Reviving VT220?

Tony Duell ard.p850ug1 at
Fri Apr 28 11:01:47 CDT 2017

On Fri, Apr 28, 2017 at 4:38 PM, Aaron Jackson via cctalk
<cctalk at> wrote:
> Yes we checked for the glow. It's very dim but it is there. I will bring

What do you mean by 'very dim'? It should be a distinct orange glow,
best seen looking
up the base of the CRT.

I normally start by checking the CRT pin voltages. There's nothing on
the pins of
a monochrome CRT that can't be measured with a normal multimeter. The sort of
voltages I would expect are :

pin 2 -- cathode -- 70V-100V
pin 1 or pin 5 -- control grid (the 2 pins are linked inside the CRT) -- perhaps
10V-20V less than the cathode

pin3,pin4 -- heater. One will be grounded (0V). The other will be around 11V-12V

pin6, pin7 -- anodes. I forget which is the first anode and which is the second
(focus) anode, but expect a few hundred V -- say 400V-800V on each of them.

The higher voltages (cathode, anodes) will come from the flyback transformer
in the horizontal output stage, so if they're all missing, suspect problems
there. If the heater voltage is low, suspect PSU problems.

I assume you have the printset (I think I got it from bitsavers). The monitor
section circuit looks very standard to me.


> one home this weekend and check some of the surrounding
> components. Hopefully it is something very simple to replace. I'll have
> a probe.
> Thanks!
> Aaron.
> Peter Coghlan via cctalk writes:
>>> Thanks for the information, Peter. It makes me want to try and test a
>>> few of the components around the flyback.
>> It would also be good to check the stuff like voltages on the base of the CRT
>> and that the CRT heater is glowing.  You could have a problem that is nothing
>> to do with the flyback.  (I'm assuming the brightness control is turned up).
>>> Thanks again,
>> You're welcome.
>> Regards,
>> Peter Coghlan.
>>> Aaron.

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