Classic Series Power Supply Schematic?

Hi all,

Anyone have a schematic for the power supply of the classic series (HP-35, 45, 55)?

Not the power brick, but the internal supply which converts Vbsw to Vcc (+8.2V), Vss(+6V) and Vgg (-12V).

I am wanting to repair an HP-45 and after metering the internal voltages at the testpoints, it looks like that's where the problem is.


-- Dan


I've traced out full schematics of just about all the old HP handhelds. They're available on the HPCC schematics CD-ROM (see the obvious web site for details).

This CD-ROM is produced by Dave Colver, the HPCC secretary. If you ask him nicely (I believe contact details are on the same site), he might e-mail you just the schematic for your machine.

My experience suggests that the 35 PSU schematic is somewhat different to the later ones (the later ones have a zener diode in the regulator circuit, the 35 one that I've seen simply sets Vss to be a certain multiple of the battery voltage).

Incidentally, if you're getting no outputs at all, most of the time it's the oscillator transistor that's failed. A 2N3904 is a suitable replacement. If you're getting excessive ripple on one of the outputs, or if the Vgg line is crazy, I'd suspect the electrolytic capacitors.


Thanks. I sent a note to Dave.

Looks like this is a flyback converter with the oscillator circuit to the left, 3 windings on the transformer, and 2.2 uF capacitor/diode circuits to the right providing the voltages.

On the oscillator circuit there appear to be two transistors, both Motorola, labeled 4-094 and 4-550 (NPN and PNP?). I tried looking up these part numbers but didn't find anything. It also has a 60uF cap.

Usually the least reliable components would be the capacitors. But I would agree if the oscillator is dead the transistors may be the problem.

So far I've only hooked it up to a multimeter. This weekend I'll scope it to see what I find.

Also I took the board and scanned both sides on a flatbed scanner at a ridiculously high DPI in case I decide to try to reverse engineer this thing myself.

With all of the gold plating and the gold/ceramic Mostek chips, this board is a real work of art. I'm half tempted to frame the picture.

Any other thoughts or related experiences are welcome.

-- Dan

Possibly Related Threads…
Thread Author Replies Views Last Post
  A Wonderful Classic HP Calculator BShoring 2 1,220 11-22-2013, 05:25 PM
Last Post: BShoring
  Classic battery polarity Kiyoshi Akima 2 1,415 11-08-2013, 01:31 AM
Last Post: Kiyoshi Akima
  CLASSIC/SPICE KEY LEGENDS Mike T. 2 1,254 08-21-2013, 04:53 PM
Last Post: Mike T.
  Woodstock, Classic & Spices for iPhone Matt Agajanian 8 2,568 07-10-2013, 06:10 PM
Last Post: BShoring
  HP 9845 option 200 power supply Dave F 2 1,404 05-30-2013, 08:10 AM
Last Post: Dave F
  Classic calculator display chip type number? Juergen Hofstaedter 3 1,401 01-22-2013, 12:47 PM
Last Post: Eric Smith
  Classic Series Enter Key Dan Lewis 15 3,840 12-19-2012, 09:10 AM
Last Post: Thomas Radtke
  HP-87 power supply repair philippe_elie 0 823 11-18-2012, 11:12 AM
Last Post: philippe_elie
  HP-48: extract coefficient of power series Thomas Klemm 3 1,363 10-21-2012, 11:42 PM
Last Post: Han
  Classic (HP-67) charger issues revisited Matt Agajanian 6 2,070 05-20-2012, 10:50 PM
Last Post: Matt Agajanian

Forum Jump: