13 Feb
2006
13 Feb
'06
7:58 a.m.
I'm trying to restore an Osborne Executive, but there seems to be
something wrong in the CRT circuitry. The machine powers up and beeps,
but I see nothing on the CRT. Fiddling with the CRT pots does nothing.
I hear none of the expected noises from the CRT circuitry, nor do I see or
smell anything burnt. Can someone here point me in the right direction?
Do you have schematics? I don't, I've never seen one of these, so the
below is the sort of tests I'd do on any small, conventional, mono monitor.
My first thing would be to look at the CRT heater. If it's not glowing,
then either it is open circuit (:-() or its power supply has failed.
Yiu can test the former by carfully unplugging the CRT base connector and
checking the resistance between the heater pins (3 and 4 for a 7 pin CRT,
1 and 8 for an 8 pin one, at least 99% of the time).
If the heater is OK, the supply is missing. It will either come from one
of the main PSU outputs (in which case, check for things like a fuse on
the monitor mainboard), or from the horizontal output stage.
The next thing I would do is to check the CRT voltages, including the EHT
(I have an EHT meter :-)). This may point to the fact that the horizontal
output stage isn't doing a lot -- maybe the transistor is open-circuit,
maybe it's not geting any drive, and so on.
-tony