ADM-3A question

Charles charlesmorris800 at
Sun Aug 11 08:58:27 CDT 2019

Thanks Bill, I hadn’t seen that particular page. As I mentioned already, the 
formerly clear “stuff” was so deteriorated I could just pull the glass plate 
off with gentle fingertip pressure.
I ran a bead of clear silicone around the outside of the clean plate and CRT 
face and bonded them back together.
Display looks great!

However, I can now see that every other line, starting with line 2, is 
showing a full line of double quotes (0x22) instead of spaces (0x20). I read 
the circuit description and schematic, and it appears that bit “2” is stuck 
high on the even-line RAM – for some reason the designer decided to call the 
LSB bit 1 instead of bit 0.

Typing (for example) “abcdef123” shows the correct text on the blank odd 
lines, but on the even lines it echoes as “cbcfef323”. Confirming that stuck 
Looks like the RAM at location H15 should be the bad one... we’re having a 
heat wave and it’s too hot upstairs to work on it until tomorrow morning at 
the earliest.

ETA: Now it's tomorrow morning and just cool enough (although 100% humidity, 
at least outside) but my replies aren't showing up in the archive - filtered 

Anyway. I did a bit more Googling and discovered that plain water dissolves 
the PVA goop just fine. No need to use a lot of expensive alcohol which 
seems to be a less effective solvent anyway!
So I took the board out and scrubbed it in the kitchen sink with running 
warm water and an old toothbrush. Rinse with distilled water, now gently 
baking in the oven at around 140F to get the water out of the keyboard. Then 
onto the RAM replacement.

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