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I just obtained an HP 25 with a new style of internal power supply I've never seen before. It's on a daughterboard, and supplies +6, -12 and 4 volts, as the old style does, but has fewer discrete components. It's got a couple of electrolytic caps, and 2 inductor circuits. The calculator chip complement is different, too.

Can anyone enlighten me?

Regards,

Dave

There're a couple of HP-25 schematics here.
http://www.hpcc.org/cdroms/schematics5.0.zip

Looks like the batteries have leaked.

(05-01-2021 12:19 AM)Dreato Wrote: [ -> ]I just obtained an HP 25 with a new style of internal power supply I've never seen before. It's on a daughterboard, and supplies +6, -12 and 4 volts, as the old style does, but has fewer discrete components. It's got a couple of electrolytic caps, and 2 inductor circuits. The calculator chip complement is different, too.

Can anyone enlighten me?

I found this daughterboard power supply also in an HP-21 and HP-29C. It seems that assembly was optimized by producing these boards. As the power supply had many discrete components, I can imagine that automatic assembly was more efficient with this method.

Bernhard

Thanks for the info and the schematics. I noticed that the LC oscillator is on that board, too.

I think I have problems with this power supply. The voltages are OK, but are noisy. I'll have to remove the daughterboard to checks caps, diodes etc. It looks like this board is soldered to the main board in 5 places. Is this correct?

Thanks again,

Dave