Lear ADM-3A repair

[chuckcmagee]

See attached JPG for example of what my screen looks like. I dug up a lower-case ROM and stuck it in there, the lower case letters show up PERFECT. The diagonal lines and those half intensity vertical bars are still there but you can plainly see the lower case letters. The upper case stuff is another story - they all end up being extended ASCII "form" drawing graphics, you know, those bar graphic dealie-bobs that you can use to draw boxes on forms (all the rage in 79).

Anyway, I have another 3A with a different set of problems, I can unsolder the Upper Case ROM from the "good" one and put it in this one. Fair amount of work to do that. Before I do that, DO YOU THINK IT WILL FIX MY CHAR. DISPLAY PROBLEM??? I would hate to go through all that and get no change when I'm done.

 

[nige the hippy]

Hi chuck,
Had a look through the circuit diagrams, inclined to think it may be a retrace blanking problem, as there doesn't appear to be any way to get half-intensity video. rather too close to sleep tonight to be really useful, but I'd be inclined to try to get the other beastie going with a 1489 line receiver. they pop occasionally especially when people plug in stuff that's not earthed too well, a little spark jumps across....

Would be nice to see a really hi res picture, or a bit of detail on the text & blocks of the one with the vid fault.

I'll try and be more useful tomorrow.

 

[chuckcmagee]

I will look at the other 3A some more. At this point, it might be easier to fix the "receive not working" than my yummy checker-board video. And yes, with my current tools and great chip handling abilities, I do think I will leave the good Upper Case Rom just where it is (in the one with the receive problem). I was hoping for one of the 2 to be repairable by me. I didn't feel like back-tracking the receive signal from the RS232C connector but that does sound easier to fix now. Thanks for the info. I had some docs on the 3A but figured out the docs were for a much later model, the one with no more dip switches. Both of mine have my fave dip switch hole on the left by the keyboard.

 

[chuckcmagee]

I played around with the Upper Case character ROM. I never could get all the background garbage to go away. Conclusion --- parts machine. The video board and CRT are in excellent shape. No "PRESS MENU" burned into the phosphor. Found a "gov surplus" Lear with some simple 2102 stuck bit problems. Now those I know how to diagnose and fix. Did it on a Lear my ex-boss gave me. Same symptoms as the one in 1977, blanks turn into a "!". Repeat after me "low order bit stuck on". I know just the correct 2102-1 to attack and now, with my new "parts machine", well.....