TRS-80 Model 12 crt substitution

From: Tony Duell <>
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!

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

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