Slowly blowing a CRT?

From: Geoffrey G. Rochat <geoff_at_pkworks.com>
Date: Thu Sep 28 14:53:59 2000

Good thing you didn't take the cap off the side of the tube, as you're
looking at something like 18KV there. DON'T EVER GO INSIDE A RUNNING OR
RECENTLY POWERED OFF MONITOR OR TV WITH A CRT UNLESS YOU KNOW WHAT
YOU'RE DOING. You could easily be electrocuted, and that would be, as
they say, "bad."

It sounds to me that when you pulled the socket off the end of the tube
or put it back on you broke the vacuum stem. Most of the connections to
the innards of a CRT go through the pins at that end, and in the middle
is usually a sealed thin glass tube (which may be buried under a
cylindrical plastic key), which is where the manufacturer attached the
pump to evacuate the whole thing during manufacturing. Alternately, you
may have cracked the glass frit seals around one or more of the pins
when you pulled off or put on the socket. If you've cracked the stem or
a seal and lost the vacuum the CRT is gonzo and will have to be
replaced.

-----Original Message-----
From: Douglas Quebbeman <dhquebbeman_at_theestopinalgroup.com>
To: 'classiccmp_at_classiccmp.org' <classiccmp_at_classiccmp.org>
Date: Thursday, September 28, 2000 3:15 PM
Subject: RE: Slowly blowing a CRT?


>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 - 14:53:59 BST

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