HP80: blank display, low Vcc

[teenix]

(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