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Restoring a Lear Siegler ADM-3A


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  Restoring a Lear Siegler ADM-3A by Bill Degnan - 06/13/2016 14:53
Ian Primus clued me in about the potentiometer just behind the yoke of the ADM-3A display CRT. These are used as a secondary control used to adjust the brightness. The tube display is weak and needed more voltage to the tube. Click image for larger view.

Basically this ADM-3A needs some help. I am not sure if this is the same ADM-3 I wrote about in 2008, or if I gave that one to VCFed for their museum. At one time I had three. This is the only ADM-3A I have now, so I decided to bring it to the VCFed Workshop this past weekend, June 12th (13th).

Two problems. First the terminal logic is sending zero's to the video memory (2102's), and the second problem is with the column counter. Note how the zero's "wrap" past the 80th column. Note there is a bit of screen rot, but it's not too bad yet. Click image for larger view, and here is a second pic of the screen that illustrates the column overlap.

Here is the basic data path to test.
74LS157 --> 2102 RAM --> 74LS175 --> 74LS166 --> character ROMs --> signals sent to screen hardware.

Logic probe showed signals going to the display:

L6 and K6 74LS157 - OK

2102 RAM testing - moving around no effect. Removing entirely screen showed ?? characters (correctly)

L13 and K13 74LS175 - OK

All LS166's OK

I went ahead and tested all the TTL remaining TTL chips and found there were three bad chips on the motherboard.

K9 74293 (up counter)
J2 7442 (BCD to DECIMAL)
B11 74293 (up counter)

There are three 74293's, one was still good. The rest of the chips were at least good enough to pass my tester's test. Hopefully replacing these chips will restore service to the unit, but further study is necessary. At this point I can't say there is anything wrong with the RAM chips, the symptom of all zeros does not point to a RAM problem. I also have no way yet to test the UART (TR1602B). Hopefully it's OK.

New pics (dated 6/13/16)

Oh, and I found the keyboard option switch cover...nice.



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