HP-19C Repair Odysee

[Don Williams]

I believe your observations are correct. There is polarity protection in the PSU. However I made a rather brilliant move during this procedure. I kept flipping the boards over and over again trying to trace the connections. I finally got tired of reattaching the power supply clips which leaped off the battery connections every time. My solution was to take some small jumpers cables that would stay firmly attached to the power and ground pins on the IC's. So I dialed up +Vcc and -Vcc and avoided those pesky protection diodes. I did all the trouble shooting in that mode and every thing went well until I made that last fatal test. PMOS and CMOS chips can really smoke when you reverse bias the power connections.