DEC Terminal VT420 not too bright

[drg]

Hi,

I feel like my ,,new'' VT420 isn't too bright. I attach the picture, however on that picture it looks brighter than it really is. I don't have live comparison with other VT420, however it's a lot darker than IBM 5151. Also when one of brightness and contrast knobs are in the middle, the screen goes black - no glowing characters visible. Is it normal or not and if not, perhaps it can be fixed somehow?

Best regards,
Pawel

[DECtechGuy]

I've got a white VT420 and it's about the same. However, mine starts off fairly dim when I first turn it on and it gradually reaches a moderate brightness. I'd imagine that's just because it's, well, old.

They may have not been very bright terminals to begin with, but I have no personal experience with them when they were new. (would have liked to of course )

[daver2]

I have a few VT420s with the same issues. This problem appears to be fairly generic from what I read.

One of my VT420s is nice and bright.

One is very dim. You can just see it in ambient light with the brightness control turned fully up.

One appeared to be non functional (apart from the tell-tale power on beep) until I switched out the light and viewed it in the dark!

I have found what claims to be the VT420 schematics at:

http://vtda.org/docs/computing/DEC/T...g-drawings.pdf.

The circuit looks straight forwards and there appears to be a few capacitors to checkout.

Could do with a monitor expert to checkout the schematic and give us some words of wisdom...

Dave

[daver2]

It would be good to know what the voltage is across C626 (10uF 250V) is on a working monitor.

Places to look would be C626 itself, R661 (the brightness potentiometer itself), the fixed resistors on either side of the brightness potentiometer, C308 (1000 pF) and R314 (47 Ohms) on the little PCB on the rear of the tube.

The brightness control should adjust the negative voltage on G1 (grid 1) of the tube - green wire.

Dave

[daver2]

C308 could be C701 and R314 could be R702 depending upon the revision of VT420 you have.

Dave

[iz8dwf]

I've just seen the call daver2 put on another forum
I had a VT420 too many years ago, on mine the flyback transformer was cracked and leaked some brown substance (and I've never understood what that was). However at that time I had no idea where to find a replacement transformer and scrapped the whole terminal. I still have the keyboard however
I have an Olivetti WS-685 which is basically a VT-220 kind of clone. It's not super-bright too, but I've never investigated too much since it's perfectly usable.
I'll check the schematics of the VT420 and get back with some suggestions.
Frank

[iz8dwf]

Ok stupid question, but did you try the "screen" potentiometer (I think it's either a trimmer or more likely a small-ish insulated potentiometer inside the monitor or sticking out of the back, it's been a while since I had the 420 now).
It's labeled R657 and should have a rather big effect on overall brightness. Set the brightness control at middle of its range and adjust screen to completely extinguish retraces, assuming that they can get visible.
The schematic isn't labeled with all the voltages that are supposed to be coming from the various flyback rectified outputs, but if someone can measure them and report, we can understand if they are in the ballpark for a 12" CRT.
HTH
Frank

[bobaboba]

I think (of course as well as all the possibilities of failing components etc.) that if these terminals have seen a lot of use the loss of brightness could be down to a loss of emission, which is a problem common to all types of thermionic valve or tube. Again I'm not familiar with the construction of the tube in the 420 particularly but generally speaking over operating time cathode coatings deteriorate, their emission falls and eventually there isn't enough for the available acceleration voltages to give a bright image. If that's the problem here then there isn't really an amateur cure. TV tubes once used to be 'regunned' but I don't know of anywhere able to do that now. It's possible to improve the cathode emission by increasing the temperature (higher heater voltage) but I don't know if the circuit lends itself to that and ultimately of course the problem worsens. Once upon a time I used to work in a TV repair shop in my high school holidays and we often used to fit a separate small transformer just for the tube itself for people who hadn't the cash for a new TV warning them that it was a time-limited fix. Frank's post above is another approach to trying to get more out of the cathode but again, if it's due to cathode degradation, it's a downhill run.

Just an idea.

[daver2]

Hi Frank,

Simple answer is no. I have only just seen it on the schematic and found where it is on the PCB (http://www.vaxhaven.com/images/5/5d/EK-VT420-PS-002.pdf - coded blue). This manual also has the adjustment procedure.

Bob,

Agreed. I will assume the simple things in life first and try an adjustment.

I too worked in a TV repair shop when I was in school! I also remember the transformer modification...

Dave

[drg]

What an interesting thread! Thank you guys, monitor adjustment procedure from manual (4.2.3 and 4.2.4) worked like a charm for me! For the reference:

4.2.3 Screen Control
To adjust the screen control:
1. On the front of terminal, set the brightness control to maximum and the contrast control to minimum.
2. Adjust the screen control (R657) until the white background (raster) almost disappears, but is still visible.
3. Readjust the brightness control to the user’s preference

4.2.4 Subcontrast
To adjust the subcontrast control:
1. Display the alignment pattern of E’s (Section 4.2.2).
2. Set the contrast control on the front of the terminal to maximum.
3. Turn the brightness control on the right side of the terminal until the raster just disappears.
4. Adjust the SUB CONTRAST control (R647) to the maximum intensity, maintaining the focus of screen characters.
5. Readjust the contrast control to the user’s preference.

This time, the picture looks brighter than monitor really is


Cheers,
Pawel