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HP Forum Archive 17

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Classic Series Power Supply Schematic?
Message #1 Posted by Daniel Weed on 22 Mar 2007, 5:36 p.m.

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

Re: Classic Series Power Supply Schematic?
Message #2 Posted by Tony Duell on 23 Mar 2007, 6:10 a.m.,
in response to message #1 by Daniel Weed

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.

Re: Classic Series Power Supply Schematic?
Message #3 Posted by Dan on 23 Mar 2007, 12:03 p.m.,
in response to message #2 by Tony Duell

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

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