IIRC) from the CPU board.
This does sound like an intermittant connection. I'd start by re-seating all the cables and connectors in the unit (especially those darn tape-like ribbon cables). Then I'd get the service manual (it's on the MoHPC CD-ROM set), and check the PSU voltages when the machine fails. Find out which one(s) you're missing, and from which PCBs.

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