Reviving VT220?

[Aaron Jackson]

The heater and control pins appear to be giving sensible voltages. The
logic of the board is fine, I can type on the terminals keyboard and I
get the correct characters on the other end of the serial cable. The PSU
is putting out 31v which seems fine?

I'm used to discharging the tube before fixing things inside CRTs
(usually I only attempt to fix simple things like a broken toggle
switch), but I have not managed to get a spark off this monitor. The
tube doesn't seem to get charged up at all. I've measured the
capacitance of nearly all caps and they seem fine, diodes seem to be
working fine. Does this mean it is most likely the flyback transformer?
Are there any other bits I should be wary of and test properly?

Thanks again for your help. Your voltage listing and advice in general
has been very useful.

Aaron.


Tony Duell writes:

> On Fri, Apr 28, 2017 at 4:38 PM, Aaron Jackson via cctalk
> <cctalk at classiccmp.org> 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
>
>> 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.
>>>>