[Coco] CM-8 Service Manual

[Gene Heskett]

On Monday 09 July 2012 19:13:25 tonym did opine:

> Haven't had luck finding someone who will look at it.
> The guy I used to use closed up, and moved to Texas.
> 
> Maybe a description might ring someone's bell as to what it is?
> I can see the screen, but it's trapezoidal - narrower on top, about 2"
> of black border on both sides on bottom, about 3" on both sides up top.
> Screen is legible, just wierdly shaped, and packed to the middle. Kinda
> reminds you of the old 80's driving video games, trying to do a
> perspective view of a road going into the distance.
> 
That is waddling and quacking like a duck called a bad capacitor.  The 
electrolytic caps have a plastic sleeve shrunk to the can they are 
assembled in in, and the top of the can is usually creased to make weak 
places in the alu can.  If they get too old, they start suffering from ESR, 
Equivalent Series Resistance, which is an actual bad connection between the 
solder lug on the bottom, and the edge of the alu foil inside it.  So any 
circulating current makes heat and eventually it opens the connection.  
This heat will create pressure in the can, enough to make the top of the 
can start to bulge.

We have a measuring instrument called a Capacitor Wizard to check ESR with, 
but it sells for about $175, worth every penny to a busy shop but not for 
fixing a onesie.  So, check every electrolytic in it for signs of a bulged 
top and replace those that show the magic sign.  If you're good with a hot 
soldering iron, you just might fix it.

> Roy is backed up with the VGA converters, and I can't get anyone to fix
> it, and the 80col screens aren't really legible, especially as my eyes
> aren't as good as they were 25Y ago :) So, I got this nice, shiny, new
> SuperIDE and a Sardis FDC and a repaired J&M FDC... and I can't use it.

80 col screens on a cm8 always were pretty poor.  Back in the day, the 
other monitors were scads better.  But they have all died too, usually from 
a shorted scan transformer, which by the time they started failing, had 
been made out of pure unobtainium for 5 years already.
 
> Ideas on what the issue might be?
> 
> I have a referral, but they guy says he doesn't do monitors, because
> it's impossible to get parts. However, cracking this thing open last
> night, it's mostly discrete components, not a bunch of proprietary
> parts. Wonder if I shouldn't just swing by with it to see if he'll take
> a crack at it if it's not proprietary stuff?

That is why the adapters like Roy's are now being used to drive the modern 
10x sharper color lcd computer monitors.  Put your name on Roy's waiting 
list.  Its worth it.
 
Cheers, Gene
-- 
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