[Coco] Screen roll on model 1

[Gene Heskett]

On Monday 24 September 2018 20:27:19 rietveld rietveld wrote:

> Hi. Not coco but trs80 anyway
>
> I am getting a constant vertical screen roll on my model1's original
> monitor. Thinking it was a problem with the monitor I just built one
> of Mav's composite cables. I am still getting the same vertical roll
> on an old Tandy vm2 as well as a newer flatscreen monitor using the
> comp in
>
> It seems that the model 1 is the problem.    What should I be looking
> at?
>
> Sent from my BlackBerry


With a scope, check the vertical synch pulse. As thats a lower frequency 
pulse, it would be highly subject to getting lost in old capacitors with 
high ESR, so they would need to be replaced. There is simply no way that 
a sub 50 uf cap rated at 6.3 or even as high as 25 volts, is still 
surviving in working condition after 30 years. That 50 uf, or even a 220 
uf low voltage cap will likely measure less than .1 uf today, and 
correspondingly lower, down to perhaps .01 uf for one marked as 5 uf.

One of the hazards of trying to keep 30+ yo digital circuitry working.
An oscilloscope is probably the best tool since ESR will cause a loss of 
amplitude, but the loss of capacitance is much more important in this 
sort of circuitry.

DVM's with capacitance scales are generally worthless for "part in 
circuit" tests, the scope is king because it shows you the distortions. 
All you need to is recognize the distortions.

You'll need soldering and desoldering tools and decent rosin cored 
solder.

-- 
Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>