TRS-80 Model 12 crt substitution

From: jpero_at_sympatico.ca <(jpero_at_sympatico.ca)>
Date: Thu Mar 4 15:06:24 2004

> > > 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'.
> >
> > Ah, I didn't know that - I thought they discharged naturally.....
>
> Rememebr that the dielectric (insulator) in this capacitor is the glass
> of the CRT itself. And that glass is a darn good insulator. I suspect
> that most of the leakage occurs via dirt on the surface of the CRT
> (forming a conducting path between the anode connector and the outer
> coating), via atmospheric moisture, etc.
>
> -tony

I can confirm that glass is very good insulator too. Mere dirt even
greasy smear even a incomplete air-seal (HV cup) is enough for HV to
escape with a (SNAP!, crackling or hissssing). Even a tight pinhole
in rubber/silicon/plastic insulations that can hold back water
didn't stop HV's dramatic exit. That one I had one, had to find it
with HV probe waving around each HV cup (three in a set for projector
and that particular set has secondary HV hat loosely placed over each
HV cup and kept in place by wedge force) till I see some deflection
from that analog meter. The pinhole wasn't easy to see, had to take
it apart and finally found the burn spot on the metal HV clip that
matched a microsopic pinhole in the rubber cup.

Oh, as other have said about the "power" of the HV it is very
powerful enough to "eat" through if insulations has flaws in it. I
see this often in flyback transformers & botched HV
splitter installions. One time I seen "tufts" of impossibly
fine magnet wires for HV stuck out of crack encircling flyback's
round body where flyback transformer housing split open with a BANG
according to owner whom heard the noise.

For this reason, manufacture of HV stuff had to be super clean and
quality stuff, HV resistors, caps, diodes, very carefully
manufacturing steps, vacuum vessel to "boil" potted units prior to
curing. That's the reason for not filling potting stuff to the brim.
 It's below the rim of housings about 1cm depth. Deduced from my
reasonings.

33 to 35KV is "asolute" max for consumer stuff including projectors.
Anybody care to raise reason behind this HV "wall"? Not just for
x-ray reasons.

I use is I think 180 Ohm 20W sandbar resistor for both HV and main
filter caps as discharger. Much gentler on stuff and sanity.

Cheers,

Wizard
Received on Thu Mar 04 2004 - 15:06:24 GMT