compucolor

[Tony Duell]

> 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