VT100 preventative maintenance?

tony duell ard at p850ug1.demon.co.uk
Sat Aug 22 14:14:44 CDT 2015

> Oooh, didn't know about the diodes. Are there any specific ones that you

Err, no... There is this witch-hunt against capacitors, but other components are
never considered

> think should be replaced no matter what? If not, are there any tests you
> would recommend?

There are several supplies derrived from the flyback transformer. Let's take as 
an example the Ball Brothers VT100 monitor on page 43 of the printset on bitsavers.

There is a -120V line, used for the control grid (g1), produced from the primary
side of the flyback by CR110 and C117. I think if this failed it wouldn't damage the
flyback, though

There is a +32V supply for the video amplifier, produced by CR114 and C121
A 400V supply for the the first and focus anodes (OK, g2 and g3 across
the Pond [1]) -- CR113 and C120. If either of those play up in can cause the
flyback to overheat, 

The 11.5kV final anode voltage is produced by CR1 and smoothed by the
capacitance of the CRT. I think CR1 is integral with the flyback transformer

[1] In the UK we call all CRT elecrodes after the control grid (first grid) 'anodes' which
makes sense as they are run +ve wrt the cathode. In the states, everything between the
cathode and final anode (the aquadag coating on the CRT flare running at 10kV or
more) is a 'grid'. Now something has just struck me. The adjusment for the 
first anode, if present (it isn't in this VT100) is known as the 'screen' control. What I 
do not know is if it's called that because it controls the overall brightness of the CRT
screen, or because it sets the voltage on what is called G2 across the Pond, and which
 would be the screen grid of a tetrode or pentode. Does anyone know the reason?


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