2 Nov
2005
2 Nov
'05
10:52 p.m.
Okay, I'm going to try and take another crack at replacing the flyback on my
VT100. A few months ago I almost did, and chickened out. I'm very very
afraid to do so, as the idea of trying to discharge the CRT scares me very
much. The terminal has been off and disconnected from power for at least
2-3 months now.
This is what I've learned so far from reading:
Put one hand in your back pocket. Use an insulated screwdriver with
alligator clips on either end, one to the screwdriver blade, one to the
frame (VT100 service manual says the frame right above the CRT is where you
should ground to). Place the screwdriver under the anode cap and touch the
metal connection underneath. Wait till the crackling stops. Remove the
anode cap.
Can anyone reassure me that this is a semi-safe procedure? Has anyone done
this sort of thing before?
Thanks
Julian
2 Nov
2 Nov
11:13 p.m.
Wolfe, Julian declared on Wednesday 02 November 2005 22:52:
Okay, I'm going to try and take another crack at
replacing the flyback
on my VT100. A few months ago I almost did, and chickened out. I'm
very very afraid to do so, as the idea of trying to discharge the CRT
scares me very much. The terminal has been off and disconnected from
power for at least 2-3 months now.
This is what I've learned so far from reading:
Put one hand in your back pocket. Use an insulated screwdriver with
alligator clips on either end, one to the screwdriver blade, one to
the frame (VT100 service manual says the frame right above the CRT is
where you should ground to). Place the screwdriver under the anode
cap and touch the metal connection underneath. Wait till the
crackling stops. Remove the anode cap.
Can anyone reassure me that this is a semi-safe procedure? Has anyone
done this sort of thing before?
Yeah, I've replaced CRTs and flybacks before.
A better idea is to place a current-limiting resistor in series with the
alligator clip to ground (twist one lead around the screw driver, and
tape it on, attach the allegator clip to the other lead). 100k to 1M
ohm should do the trick, and hold it on the metal part of the anode cap
for 5 seconds or so. Not using the resistor will still discharge the
tube, but it runs the risk of damaging it.
If you have the tube powered off for a couple days, the charge will most
likely be quite dissipated. In fact, having it off that long means it's
unlikely to have any charge on the tube at all, but it's still best to
still discharge it, just in case.
Of course, I've worked on monitors live before. Trick there is to stay
clear of the PSU and HV parts alltogether, and use insulated tools.
Once you do it once or twice, you'll start feeling a bit more
comfortable doing it. It isn't a good idea to *replace* the flyback
with the terminal live, but you'll probably need to have the power on
when you adjust the screen/focus controls on the new flyback you
install.
Pat
--
Purdue University ITAP/RCAC --- http://www.rcac.purdue.edu/
The Computer Refuge --- http://computer-refuge.org
3 Nov
3 Nov
1:42 p.m.
Yeah, I've replaced CRTs and flybacks before.
As, of course, have I.
A better idea is to place a current-limiting resistor in series with the
alligator clip to ground (twist one lead around the screw driver, and
tape it on, attach the allegator clip to the other lead). 100k to 1M
ohm should do the trick, and hold it on the metal part of the anode cap
for 5 seconds or so. Not using the resistor will still discharge the
tube, but it runs the risk of damaging it.
Agreed. Actually, I have a Heathkit EHT probe. It's just an 800M resistor
and a microammeter in series. I connect the ground lead of that to the
CRT earth, then hold the probe end on the anode connector for a minute
after the meter has got to 0. That seems pretty reliable.
If you have the tube powered off for a couple days, the charge will most
likely be quite dissipated. In fact, having it off that long means it's
The semiconductor rectifiers used in computer monitors, etc will leak
enough to discharge the CRT. Watch out if you work on old TVs with valve
rectifiers, they are much less leaky.
unlikely to have any charge on the tube at all, but
it's still best to
still discharge it, just in case.
Of course, I've worked on monitors live before. Trick there is to stay
clear of the PSU and HV parts alltogether, and use insulated tools.
I once had the 30kV EHT in a colour monitor arc over to my fingers.
Suffice it to say I then used langauge that I would not repeat on this
list :-)
If you're working on a 'live' unit, don't work alone. Have somebody
nearby who knows how to turn off the power if things go wrong.
And rememebr that (at least in Europe), the mains is a lot more dangerous
than the RHT due to the former's much lower impedance. Don't take great
care to avoid the (generally well insulated) EHT and then put your hand
on a live heatsink in the SMPSU.
Once you do it once or twice, you'll start feeling
a bit more
comfortable doing it. It isn't a good idea to *replace* the flyback
with the terminal live, but you'll probably need to have the power on
when you adjust the screen/focus controls on the new flyback you
install.
