|Re: My HP 55 died|
Message #8 Posted by Viktor Toth on 18 May 2001, 6:47 a.m.,
in response to message #7 by Walcott Hamilton
A single, unusually bright digit and a dead keyboard together are clear indicators that the display and keyboard are not scanned. Possible reasons include a dead main oscillator, power supply problems, dead CPU, even possibly (but not probably) a dead cathode driver. This is usually when I push the ON button on my oscilloscope...
If the digit appears with normal brightness, the problem is probably something more subtle, since obviously, the display is then still scanned properly, so the clock frequency is present. In other words, the problem is likely a faulty CPU or ROM chip, neither of which is good news.
You were already advised to check for bad/dirty contacts; another routine thing I do is to check for faulty semiconductor junctions using the diode setting on a DMM. (This measures the 'forward bias': basically, it lets a small current flow through the junction in the 'open' direction, and measures the voltage drop, typically around 0.7V for a silicone junction.) Most diodes and transistor junctions can be checked in-circuit this way, and if there's, say, an open diode (measures as infinity) or a shorted pn junction (measures as 0 or something very close to 0) in a transistor, they can be detected. If you get lucky, you might find that the problem is only an easy-to-replace diode that has gone bad.