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Vertical line on a Lear Siegler ADM-3A Terminal

alan8086

Experienced Member
Joined
Oct 1, 2016
Messages
325
Location
Burnley, UK
Hi All - I'm thinking of buying said terminal. It appears functional - displays text on the screen etc. However - there is a vertical green line to the left of the text from top to bottom. I have some experience of repairing TVs from the 1970's/80's at a hobbyist level, as a teenager - its been a while though.

What would cause one green line on an otherwise flawless raster? At first I thought rather than the CRT geometry circuitry, could it be perhaps the VDU IC/circuitry or perhaps a RAM issue if the screen layout is bitmapped or something?

Having since read some of the service manual - the screen is 'drawn' per horizontal sweep according to the layout of the character map currently in use. As the vertical line is to the left of the normal block of 24x80 characters, perhaps it is more of a CRT control geometry issue - rather than a logic issue on the motherboard?

I have an IMS 8000 S100 box, bought off its original owner about 12 years ago. He had a Lear Siegler ADM-3A terminal originally but that wasn't part of the sale. Its always seemed a shame to use HyperTerminal on an LCD screen with this venerable machine from 1980.

If anyone has any suggestions from the scant info I've provided -I'd appreciated it!

Thanks ;-)
 

alan8086

Experienced Member
Joined
Oct 1, 2016
Messages
325
Location
Burnley, UK
Forget it - the moment I post a question to the seller (or maybe post a question here) the listing is taken down :-(
 

Hugo Holden

Veteran Member
Joined
Dec 23, 2015
Messages
2,622
Location
Australia
Most of the time with a CRT monitor, it is usually fairly fairly easy to deduce where the problem is looking at the screen (though there are a few exceptions). Generally speaking if there is a change in the intensity of the beam, it will correspond to a change in the CRT's beam current, which is set by the relative potential difference between the CRT Gun's grid an cathode electrodes. And in this case the disturbance would be said to be in the "video" or part of the signal source coming say in this case from the digital electronics in the terminal.

The other way the beam intensity can get modulated is by the scanning velocity of the beam, if it slows down, or tracks back over the scanned area nearby, more energy is delivered to the phosphor over that time. So if there is an oscillation in the yoke's scanning current, typically seen after flyback in the H scanning, it can cause bright vertical bars on the left side of the picture. Or perhaps other decaying oscillations. In this case the fault is said to be in the "Raster" and is independent of the video signal content or video signal.

Also, some brightening of the image in parts can be caused by the beam tracking back over the scanned area during beam flyback. In this time frame, the CRT's gun is supposed to be turned off. This is achieved by applying a relative negative voltage to the CRT's grid, with respect to its cathode, which repels electrons back to the cathode. If the V blanking is missing, the defect appears as tilted horizontal lines scattered over the raster. If H blanking is missing the effects are variable and often appear diffusely and generally more on the left than the right of the raster.

So the first move in deciding where a display defect is , is to work out is the defect in the scanning raster, or is it in part of the video signal.

Without looking or a photo of it at least, it would be impossible to guess.

I have seen those ADM-3A terminals before, they are very cool looking and appear to capture the magic of the 1970's.
 
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alan8086

Experienced Member
Joined
Oct 1, 2016
Messages
325
Location
Burnley, UK
That Hugo, was a very comprehensive reply. Some weeks later - I thank you!
I did go on to purchase a non functional unit off the same seller. Its likely I could have fixed it. He had trouble finding a courier that could get it through the Brexit nonsense in to the Uk, so we agreed to cancel.

Probably a good thing in the end.
 
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