VT100 terminal revival

We found an old VT100 terminal that someone in the building was throwing away.  Thinking of the historical value, we rescued it and tried to turn it on… but no dice.  The screen was dead and there were no hints of life (no hums, beeps, or status LEDs).

We opened it up and saw the first problem, an exploded capacitor on the video control board (the black shell to the right of the white plastic connectors in the middle of the board):

Some digging online revealed that this capacitor, C439, was a common problem in these units.  The full schematic and parts list was very helpful as well.  We replaced it with a new, higher rated, part.

Now more hopeful, we powered the machine on, and had a bit more success – the keyboard beeped and lit its LEDs, indicating successful self-tests.  The user guide online helped understand the boot procedure.  However, the CRT was still dead.

Some more research revealed that a microfuse on the board (F401, 2A) was another common failure.  We had tested the main power supply fuse as soon as we got the machine, and it was fine.  I’ll reproduce this extremely helpful email from 1992 here:

Date: 25 Feb 1992 16:25:30 GMT
From: hubich@mercury.cs.uregina.ca (Chad D. Hubich)
Subject: VT100 Repairs

Over the years we have repaired a few DEC VT100 terminals with 
various problems.  Typically the same failures occur repeatedly 
in different machines.  Below is a short summary of some of the 
most common failures and their fixes.  If anyone has any other 
repeair info, especially with the last problem, please post it 
to the net.

	Symptom				Possible Fix
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No HV or filament voltage	check/replace F401 2A picofuse
				(usually caused by some other failure)

No video or only vertical	check/replace C441 10uF, 35V bi-polar 
line down center of screen
(no horizontal deflection)

No video, middle of screen 	check/replace C439 100uF, 25V
compressed or warped along 
vertical line or screen too
wide.	

No video			check/replace CR408 1N5408
R478 hot or smoking

Low HV or Jittering video	check/replace flyback transformer

Error '2' (NVR) at power	replace E24 (logic board) ER 1400
up or when Setup is saved  	

Bright raster along left 	?????????????????????????????????????
side of screen	

---------------------------------------------------------------------
Chad D. Hubich
Technician				hubich@cs.uregina.ca
University of Regina			chubich@ureginav.BITNET
Department of Computer Science
---------------------------------------------------------------------

We had encountered (and hopefully fixed) the C439 issue, and the F401 issue also seemed to match our symptoms (and was the easiest to test).  It turned out the fuse was tripped, and so we jumped over it as a quick workaround test:

You can see the jumper and our shiny new blue capacitor (rated at 100V instead of the 25V part that failed, which was 6V in the original specs).

After this quick hack, the terminal powered on and showed full signs of life:

Great success!  Now we’ll order a replacement microfuse to have a proper repair, and then we can hook the terminal up to our modern systems for true vintage nerd fun.  We might also clean it up a bit, like others have done.

Thanks to my co-conspirator Tatsu for brainstorming, soldering skills, and high-voltage safety supervision.

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