Search Posts:

Restoring a Lear Siegler ADM-3A


Return to Threads

  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.



Buy a Commodore Computer Poster

Popular Topics and FAQs

  • Commodore B Series Tips and Tricks
  • Aerocomp TRS 80 M 1 Expansion Unit DDC
  • Items Wanted
  • Lobo Max 80
  • Zenith Z-19-CN
  • Prototype PET 2001 photo
  • Using Toggle Switches to Analyze Memory
  • Commodore Disk Archive Project
  • PET 2001 Prototype at Gametronics 1977
  • Jim Butterfield Photo
  • IMSAI 8080 With Processor Tech. Cutter
  • Secrecy is the keystone of all tyranny
  • Cromemco System Three
  • Northstar Horizon - Boot Problem
  • Computer History and Restoration Links
  • Commodore BX-256-80 - 8088 Co-processor
  • S-100 board testing with Z-80 ICE
  • Donner 3500 - an early portable computer
  • Digital (DEC) PDP 11/05 NC Assembly
  • Univac 1219 rescue
  • Fido BBS listing node list 6-13-1986
  • PDP 8e
  • MITS 88-2 SIO (2SIO) for BASIC
  • Visual Technology Inc Model 1050
  • Amiga 2500 Restoration
  • The Evolution Of IBM Computers
  • Replacement teletype print hammer head
  • Archiving and Copying Software 101
  • Computers Built 1940 - 1950
  • CBM B-520 (a.k.a B256-80 or B500 256)
  • RCA COSMAC Microkit
  • Commodore 64K C-116 Mods
  • MITS 8800b Turnmon 9600 baud
  • Catweasel, 8in and 5 1/4
  • Raspberry Pi as Gateway to Internet
  • Digital PDP11 late 1969 early 1970
  • PDP 11/40 72 inch cabinet model
  • PDP 11/40 Industrial 11 model
  • Digitial MicroVAX 3100 30 System
  • Digital VAX 4000-200
  • Commodore 64 / 1541 DRIVEKNOCK
  • Booting the System Using RL02 drive
  • PACS: Reflections by Kathleen Mauchly
  • Tele-Graphic Computer Systems Inc.
  • Commodore B Series SID Jukebox?
  • Installing Core into PDP 11/40
  • Setting Up OpenVMS 7.1 DNS CLERK
  • Felt-Tarrant Comptometer Model J
  • NextStation Color
  • Digital Rainbow (PC100-B2)
  • 1970 Compusad Compulogical Tutor
  • Archiving Papertapes Using DSI NC 2400
  • 1976 P.C.C. Features the MAI JOLT 6502
  • 1961 Beckman DEXTIR Computer
  • UNIVAC 1 and UNIVAC File Computer 1
  • Past Issues:

    Cinematographer Colossus 1980

    This image was selected at random from the archive. Click image for more photos and files from this set.