Lear ADM-3A terminal repairs - RAM

This Web page last updated date April 1 2013.

Introduction

This is a Web page on RAM repairs of an ADM-3A terminal. This is part of other vintage computer repair and restoration work I've done. I also offer some of these terminals for sale on occasion, Check this terminal page for details.. - Herb Johnson

ADM3A with bad RAM

One of my ADM-3A terminals seemed to work, except some of the characters were either the wrong character, or blank areas of the screen had characters. Some of this occurred intermittantly. These sorts of errors are clearly products of character RAM. the ADM-3A stores characters for the screen 80 X 24 display, in two banks of 2102 RAMS. The RAMS banks, rows of 8 2101 1K by 1 RAMS, correspond to even and odd rows of the display; each RAM chip is one bit of the 8-bit ASCII character. Diagnosing these problems consists of typing various characters into RAM and looking for patterns of errors. I saw two patterns of errors, and this note describes diagnosis and repair.

First, some A's became I's, B to J, C to K... If you look at the ASCII codes for these, that corresponds to bit 3 of memory (bit 0 the least signifigant bit). As the problem was in only one row - that lead to a specfic 2102 postion in the schematic and on the board.

I removed the chip carefully, using a solder sucker to remove most of the solder, than "solder wick" to remove the rest.It's very important to avoid damaging the PC board, and to make sure there's no debris around the pins after desoldering. Also, it's important to remove the IC carefully - it's easy to pull up the PC board traces. It's an art to do this properly; I'd recommend that you CUT the chip out from its pins before desoldering, and remove each pin separately - that's the safest route. I did not do that, but I am experienced and I was very slow and careful. I decided to socket the replacement chip.

Look at the photo in the next section for the results on the display. None of the B's are shown as K's. ALso note in the above board images, the original RAMS are 2102-4, I presume 400ns. The replacment is also a -4; to use a slower chip risks intermittant or heat-related failures.

Second, intermittantly some B's became C's, or some blank areas had characters If you look at the ASCII codes for these, that corresponds to bit 0 of memory. By probing the RAMs, I found that touching pin 12 (data out) of the affected RAM caused the added bit to come and go. I could not decide if one bad RAM was causing the other RAM to be intermittant. So i desoldred BOTH pin 12's sufficiently, to see which one when disconnected ended the problem. In this way I decided which RAM to remove.

When I replaced the RAM by simply inserting the chip into the unsoldered board, the problem went away. So I soldered the replacment in place and the terminal displayed the "proper" characters. IN this photo, all the B's are shown as B's not K's, and there's no occasional "!" characters elsewhere.

- Herb Johnson

Copyright © 2013 Herb Johnson