Restoring a VT50 (VT52 actually) .

Mattis Lind mattislind at
Tue Dec 26 05:25:44 CST 2017

2017-12-24 13:09 GMT+01:00 Noel Chiappa via cctalk <cctalk at>:

>     > From: Mattis Lind
>     > Unfortunately I am not having any lutfisk this year. The rest of the
>     > family is not very fond of it.
> I"m glad to see there are _some_ non-crazy people there! :-)
> Anway, I can way top that - the tradition Bermudian Christmas dish is
> cassava
> pie: make that wrong (starting with raw cassava root), and it will _kill_
> you! :-)
> (Cassava root contains a cyanide precursor. That's what cassava root is
> banned for human consumption in Japan. Somewhat ironic, the land of 'fugu'
> banning it! :-)

After too much christmas food I spent some more time in the garage fixing
the VT50/Vt52.

The tip from Josh about the PROM socket was a good one. Replacing those
together with the two ITT made 7400 chips I already replaced there were
nice click sound when pressing the keys on the keyboard. Setting the
terminal mode to OFF-line so that it loop-backed the serial signal gave a
some kind of pattern on the video signal. But was it an OK signal or were
there any more problems in the data path? Hard to tell without reassembling
the entire terminal. But if the there actually are more faults in there I
had to disassemble it again, fix and repeat. Instead I came up with a small
circuit (well stole actually from the VT100 schematics) that generated a
composite video signal from the TTL signals generated by the VT52 and feed
it into an old  Philips CM8533 monitor.

Yes! All the logic seems to work just fine. So now I can quite confidently
reassemble the thing and test the last step. HV generation, Horizontal
deflection and CRT.

Since the terminal appears to work I also dumped the firmware out of the
four bipolar PROMs.

@Fritz: You don't need to get a dump of the PROMs in your terminal. I think
the ones I have is in pretty good shape.

>         Noel

BTW. Comparing the VT52 and the HP2640 I worked on previously is like
comparing a Skoda with a Mercedes or something. Single layer circuit board
made of some cheap material (pertinax?) versus double sided gold plated
boards. Very simple quite hardwired architecture versus generic
microprocessor based design with a back plane bus. Was it just that DEC
wanted to be as cheap as possible? The predecessor VT05 had four standard
quad sized boards in a small backplane.


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