Slowly blowing a CRT?

From: Paul Williams <celigne_at_tinyworld.co.uk>
Date: Sat Sep 16 14:45:46 2000

I spent an half an hour today operating beyond my level of competence,
but I seemed to get away with it.

I took apart an old VT320 terminal because I wanted to measure the
display refresh rate. From disassembling the ROM I know that a frame
interrupt is passed to the microcontroller, so I thought I'd open the
case, prise apart the tube and the main PCB, connect an oscilloscope
probe to the appropriate pin on the microcontroller, power up the
terminal and get my timings.

I was reassembling the terminal when an odd rattling and whistling sound
made me dive for cover. This noise went on for about 15 seconds and then
all went quiet. I very nervously approached the terminal and finished
snapping the case together. I've not attempted to power it up since.

I had noticed that the tube bore a label claiming that it had an
implosion protection system, so several questions occur to me.

1. Is it possible that I've somehow blown the tube by shorting one of
its connections soon after disconnecting the power? I noticed that there
are a number of large capacitors on the circuit board, but I don't know
how long they take to discharge.

2. There are two sets of wires going to the CRT. One set of five (six?)
goes to a cap on the end of the tube, and another wire (HT?) goes to the
side of the tube. I took the cap off the back of the tube, but didn't
touch the wire at the side. Can that be safely disconnected?

3. Why would a mere software engineer be doing something this foolhardy?
Don't even attempt to answer that.
Received on Sat Sep 16 2000 - 14:45:46 BST