7 May
2005
7 May
'05
3:23 p.m.
The first step was to check for any damaged components
or traces, but
none were apparent, so I just gave it a go. Before the variac police
come and get me, it was powered up just before I received it.
I'd be more worried about damage to the rest of the machine (avoided by
testing the PSU on dummy load before connecting the logic) than damage to
the PSU (possibly avoided by running it up slowly on a Variac).
I've never found a Variac that useful. Light bulbs in series as a current
limiter (when fixing SMPSUs), sure. But I am not sure how much good
running up the voltage slowly does (and with some SMPSUs it seems it
could do some damage!)
Miraculously, it simply just worked. I imagine the
smoke that the
original owner experience was due to dust on the CRT burning off. I
have no other explanation.
Anyway, the disk drive wouldn't read anything. I tried to INItialize a
blank disk but that failed too. Lacking a strobe or even fluorescent
lights, I couldn't tell from the tach disk on the drive if the speed was
off or not, but it seemed like a plausible cause for all the disks to be
'Scope or frequency counter on the index testpoint?
dead (I should mention I scoped the read logic and
it was detecting
It's one possibilty. It could also have been a lot of other things, drive
or controller related. Personally, I'd have done a lot more tests before
twiddling anything (but then again, I once spent an afternoon figuring
out why a CBM 8250 was ubreliable on one drive, only to find the cause
was dirty heads...)
transitions). I finally just twiddled with the speed
pot to find that
the speed was way off. Although incandescents have a lot of glow during
the power line's zero crossing, near the right speed I could see the
strobe pattern well enough. The real reason I couldn't see the pattern
was that the speed had been so far off.
Did it look like it'd been twiddled before? If not, then I wonder if some
other component (capacitor?) is failing, and the speed will therefore
drift again.
After adjusting the speed I was able to read most of the disks, although
sometimes with retries. Fortunately, all the disks that ISC put out for
the compucolor recorded all the programs on both sides (it was a single
sided drive) so even with hard sector failures I was able to get everything.
Now the main problem is pincushioning and color convergence. I did some
simple adjustments to improve color convergence, but without doing
something much more involved, it isn't possible to get all regions to
converge at the same time. For now I'll just live with the problem.
Assuming this is an in-line gun CRT, you normally use the ring magnets on
the back of the yoke to do the static (centre) convergence, then
tilt/wedge the yoke to get the edges right. But sort out that
pincushioning -- which could well be a component failure in the raster
correction circuit -- first.
Most colour monitors, at least over here, followed pretty standard
designs at that time. If the monitor was built by a consumer electronics
compeany (NEC, Hitachi, Zenith, etc), look at the service manuals for
contemporary TVs from the same manufacturer.
-tony