HP-91 power supply problems


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.




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.




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.


Possibly Related Threads...
Thread Author Replies Views Last Post
  HP Prime Save Power Turn Off Not Working Timothy Roche 12 2,750 10-27-2013, 01:41 PM
Last Post: Michael de Estrada
  Low power warning for HP-15C LE and batteries Nick_S 1 971 09-16-2013, 09:34 AM
Last Post: Hans Brueggemann
  HP 67 problems Manel 1 751 06-11-2013, 09:15 PM
Last Post: BobVA
  HP 9845 option 200 power supply Dave F 2 1,131 05-30-2013, 08:10 AM
Last Post: Dave F
  HP-87 power supply repair philippe_elie 0 671 11-18-2012, 11:12 AM
Last Post: philippe_elie
  HP-48: extract coefficient of power series Thomas Klemm 3 1,041 10-21-2012, 11:42 PM
Last Post: Han
  HP-41CV power problem Don Davis 2 870 10-11-2012, 09:13 PM
Last Post: Don Davis
  What power adaptor does an HP-65 use? Robert Morton 3 1,025 09-30-2012, 02:32 PM
Last Post: Kees Bouw
  Update (Fixed): Problems concerning HP 82104 card reader Edward Cheng 0 718 09-05-2012, 04:58 AM
Last Post: Edward Cheng
  Problems concerning HP 82104 card reader Edward Cheng 3 1,044 08-21-2012, 07:52 PM
Last Post: aj04062

Forum Jump: