At 08:45 PM 9/16/2000 +0100, you wrote:
>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.
If the tube vacuum is gone you MIGHT see a clear area on the
screen where the phosphor is blown off. On the old tubes there was a tip at
the end of the neck where it was sealed off, and this is easily broken.
Other than that breakage might be caused by incorrect alignment, with the
deflection yoke applying a twisting force to the neck. I had one new tube
that had been correctly installed and self destructed when warmed up,
apparently due to stresses in the glass. The client was not amused.
Foolhardy? Don't feel bad. Back in my TV repair days I once had a
customer come into the shop carrying a large picture tube by the neck and
asking "Do you test tubes?"!
Cheers
Charlie Fox
Charles E. Fox
Chas E. Fox Video Productions
793 Argyle Rd. Windsor N8Y 3J8 Ont. Canada
email foxvideo_at_wincom.net
Check out "The Old Walkerville Virtual Museum" at
http://www.skyboom.com/foxvideo and
Camcorder Kindergarten at
http://www.chasfoxvideo.com
Received on Sat Sep 16 2000 - 16:06:39 BST