Slowly blowing a CRT?

From: Douglas Quebbeman <dhquebbeman_at_theestopinalgroup.com>
Date: Thu Sep 28 13:49:22 2000

Paul-

I can't help you with parts or theory-of-operation, but if
you need a working amber VT320, I've got one for sale.

regards,
-doug q

> -----Original Message-----
> From: Paul Williams [mailto:celigne_at_tinyworld.co.uk]
> Sent: Saturday, September 16, 2000 3:46 PM
> To: classiccmp_at_classiccmp.org
> Subject: Slowly blowing a CRT?
>
>
> 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 Thu Sep 28 2000 - 13:49:22 BST