HP-97 Blues... any advice?



#10

Hi, all.

I've got an HP-97 that won't power up. When I turn it on, it shows only the low-battery LED. The transistor and resistor near the power input get hot. I presume I've got a component or chip shorting. Any advice on where to start? Sound like a familiar problem to anyone? The voltages barely register, of course.

Thanks!
Michael


#11

Hi Michael,

Have you tried to power it up with the printer motor and/or card reader motor disconnected. I found that the printer motor caused that problem on one of my 97's. I just pulled the wires to the motor and that did the trick. (Then, I had to fix the printer, of course.)

Failing that, have you tried some board swapping with one of your good machines to isolate the problem? It could be on the printer driver board, keyboard or main circuit board.

-Katie

#12

Just from my memory of things I've read here - the 97 has a transistor for over-voltage protection that is turned on to load down the power supply if the voltage gets too high, to pull the voltage back down. I don't know if the transistor has a load resistor to absorb the energy or if the transistor drops all the voltage itself. I remember reading that the transistor sometimes goes bad, stays on, gets hot, and keeps the power supply voltage from rising to its normal level.

#13

First get out your handy dandy HPMUSEUM CDROMS. The HP97 service manual is on them. Full schematics and everything. Most of the power supply is on the display driver board. I have seen several machines where either the power resistor failed or the transistor across it. This led to a cascade of other failues taking out a total of four transistors... yummy.

Another problem that I have seen was due to a stuck print head. The machine will either not light up or displays ERROR if the head cannot go home.

Follow Katies advice and swap out the top end first to see where to start. Also remember that the machine will not work if the printer circuit board is not connected.

#14

I had a Topcat in for repair that did this (it wasn't a 97, it was a 95C, but the circuitry is much the same).
There's a circuit in the Topcats to load the power supply down if the input voltage tries to exceed the correct +ve output voltage -- this protects the machine if it's connected to the AC adapter with no battery pack in place, so you don't get IC failures like in the Woodstocks.
Anyway, if the power converter (on the CPU board) isn't running, this circuit will try to pull the inpjut voltage right down to 0V, and gets hot in the process.
I would start there. The service manual, with scehamtics, is on the MoHPC CD-ROMs. THe power converter is a simple discrete component circuit based round 2 transistors (oscillator and regulator) and a transformer. Most of the times I've had problems with this circuit (not just in Topcats), the oscillator transsitor is open. A 2N3904 is a suitable replacement.


#15

Hi All.

I have the service manual. I started out by swapping boards... it's definitely in the largest main circuit
board, even with the printer and card reader disconnected.

The service manual is a bit difficult to read, and the components are in a very different layout than the nice diagram in the manual.

I appreciated the suggestions for where to start to look for the problem, and I'll keep you posted.

Michael


#16

Oooh, bad news doc... I have never seen a bad main board had the decency to have a off-the-shelf component fail. Or an ACT chip either. I have seen a couple of bad PIK chips and a bad ROM chip though.


#17

Odd, my 97 (which I was given in a rather poor state) had a bad ACT chip (I replaced it with one from a 21 with a bad ROM...). The other chips on the CPU board were all fine.
I have also replaced many PSU oscillator transistors in Topcats (and classics)....

#18

Electrically the boards are much the same (I would guess you have the later version board with the 16 pin DIL space for the cabel for the 97S I/O module).
Start by finding the power converter autotransformer -- a little 'wound component' with 4 pins. Trace the circuitry round that to find 2 transistors. Then test these transistors out of circuit.


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