Repairing a VT240.
robert.jarratt at ntlworld.com
Sun May 1 01:14:29 CDT 2022
> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Chris Zach via
> Sent: 01 May 2022 02:57
> To: CCTalk mailing list <cctalk at classiccmp.org>
> Subject: Repairing a VT240.
> 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
> 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.
That's interesting. What did you use for the thermal camera?
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