Reviving VT220?

Aaron Jackson aaron at
Fri Apr 28 11:41:51 CDT 2017

Hi Tony,

Thanks for the information, that is all very useful! I think I will open
up my VT420 which is working nicely to compare the glow in the base
followed by checking the voltages, as you describe.

Thanks again!


Tony Duell writes:

> 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.
> -tony

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