Archives and Resources
TV Typewriter II Restoration Update
View Entire Thread
Return to Threads
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:
Manufacturer photos, software, docs
Computer History Articles Scans and Files
Misc Pictures not specific to one system
Vintage Computing Links
Popular Topics and FAQs
Commodore B Series Tips and Tricks
IMSAI 8080 Project
Prototype PET 2001 photo
Using Toggle Switches to Analyze Memory
Commodore Disk Archive Project
New York Weekly Messenger 2-13-1833
Secrecy is the keystone of all tyranny
Fortran Man - F-Man and Billy BASIC
Northstar Horizon - Boot Problem
Computer History and Restoration Links
IC and chip inventory
S-100 board testing with Z-80 ICE
Donner 3500 - an early portable computer
Univac 1219 rescue
IMSAI 1.4 BASIC vs. MITS 8K BASIC
Fido BBS listing node list 6-13-1986
MITS 88-2 SIO (2SIO) for BASIC
Visual Technology Inc Model 1050
Amiga 2500 Restoration
The Evolution Of IBM Computers
Replacement teletype print hammer head
Archiving and Copying Software 101
Computers Built 1940 - 1950
CBM B-520 (a.k.a B256-80 or B500 256)
RCA COSMAC Microkit
Commodore 64K C-116 Mods
MITS 8800b Turnmon 9600 baud
Catweasel, 8in and 5 1/4
Raspberry Pi as Gateway to Internet
Digital PDP11 late 1969 early 1970
PDP 11/40 72 inch cabinet model
PDP 11/40 Industrial 11 model
Digitial MicroVAX 3100 30 System
Digital VAX 4000-200
Dell Dimension XPS 466V
Booting the System Using RL02 drive
PACS: Reflections by Kathleen Mauchly
Process Computer Systems, Inc.
Installing Core into PDP 11/40
Slugging it through XXDP Diagnostics
Vintage Computer Festival East 3.0
Commodore B Series Prototypes
VOLSCAN - The first desktop computer with a GUI?
ROBOTS! - Will Robots Take Over?
Magnavox Mystery - a Computer, or?
The 1973 Williams Paddle Ball Arcade Computer Game
The Sperry UNIVAC 1219 Military Computer
VCF East 2007 - PET 30th Anniversary
The Electronic Brain
Community Memory and The People's Computer Company
Charles Babbage's Calculating Machine
Vintage Computing - A 1983 Perspective
Laptops and Portables
From Giant Brains to Hobby Computers - 1957 to 1977
Historic Computer Magazines
World's Smallest Electronic Brain - Simon (1950)
December 2008 - Feb 2009
Free Program Listings
Popular Electronics Jan/Feb 1975 - Altair 8800
Early Microcomputer Mass Storage
VT302 front w disks
This image was selected at random from the archive. Click image for more photos and files from this set.