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TV Typewriter II Restoration Update
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by Bill Degnan
- 08/08/2009 20:11
Evan and Bill Degnan asked me to take a look at your
TV Typewriter II, as they know I like fooling with old electronics
stuff. Whoever built it really did a great job on keeping things nice
and neat, even the mods. I'm impressed.
The TV Typerwriter II in MARCH's museum. Click image for larger version.
When I first turned it on, I wasn't getting a clock to the UART,
which also kept the character from getting sent to the display RAM.
The keybd strobe was fine. I removed a "rate multiplier" chip that
adjusts the clk to get the different baud rates, and then plugged it back in.
Now I had a clk to the UART. That got me characters on the screen.
Then I tried the cursor keys and noticed that "right" was moving the cursor
maybe 10-20 positions. I found a one-shot on the cursor bd that wasn't working.
Replaced that chip and the cursors started to do the expected thing.
Then I did a little work with the ohmmeter to figure out what pins on the
RS-232 cable were tx and rx, and made an adapter for the PC's 9-pin
connector. Now I can run a terminal program and send and receive chars
from TVT II. I had to change the PC to 7N1 instead of 8N1 to get it to
Your unit appears to have these mods:
1) 64 columns instead of 32.
2) Extra RAM chip added to store the 7th bit.
3) Extra character generator ROM chip added to support lower case.
4) Speaker for control-G beeping. (works)
I am not sure what the circuitry on the extra board that the speaker is
hooked to is for. Maybe that's used for some of the above mods? Evan mentioned
something about auto-repeat for the keyboard. Not sure where that is done.
One strange thing I have noticed is that the bit map for lower case j looks wrong.
The character ROM is supposed to have one blank row per character. When it's time
to blank the display (vertical or hor blanking) the circuit selects the blank row, and the
display is blank, even though the display RAM is being accessed during the blanking time.
The lower case j seems to have a bit set in the row that is supposed
to be all 0's. As a result, if there are j's in the display memory, they show up as vertical
lines in the part of the screen above and below the 16x64 normal character display area.
Does this sound familiar? I was wondering if you had this problem before, or if maybe
the character generator ROM has gotten a little corrupted from age. All of the other
characters look fine and have correct blanking.
To fix the lower case j blanking
issue, one would copy both 2513 ROM's (all of 64 x 8 x 5bits!)
and program a new CMOS EPROM with both images, after
fixing the j pattern of course. Any EPROM with at least 1K bytes
would be big enough. Something like a 27C64 or 27C32 would work.
I would attempt it, but the pinout of the 2513 is totally non-standard
and it takes 3 supply voltages, so I don't think my EPROM
programmer could read it. It works fine as-is, just has some extra
pixels in the blanked area of the screen if there are lower case j's
on the screen. Also, the lower case j looks weird:
and it should be:
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