Perq 2T1 display fault

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Feb 19 15:50:57 2004

>
> On Sat, 2004-02-07 at 23:35, Jules Richardson wrote:
>
> > Oh, and if anyone's dishing out Perq wisdom there's also a Perq 1 with
> > a
> > display fault too - on that one the image is centered on the screen,
> > but
> > squashed into about 80% of the normal width, and maybe 10% of normal
> > height. It's also very non-rectangular in nature.
>
> Chatted with my good buddy who still makes a living repairing
> terminals. His guess is that the horizontal yoke isolation cap is
> leaking/bad. This will definitely make the horizontal squoosh. Many
> terminals derive the drive voltage for the vertical from the horizontal
> flyback - thereby causing the problem you are seeing.

Ve _very_ careful. The VMI monitor used on the PERQ 1 is unconventional
in a lot of respects (I have an idea that the EHT doesn't come from the
horizontal output stage, but from a second transistor/transformer circuit
also driven from the horizontal drive signal). I will have to dig out the
schematics sometime (yes I have the full manuals...)

I would start, though, by checking the 55V line from the PERQ's PSU to
the monitor. If that's low or noisy, you'll have all sorts of problems...

>
> His recommendations:
> 1. look for an oversized cap (electrolytic - yup one of those
> beasties...) near the horizontal drive transistor (generally
> heatsinked) that has a rating between 35 and 60 volts and a value

Eh? The coupling capacitor is normally a lot higher voltage than that
(600V and above in my experiece!). It's also likely to be a non-polarised
electrolytic (this is similar to having 2 normal electrolytics 'back to
back'), and no, those can't easily be re-formed.

> between 2.2 and 20 microfarads. This is the isolation cap. If it is in
> the 2.2 to 4.7 volt range replace it with a film cap - you will never
> have a problem again. You can't reform this one since they put AC
> (albeit mostly DC) through the cap and once it goes it's gone.

-tony
Received on Thu Feb 19 2004 - 15:50:57 GMT