Threaded Mode | Linear Mode

andrewsiegel · 10-23-2020, 02:19 AM

I bought a non-functional HP80 off Ebay. Aside from some corrosion from battery leakage, it's in good physical shape. The display is totally blank on power-on. Using known good batteries, I did a quick test of voltages: Vss and Vgg are good, but Vcc is showing +3V (it should be +8.2V), which might explain the blank display. I don't have a scope yet, so I can't check waveforms, but I wonder if anyone has a suggestion as to what the problem might be.

teenix · 10-23-2020, 03:52 AM

(10-23-2020 02:19 AM)andrewsiegel Wrote: I bought a non-functional HP80 off Ebay. Aside from some corrosion from battery leakage, it's in good physical shape. The display is totally blank on power-on. Using known good batteries, I did a quick test of voltages: Vss and Vgg are good, but Vcc is showing +3V (it should be +8.2V), which might explain the blank display. I don't have a scope yet, so I can't check waveforms, but I wonder if anyone has a suggestion as to what the problem might be.

Maybe the power supply Vcc output diode or 2u2 cap may be faulty, poor coil connection for the Vcc rail.

Corrosion somewhere on the Vcc PCB tracks on CPU or display boards.

Does the cathode driver chip get warm, or any other display components, inductors etc? This may indicate faults.

cheers

Tony

andrewsiegel · 10-24-2020, 01:47 AM

Thanks Tony. There was a lot of corrosion around the Vcc trace on the keypad board, though I had scrubbed it with IPA, and a visual inspection now shows no apparent bridging. The resistance between the Vcc trace and the neighboring ground trace is about 1.3 megaohms.

Neither the cathode driver nor any other display component gets warm. I do have a scope now, and I see signals on all of the test points and board connectors where I think they should be, so this calculator isn't brain dead.

Does a schematic of the HP80 power supply exist anywhere? I am working off of Tony Duell's HP35 and HP45 internals articles.

teenix · 10-24-2020, 09:36 PM

(10-24-2020 01:47 AM)andrewsiegel Wrote: Does a schematic of the HP80 power supply exist anywhere? I am working off of Tony Duell's HP35 and HP45 internals articles.

http://www.hpcc.org/cdroms/schematics5.0...c/hp80.pdf

or lots of redrawn schematics at

http://www.teenix.org/classSchm.zip

cheers

Tony

andrewsiegel · 10-24-2020, 10:30 PM

Fantastic, thanks!

andrewsiegel · 10-24-2020, 10:35 PM

Next question: do you think I can use a 1N914 or 1N4148 diode in place of the Vcc output diode, or in place of any diode on these boards for that matter? If not, then what would you suggest?

teenix · (This post was last modified: 10-25-2020 04:37 AM by teenix.)

(10-24-2020 10:35 PM)andrewsiegel Wrote: Next question: do you think I can use a 1N914 or 1N4148 diode in place of the Vcc output diode, or in place of any diode on these boards for that matter? If not, then what would you suggest?

1N34A was mentioned in Jarques Laporte's notes. Larger current capacity.

1N4150

cheers

Tony

andrewsiegel · 10-25-2020, 04:51 PM

Thanks!