HP-45, PSU and testing points

[teenix]

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

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 ?

More in general, has anyone found a good replacement for the diodes, capacitors and transistors for a PSU ?

The transistors are marked F4 094 427 and 0550
The diodes should be 1N4148 except for the zener
we don't know the voltage value for the zener
Capacitors should be replaced with tantalium or with Electrolytics ?
The disc capacitor I think is a ceramic one

Thanks for any advice on this subject !

Try
https://www.keesvandersanden.nl/calculat...supply.php

Perhaps if the oscillator is not working, the transistors may be sinking too much current from the battery directly through the coil which is not what it is supposed to do.

cheers

Tony