IBM 5151 (MDA display) - common faults / gotchas?
ard at p850ug1.demon.co.uk
Fri Nov 7 23:49:28 CST 2014
> Of course working condition is unknown, so before I commit to it, are there
> any common showstoppers with these displays (failed CRT, yoke windings,
> LOPT etc.) - or are whatever faults that may be typical with them all in
> components that are still available?
It's a very simple circuit, the schematic is in the TechRef. One odd feature is that
there is no horizontal oscillator, it just drives the horizontal section from
the pulses on the Hsync (more correctly HDrive) input. As for failures, the CRT
is a common 7 pin one (although finding a spare with the long persistance
phosphor might be interesting), I have never had one fail, though. Screen
burn can be a problem (particularly if the old owner used it for spreadsheets
a lot :-)). I've never had a yoke fail. At one time the LOPT was available
from 3rd party suppliers, no idea if you can still find one.
Biggest Gotcha is getting inside. You can end up dropping the mains transformer
(linear PSU) on the back of the CRT if you do it wrongly!. Take out the 2 square caps on top
and the screws under them. Put the monitor face down, free the cables from the back case
and take out the 2 screws on the flange that obviously join the case parts. On the bottom
are 6 further screws, often covered by paper circles (!). Take out the 4 on the back case part
only. These hold the chassis in place, with the heavy transformer on it, you do not want it
totally free. Now lift off the back case.
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