2 Jun
2013
2 Jun
'13
11:14 a.m.
Has anyone managed to create schematics for the display and power supply for the
HP-85A/B? I don't know yet if Tony's schematics for the 87 are applicable or
possibly close enough for these boards.
I have both an 85A and 85B with identical display issues, which I
expect are due to old capacitors causing poor voltage regulation and/or
ripple. The pincushion test looks pretty good, except for a bit of
vertical displacement (think low amplitude waves) on the horizontal
lines. The second issue is when a lot of white is being displayed (i.e.
reverse video), there is a distinct narrowing in the scan line width in
the top inch or so, with the narrowest at the first scan line. Then
there is a gradual widening towards the bottom, with the lowest scan
lines ending up off the sides of the phosphor.
That does sound like poor PSU regulation, dried up capacitors being a
very likely cause.
I didn;'t reverse-engineer the 85 because the service manual is already
avaialble. It's called the 'assembly level' service manual, but it does
include full scheamtics (but no step-by-step comonnet level
troubelshooting). I know it's o nthe MoHPC disk (DVD-ROM now, I think,
which is why I hav en't updated...), it may be on the Australian museum site.
-tony