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HP-97 Printer Troubleshooting - Printable Version

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HP-97 Printer Troubleshooting - aj04062 - 10-15-2013

Seeking suggestions:

When I went to retest after cleaning the printer and rebuilding the card reader, the unit failed to come on and started to get warm somewhere in the printer circuitry (I think) enough that I could smell the heating component. I quickly disconnected from power. I also noticed that the low battery led was on. In the past on other units, I have noticed that a unit will not turn on if the printer PCB is not fully engaged or forgotten. I thought this might be the case and reassembled it. However it failed to remedy the fault. No I fear that I have permanently burnt out something, but I don't know what or how I did it.

It seems that something in the printer circuitry is getting warm and is shorted. Any suggestions? Shorted transistors? I have pulled all other items off the board (motor, head, etc) and retried but behavior did not change.

Re: HP-97 Printer Troubleshooting - Randy - 10-15-2013

Make sure your supply voltage is below 6 volts as it sounds like the over voltage clamp turned on. If it activated, the heat is from the to-225 case transistor in the lower right corner of the main board being saturated and dumping the supply voltage.

Any Topcat with a printer will not power up with the printer control board removed. This is due to the LC tank components of the ACT clock having been located on the printer board. I would expect this is an interlock of sorts and bad things would happen if it was powered without the board in place. This is speculation on my part but given that there is a cost in having located the components where they did (connectors, extra board holes, etc) rather than right next to the ACT, there has to be a reason for having done so.

Re: HP-97 Printer Troubleshooting - aj04062 - 10-15-2013

The unit was powered on with a rebuilt battery pack (nicads), so the supply voltage is under 6 volts.

I'll pull the transistor and test.


Re: HP-97 Printer Troubleshooting - Eric Smith - 10-15-2013

For years I've been wondering why the LC components are on the printhead driver board. I've operated a 97 with the printhead driver board removed and LC components kludge in, and experienced no noticeable ill effects, but YMMV.

I was doing that to make sure that none of the abuse I was perpetrating on my poor unsuspecting 97 could put its PIK chip into the state that burns up the print elements, as can happen with some PIK chips if you print NNNs. PIK chips were made by AMI and Mostek, and there were reports in PPCJ that only the PIK chips from one of the two vendors will do that, but I don't recall which one, and I wouldn't care to risk it with either.

Re: HP-97 Printer Troubleshooting - aj04062 - 10-15-2013

Okay bear with this dumb M.E. question.

I've pulled the transistor and checked with my PEAK DCA55 component analyser (yes it's a British one).

NPN Silicon Transistor

Current Gain Hfe=69

Test current Ic=2.5mA

Base-Emitter V Vbe=0.72V

Test Current Ib=4.68mA

Leakage current Ic=0.00mA

This checks out okay, correct? I don't know what specs to compare these values against. This transistor is part of the "protection circuit" as outlined on the HPCC circuit diagrams, correct?

Edited: 15 Oct 2013, 9:05 p.m.

Re: HP-97 Printer Troubleshooting - Randy - 10-17-2013

The HP to industry device cross reference I have does not list the 1854-0713 transistor so I have no basis for what the device specs would be.

Since you can measure the gain with your tester and there is no leakage, you should be good to go...

Re: HP-97 Printer Troubleshooting - Ignazio Cara (Italy) - 10-18-2013

Hi Adam, ciao.
have you checked the -0668 and -0071 transistors?
Maybe one is bad, and you must find a spare.