HP-45, PSU and testing points

[albertofenini]

Hi everyone, today we dealt with a two HP-45.

Both were not running, we fixed one and we are looking for help for the other.
Doing this, we recovered some testing points from the schematics and we are happy to share
them, may be could be useful for someone else.

The one that we fixed had all of the Voltages correct, except for Vcc which was about 3 volts,
and all of the waveforms were correct. Display was close to dead. Also Ø1 and Ø2 were good.
We replaced the diode were Vcc is measured (top of the logic board) and it worked like a charm.

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 !

   

[brickviking]

(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 ?

...
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.

(Post 326)

[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

[teenix]

(03-03-2019 10:13 PM)albertofenini Wrote:  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 !

Jarques Laporte mentions

0550 - 2N3904
other - 2N3704
tantalium or electrolytic, either is fine, but tantalum more compact.

cheers

Tony