Repairing a VT240.

[Chris Zach]

So I'm clearing out projects on my bench. The latest is a Dec VT240 
which did not work. Has been dead for 20 years, the usual so I figured I 
would see if I could get it to run. This is a quick summary of what I 
did in the hopes it can help someone else out someday.

First I got the service manual. It's full of good information. Then I 
opened it up and checked out the power supply. 5 volts was fine, -12 was 
fine, +12 was a bit off (10v) but I didn't have the unit under any load. 
Putting a small load on 5 and 12 got me a solid +12v and 5.1v so all was 
good there.

Then I hooked it up to the board and fired it up. Voltages were still 
good (nothing big shorted) and the VR201 would come up, display a screen 
filling up with scan lines 3 times, then go black with nothing. Keyboard 
had WAIT lit and didn't work. So something was wrong.

Looking through the manual I saw that this thing is nuts: It has a full 
pdp11 in there (a T11 CPU) with something like 256kw of memory, 32k RAM 
and the rest ROM chips. Which is impossible, however they built a bank 
switching system into the terminal so the T11 could access more than 
32kw of memory. Yes, they literally built a MMU just so they could use a 
pdp11.

The 11 seems to control the basic functions and the graphics modes (4014 
and REGIS/VT220/VT125) through the graphics chip controller which looks 
like the same one on the Rainbow's expansion video board. However there 
is also an 8085 in there that apparently generates the shape of text 
characters. So they tossed in another processor because it's DEC....

Anyway I got out the thermal camera to see if there were any unusual 
shorts or chips drawing a lot of current. And sure enough the 8085 was 
glowing cherry red in the center. Felt it, it was hot and probably blown.

So I bought a new CPU on Ebay, realized it was an 8085A instead of the 
8085-2 on the unit, bought an 8085-2 that will come next week, got the 
8085a and figured might as well try it.

Popped it in, and the unit comes up! Set up works, ports work, I haven't 
tested the 20ma current loop but that probably works too. This is pretty 
much the most super terminal I've seen, and is classic DEC 
over-engineering. But it works, and now I can spend some time thinking 
about what I might want to DO with it.

Moral: After testing the power supply try checking the board with an IR 
camera. You can see a lot of interesting things with one.

Chris