Teletype 56d battery replacement

I got a teletype model 56d from last VCF Midwest for free.  It was just labeled ‘amber CRT’ so I figured I’d take a crack at fixing it/replacing the missing keyboard.  I have found no real information on it, no manual, no schematics, no rom dumps.  There is a small entry on the terminals wiki, but it contains very little info.  There’s an instance of someone repairing the power supply for one, and someone else trying to get it to play nice with Sun hardware, but that’s all I find.  I decided to tear into it to at least see what’s up.

Yes, I added the tape over those windows

I feel like I am super justified in being angry about windowed UV eproms not being covered in a production device.  I dumped all those roms (link forthcoming) and noticed that this runs on a z80 so I have some hope of reverse engineering the code.  There’s also a custom teletype branded chip so maybe not… Among the other stuff I found was one too many rs232 transceiver chips meaning that the keyboard probably uses that as well.  

Three cells

The focus of this entry is on the leaking battery though.  I tore it apart and found three mystery cells.  The markings originally read ‘8711japan’ and ‘406099’ and each cell weighed 6.328 grams.  The battery measured 18M ohms in each direction so it’s effectively an open circuit to the terminal and the terminal put out 8-ish volts to the battery terminals.  With a 51 ohm resistor current limiting it, it was proposed that these may be NiCd cells, much more tolerant of having some current run into them at all times than NiMH (which I was planning on using).


I picked up some AAA sized batteries from amazon and used one of those triple AAA holders from cheap LED flashlights as the place to park them.  With these cells installed the terminal puts 9.5mA into them and it started falling, so I think this was the right fix.

Looking pretty sharp

Now I just need a keyboard, unless I use it solely as a display…

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