Actually, in a VT100 the presets are all on the video bard. It's still
worth rememebring there's about 800V on some of them, so use a
well-insulated screwdriver to tweak them.
-tony
3:04 p.m.
You were dead on about this Pat. Absolutely NOTHING happened. I sat there
with the screwdriver for about 3 or 5 minutes and nothing happened, then I
took the anode cap off. However, I had my vet tech friend there in case I
needed CPR or any other sort of medical assistance (she's trained in basic
human medicals too)
And Pat, you're also dead on about the controls, I had to readjust those
twice before when the flyback was going in a hurry, and had to do it live,
but I was a lot less scared of that - I knew as long as I stayed way from
the CRT I would be fine.
It's funny, my particular life's experiences have pretty much told me that
if I have some sort of safety net with me, I never need it ;)
Thanks again to everyone who offered assistance.
Julian
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Patrick Finnegan
Sent: Wednesday, November 02, 2005 10:13 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Discharging a VT100 CRT
Wolfe, Julian declared on Wednesday 02 November 2005 22:52:
Okay, I'm going to try and take another crack at
replacing the flyback
on my VT100. A few months ago I almost did, and chickened out. I'm
very very afraid to do so, as the idea of trying to discharge the CRT
scares me very much. The terminal has been off and disconnected from
power for at least 2-3 months now.
This is what I've learned so far from reading:
Put one hand in your back pocket. Use an insulated screwdriver with
alligator clips on either end, one to the screwdriver blade, one to
the frame (VT100 service manual says the frame right above the CRT is
where you should ground to). Place the screwdriver under the anode
cap and touch the metal connection underneath. Wait till the
crackling stops. Remove the anode cap.
Can anyone reassure me that this is a semi-safe procedure? Has anyone
done this sort of thing before?
Yeah, I've replaced CRTs and flybacks before.
A better idea is to place a current-limiting resistor in series with the
alligator clip to ground (twist one lead around the screw driver, and
tape it on, attach the allegator clip to the other lead). 100k to 1M
ohm should do the trick, and hold it on the metal part of the anode cap
for 5 seconds or so. Not using the resistor will still discharge the
tube, but it runs the risk of damaging it.
If you have the tube powered off for a couple days, the charge will most
likely be quite dissipated. In fact, having it off that long means it's
unlikely to have any charge on the tube at all, but it's still best to
still discharge it, just in case.
Of course, I've worked on monitors live before. Trick there is to stay
clear of the PSU and HV parts alltogether, and use insulated tools.
Once you do it once or twice, you'll start feeling a bit more
comfortable doing it. It isn't a good idea to *replace* the flyback
with the terminal live, but you'll probably need to have the power on
when you adjust the screen/focus controls on the new flyback you
install.
Pat
--
Purdue University ITAP/RCAC --- http://www.rcac.purdue.edu/
The Computer Refuge --- http://computer-refuge.org
3:10 p.m.
And Pat, you're also dead on about the controls, I
had to readjust those
twice before when the flyback was going in a hurry, and had to do it live,
but I was a lot less scared of that - I knew as long as I stayed way from
the CRT I would be fine.
Hmm.. A little knowledge can be dangerous.
The EHT -- the 15kV to the final anode -- is pretty well insulated when
the terminal is in operation. The flyback is potted, there's a
well-insulated lead to the anode connector, then the connector itself.
It's pretty hard to touvh it by accident.
But the 'screen' and 'focus' controls set the voltages of 2 of the
electrodes in the gun. Typically there will be a few hundred volts on
them. And those voltages are on the PCB tracks, on the terminals of the
presets, and so on.
No, they're unlikely to be fatal, just unpleasant. But they are much
easier to touch by accident.
-tony
5:32 p.m.
Understood, and yes I respect the voltage those things put out. That is the
reason I use a long plastic adjuster for this and not a screwdriver.
However, the good part about those adjustments is that you do not have to
directly work with high voltage like you do on a freshly turned-off flyback,
only avoid it.
In any case, I'm very careful and methodical about it, and that's what
counts.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Tony Duell
Sent: Thursday, November 03, 2005 2:11 PM
To: cctalk at classiccmp.org
Subject: Re: Discharging a VT100 CRT
And Pat, you're also dead on about the controls, I
had to readjust those
twice before when the flyback was going in a hurry, and had to do it live,
but I was a lot less scared of that - I knew as long as I stayed way from
the CRT I would be fine.
Hmm.. A little knowledge can be dangerous.
The EHT -- the 15kV to the final anode -- is pretty well insulated when
the terminal is in operation. The flyback is potted, there's a
well-insulated lead to the anode connector, then the connector itself.
