ADM3 terminal repair

[RolexTM]

Good day folks,

here we will document our efforts to repair our ADM3 Terminal. I think it is beautiful and once repaired will live its life happily in the museum.

What we have done:
1) The reset circuit was faulty, one of the consumers of the CLEAR line permitted a reset.
2) something was not okay with the display circuit. After disassembling the monitor board, re-flowing some suspicious looking solder joints and giving all potentiometers some deoxit we have a picture:



What we see here are a bunch of '0' with occasional space in between.

Besides the probability that every display ram is bad.... I think what to look at to progress is clear

- Check if the initial write to RAM on reset/clear is triggered (Schematics page 4 D1). Maybe the RAM is garbage because there is no write.
- Check if the buffers from RAM -> character ROM are working (Schematics page 8)

I think that the machine is able to display at least 2 different characters (and one of them is blank that is supposed to appear) is a indicator.
The character generation itself works and the lines are correctly built beam by beam.

Does someone have a clue how the character ROM is organized? Is it ASCII?
Because if so, SPACE and '0' are only 1 flipped bit apart 0b00100000 vs 0b00110000

Regards
Alex

 

[RolexTM]

Mini Update -

Repaired one RAM chip, now the display shows all „0“. I think this is more repaired than before.

The master UART chip is dead, the cursor will not move and no local Echo takes place in half duplex mode.i replaced it and now the keyboard is responsive and I can input characters on the keyboard that show wrong characters. I guess it is the same flipped bit that make space to „0“ …. Will need to look into it.

 

[RolexTM]

Okay,

The schematics show a „fill“ switch that will trigger exactly said bit to fill the screen.

It is the small switch to the right on the pcb. After we turned that off the terminal is operating, at least the local echo for now.

I have not seen this switch mentioned anywhere, but well…

Next steps: unstick keyboard, someone spilled something sticky on it. Gave it a bath with hot soap and isopropyl after. Will see how that works out.

Test all characters on all positions, expect to find more dead rams

Test uart


Alex

 

[RolexTM]

The terminal is repaired. All keys work and all characters at all places, and the UART works.