Fixing a HP 38C

[Dave92F1]

I picked up a 38C on eBay for parts, but to my dismay it almost works so I can't steal parts from it - instead I have to fix it.

When I turn it on, it works properly about 1 attempt in 20.

The rest of the time it does something unpredictable - sometimes a blank display, other times all zeros (0000000000), other times random segments turn on and off unpredictably. And sometimes the display is messed up but if I press a key it starts working properly.

I don't think it's the power switch - I cleaned it, and it does the same things if I switch the power externally.

I suspect the power supply isn't starting up cleanly each time.

Any suggestions? (I'm quite handy with a soldering iron, but not really an analog guy.)

[rprosperi]

Is this unit the early 'press-fit' type or the later soldered type? If you're not sure, a couple ways to narrow it down are it's weight (in grams) and serial number.

If it's the former, the problem could simply be poor contacts.

[Dave92F1]

(03-04-2021 03:24 AM)rprosperi Wrote:  Is this unit the early 'press-fit' type or the later soldered type? If you're not sure, a couple ways to narrow it down are it's weight (in grams) and serial number.

I opened it up to clean the power switch (DeOxit Gold) and the chips didn't fall out so I think it's the soldered kind. S/N is 2221S30100, Singapore.

At first I thought it was the power switch too, but as I said I cleaned the switch and tried switching it externally and it does the same thing.

Here's a video of what it does: https://photos.app.goo.gl/ngAsWU7oBhrtS99y5

One thing I've learned since my original post: If I pull out the battery for 5 minutes, then put it back in, usually the first time I turn it on I get "Pr Error", then the second time it starts up OK ("0.00"). And it works OK for the first 5 or 6 keystrokes, then goes nuts again. But if I leave the battery in I get "0.00" on maybe 1 out of 20 tries. Pull out the battery - get "Pr Error" 10/10 tries, then "0.00" 7/10 tries. Something about pulling the battery helps (resetting the continuous memory maybe).

[PANAMATIK]

(03-04-2021 04:27 AM)Dave92F1 Wrote:  One thing I've learned since my original post: If I pull out the battery for 5 minutes, then put it back in, usually the first time I turn it on I get "Pr Error", then the second time it starts up OK ("0.00"). And it works OK for the first 5 or 6 keystrokes, then goes nuts again. But if I leave the battery in I get "0.00" on maybe 1 out of 20 tries. Pull out the battery - get "Pr Error" 10/10 tries, then "0.00" 7/10 tries. Something about pulling the battery helps (resetting the continuous memory maybe).

Your image shows indeed the soldered version. Therefore the failure cannot be explained by the common contact problems in the solderless version.

When the HP-38C program starts, it tests only a part of Continuous RAM and shows "Pr Error" if it is lost. Perhaps the failure is a RAM problem, but can also be one of the other circuits.

I have an HP-38C Chip set left over in my parts box, but cannot guarantee that it is working.

Bernhard

[vassilisprevelakis]

Hi,

did you try running the self test? (press STO then ENTER^)

the screen should go blank for a few second and then display -8,8,8,8,8,8,8,8,8,8,

**vp

http://www.series80.org