TRS-80 Model 12 crt substitution

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Mar 3 18:19:26 2004

> If the M12 is anything like the M2 it just uses a standard 12" B&W TV tube.
> When my model 2 was 'handled' by airport baggage crews they smashed the
> screen to bits, so I replaced it with the screen from a standard old B&W TV.

FWIW, the cheapest way to get such a CRT is to buy a cheap portable TV --
even a new one -- and raid the CRT from it.


> If that's the case remember to change only the tube itself, ie leave ALL the
> wiring behind - you must loosen the holding screw(s) on the yoke (the mass
> of copper wiring) and fit it to the 'new' tube. Remember not to tighten too
> tightly!

The old method for this is 'tighten until the CRT implodes, then back off
quarter of a turn' :-). [Note : If you don't realise this is a joke, then
you shouldn't be attempting a CRT replacement, OK!!!]

>
> I'm assuming you also know that if you power up the replacement screen and
> it doesn't work you must either discharge the tube with a suitable EHT meter
> on the connection to the flyback transformer (very thick wire going to the
> back of the screen rather than to the neck) or leave it overnight to
> discharge itself before swapping in another one.

Unless there's a bleeder resistor in the EHT supply -- and AFAIK there
isn't in the M4, so there probably isn't in the M12 either -- the CRT can
hold a charge for a lot longer than 'overnight'.

The charge is not generally enough to be lethal, but it's a nasty shock,
and it may cause you to drop the CRT. I alwayse discharge a CRT with my
EHT probe before handling it.

Don't discharge it by shorting the anode connector to the chassis -- not
even to the CRT earth (the spring contact onto the outer aquadag
coating). The resulting spike can take out chips all over the machine.
You have been warned!

-tony
Received on Wed Mar 03 2004 - 18:19:26 GMT