HP-45, PSU and testing points

[brickviking]

(03-03-2019 10:13 PM)albertofenini Wrote:  Hi everyone, today we dealt with a two HP-45.

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The second one one is more tough.
Display is completely off, and the two transistors from the PSU get very very hot.
We replaced them with a couple of transistors from another calc but with no improvement.
Also this couple of transistors get very very hot.

What could be a possible cause for this ?

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Thanks for any advice on this subject !

I'm surprised nobody's replied before now, normally people are a bit quicker to reply than this.

If you have the ability, and have a working HP-45 to compare with, I'd be checking voltages and current around those transistors. Odds are the transistors are getting fed with something considerably outside their normal voltages. If you've got the time, start from the battery/PSU and work your way through components to the display drivers, and then the display itself. You can usually check voltage drops across components with a good multimeter, current drops are a little harder but amounts to lifting one end of resistor, and very carefully placing probes in the right place. For those without fantastic coordination, you could solder a headerpin into the socket, and test current between the end of the component and that pin. I wouldn't do this for anything more complicated than resistors, I'd figure something else out for capacitors and transistors.

Some modern multimeters have a simple transistor gain measurement in their bag of tricks (and diode tester too), some others also have capacitance testing, both of those can be useful in their own ways, but are limited.

EDIT: additionally, many of the troubleshooting suggestions made for the HP-25 will also be relevant for the HP-45. Sorry I hadn't read that post before replying to this one and shooting my mouth off...
Anyhow, I hope you get it working. Others have had fun with the HP-45.

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