HP-91 power supply problems

[Chris Dalla]

Okay, so here's the thing: I've got this HP-91 that I picked up at the flea market a while back, and it clearly _wants_ to work: Display comes up nice and bright, all the keys respond (too well, in fact: the keybounce is awful), everything's happy. Until I light too many segments of the display. As soon as I do that, the calculator resets (the display goes dark for a moment) and comes back up with 0.00.
I'm pretty sure it's a power supply problem and not some sort of bizarre short in the display module because the reset point is clearly linked to the amount of power being drawn: I can load the display up with 1's, but I can only get about four 8's on the display before it resets. In addition, setting the printer switch to ALL or NORM or pressing one of the buttons that would cause the printer to try and print causes a reset.

Now, having said all that, what I'm ideally hoping to find is a schematic of at least the power supply section of the calculator...at worst, I'd just like to know for sure what the 3-pin semiconductor with the heat sink tab is (since all the other parts in the power supply are discretes, I can just measure the ones that aren't marked to find out what their values are). If anyone can help, I'd be grateful.

-Chris

[Viktor Toth]

Chris,

That large semiconductor is just a plain NPN power switching transistor. I don't remember its pinout; I do remember that I was able to replace it with a 2N3053, which worked fine, but was too large (case didn't close), so I used a power Darlington instead, and it's working great.

The function of this transistor is to let through extra current during times of heavy loads, such as printing, so your assumption that it's the one causing the problem is quite reasonable.

The power supply schematic, by the way, is the same as that of the HP-97, which can be found on the MoHPC CD-ROM set.

Viktor

[Chris Dalla]

Viktor:

Excellent...that's what I was hoping (both about the component and the schematic: I ordered the CDs, they're just a bit behind in production). Once I figure out what's
going on, I'll post a follow-up.

-Chris