DEC Terminal VT420 not too bright

[Alegend]

My VT420 suffered from the same brightness issue, so today I decided to open it up and see if I can make it brighter, running the risk of all the plastics crumbling to bits.

For whatever reason, the VT420's plastics are stronger than my newer, half-crumbled VT520. So cracking it open did not result in any damage.

Attached are photos showing the "Before", identifying the SCREEN pot, and the "After" brightness.
It only took a maybe 1/8 clockwise turn of the SCREEN potentiometer and that's it. Anything more and the retrace lines become visible. All photos are taken with the brightness control knob adjusted to maximum.
VT420 Before.jpg20201203_122353.jpgSCREEN pot closeup.jpgVT420 After.jpg
As stated before, the photos make the screen look much brighter than it really is.

It is an easy adjustment, at the acceptable cost of somewhat reduced contrast. Further adjustments of the contrast are likely possible but that will wait for another day.

-Alon.

[daver2]

Excellent news. I know what I will be doing next...

Alon, if my screen started off as good as yours did before the adjustment I would be happy !

Thanks for the words of wisdom Frank. It wasn't a stupid question after all...

Mind you, it also helped to find the schematics and maintenance manual on the web as well. I had not found the actual schematics themselves until the other day...

This thread should really help a lot of people with the same problem as it seems to be a common issue.

Dave

[DECtechGuy]

Definitely going to tinker with mine this way as well.

BUT! Wouldn't this shorten the lifespan of the tube someway? Don't have a lot of knowledge about CRTs.

[Alegend]

Definitely going to tinker with mine this way as well.

BUT! Wouldn't this shorten the lifespan of the tube someway? Don't have a lot of knowledge about CRTs.

The failure mode you need to worry about in this case is the phosphor coating wearing down.

I don't think other failure modes are affected much.

But - if my terminal becomes usable again, and I don't use it 24/7, who cares? As long as it's working, I'm happy. And whoever inherits my collection will need to worry about the consequences in the future.

Re-coating of the phosphor material layer is something that can be done, perhaps can still be done today. But probably for the cost of an arm and a leg.

-Alon.

[Hugo Holden]

There are some general principles that apply to all VDU CRT's.

The maximum useable beam current and display brightness occurs when the cathode and grid voltages are equal. If the grid is driven, by the video signal content (call it the peak white, or peak amplitude of the video signal) more positive than the cathode, it causes the grid to draw current, then the form of the beam changes and the focus suddenly degrades. In addition if the CRT bias is pushed this way, then the H and V blanking pulses (positive going if introduced at the cathode and negative going if introduced at the Grid) may not have enough amplitude to blank the beam, so retrace lines will appear.

Most color CRT's have a screen and screen control, it's less common on monochrome VDU's.

The contrast control alters the amplitude of the video drive to the CRT and the brightness control the DC axis of that signal.

So when both are set correctly and for the max useable brightness of the phosphor on the CRT face, the peak white of the video signal is just approaching the zero grid to cathode voltage condition, and the black part of the video corresponds to the CRT beam just extinguished, and the sync pulse tips (present in composite video at least) drive the beam "blacker than Black" so to speak.

It is fairly easy to tell if a CRT is down on emission. If the combination of brightness (and sub brightness) and contrast ( and sub contrast controls) are set for max phosphor brightness, just before the beam significantly defocusses, indicating that the zero grid to cathode voltage condition exists, then if it is a dim image, the CRT is likely low on emission or has a very degraded phosphor, or is gassy or all of the above. This assumes the EHT value is normal,heater temperature normal and if there is a screen pot, that has been correctly set.

If there is any doubt, a scope can be used to check if the CRT's cathode is being driven more negative than the grid (or the grid being driven more positive than the cathode) which ideally should never happen to avoid grid current and severe defocus.

On the topic of re-phosphor of a CRT, it is extremely difficult. The phosphor is placed in a liquid suspension, and allowed to settle out , the liquid is decanted and it is dried, a binding agent , a silicate is used to help it adhere,then aluminium is evaporated on it. I tried it once with a 12LP4 CRT and failed hopelessly, it requires automated machinery. The ETF are building a CRT plant. But re-gunning and re-phosphoring CRT's is difficult. It worked in large scale industry because of the automation and controls, much like manufacturing semiconductors, very hard in a backyard operation.

[Alegend]

There are some general principles that apply to all VDU CRT's.

The maximum useable beam current and display brightness occurs when the cathode and grid voltages are equal. If the grid is driven, by the video signal content (call it the peak white, or peak amplitude of the video signal) more positive than the cathode, it causes the grid to draw current, then the form of the beam changes and the focus suddenly degrades. In addition if the CRT bias is pushed this way, then the H and V blanking pulses (positive going if introduced at the cathode and negative going if introduced at the Grid) may not have enough amplitude to blank the beam, so retrace lines will appear.

Most color CRT's have a screen and screen control, it's less common on monochrome VDU's.

The contrast control alters the amplitude of the video drive to the CRT and the brightness control the DC axis of that signal.

So when both are set correctly and for the max useable brightness of the phosphor on the CRT face, the peak white of the video signal is just approaching the zero grid to cathode voltage condition, and the black part of the video corresponds to the CRT beam just extinguished, and the sync pulse tips (present in composite video at least) drive the beam "blacker than Black" so to speak.

It is fairly easy to tell if a CRT is down on emission. If the combination of brightness (and sub brightness) and contrast ( and sub contrast controls) are set for max phosphor brightness, just before the beam significantly defocusses, indicating that the zero grid to cathode voltage condition exists, then if it is a dim image, the CRT is likely low on emission or has a very degraded phosphor, or is gassy or all of the above. This assumes the EHT value is normal,heater temperature normal and if there is a screen pot, that has been correctly set.

If there is any doubt, a scope can be used to check if the CRT's cathode is being driven more negative than the grid (or the grid being driven more positive than the cathode) which ideally should never happen to avoid grid current and severe defocus.

On the topic of re-phosphor of a CRT, it is extremely difficult. The phosphor is placed in a liquid suspension, and allowed to settle out , the liquid is decanted and it is dried, a binding agent , a silicate is used to help it adhere,then aluminium is evaporated on it. I tried it once with a 12LP4 CRT and failed hopelessly, it requires automated machinery. The ETF are building a CRT plant. But re-gunning and re-phosphoring CRT's is difficult. It worked in large scale industry because of the automation and controls, much like manufacturing semiconductors, very hard in a backyard operation.

Thanks for the detailed explanation!

[daver2]

Thanks Hugo.

A very clear and informative post.

Dave