It's pretty hard to touvh it by accident.
But the 'screen' and 'focus' controls set the voltages of 2 of the
electrodes in the gun. Typically there will be a few hundred volts on
them. And those voltages are on the PCB tracks, on the terminals of the
presets, and so on.
No, they're unlikely to be fatal, just unpleasant. But they are much
easier to touch by accident.
-tony
3:26 p.m.
Julian Wolfe wrote:
It's funny, my particular life's experiences
have pretty much told me that
if I have some sort of safety net with me, I never need it ;)
Which is why I have about 100kg of tools in the boot of my car *all the
time*. And spare spare spare sound cards and the like...
Gordon.
3:07 p.m.
Which is why I have about 100kg of tools in the boot
of my car *all the
time*. And spare spare spare sound cards and the like...
As I've said before, whenever I demonstrate one of my classic computers,
I take along enough tools and test gear [1] to repair it should it fail.
That, of course, is the best way to ensure it doesn't fail
[1] Of course I have the schematics, etc with me too. If I'm
demonstrating a machine, somebody is going to start asking nasty
questions :-)
-tony
3:59 p.m.
Rumor has it that Gordon JC Pearce may have mentioned these words:
Which is why I have about 100kg of tools in the boot of
my car *all the time*.
Which is why I like my Chevy Avalanche. I can keep about 50kg of tools
organized, hidden, and locked - so I can still fit a wife & kids in my
truck! ;-)
[[ I tend to be messy if I have less room for tools than my paranoia
requires... ;-) ]]
And spare spare spare sound cards and the like...
Dang... sounds like you had an IRQ conflict there. ;-)
Laterz,
Roger "Merch" Merchberger
--
Roger "Merch" Merchberger | "Bugs of a feather flock together."
sysadmin, Iceberg Computers | Russell Nelson
zmerch at 30below.com |
5:34 p.m.
Roger Merchberger wrote:
Rumor has it that Gordon JC Pearce may have mentioned
these words:
OT, but I drive a 1991 Citro?n XM. I can fit all of my tools in, a
comprehensive set of spares (including 5l of hydraulic fluid), and still
squeeze a couple of cases in. If I fold down the back seats I can fit
an LA36, a stack of manuals, and all the aforementioned tools, and still
see out the back window. Fitting my PDP11 rack in there was no biggie.
Now if I could get my girlfriend to let me buy a CX Break I could keep a
Smart Coupe in the back as a spare...
Gordon.
Which is why I have about 100kg of tools in the
boot of my car *all
the time*.
Which is why I like my Chevy Avalanche. I can keep about 50kg of tools
organized, hidden, and locked - so I can still fit a wife & kids in my
truck! ;-)
5:46 p.m.
Yeah, with all the hauling of crap I've been doing, I'm kicking myself for
not buying the station wagon (Magnum) version of my Charger.
Thankfully though, I was able to break down the H9612 into its component
parts and fit it in the car.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Gordon JC Pearce
Sent: Thursday, November 03, 2005 4:34 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Discharging a VT100 CRT
Roger Merchberger wrote:
Rumor has it that Gordon JC Pearce may have mentioned
these words:
OT, but I drive a 1991 Citro?n XM. I can fit all of my tools in, a
comprehensive set of spares (including 5l of hydraulic fluid), and still
squeeze a couple of cases in. If I fold down the back seats I can fit
an LA36, a stack of manuals, and all the aforementioned tools, and still
see out the back window. Fitting my PDP11 rack in there was no biggie.
Now if I could get my girlfriend to let me buy a CX Break I could keep a
Smart Coupe in the back as a spare...
Gordon.
Which is why I have about 100kg of tools in the
boot of my car *all
the time*.
Which is why I like my Chevy Avalanche. I can keep about 50kg of tools
organized, hidden, and locked - so I can still fit a wife & kids in my
truck! ;-)
4 p.m.
And then you hang on to those spare spare spare sound cards for awhile, and
all-of-a-sudden they're worth oodles of cash, like my Game Blaster card
ended up being. And the woman-folk have the nerve to call this stuff junk!
;p
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Gordon JC Pearce
Sent: Thursday, November 03, 2005 2:27 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Discharging a VT100 CRT
Julian Wolfe wrote:
It's funny, my particular life's experiences
have pretty much told me that
if I have some sort of safety net with me, I never need it ;)
Which is why I have about 100kg of tools in the boot of my car *all the
time*. And spare spare spare sound cards and the like...
Gordon.
4:04 p.m.
New subject: Discharging a VT100 CRT (now Being Prepared)
And, forgot to add, I now keep a fire extinguisher in the trunk of my car
ever since the engine of my 81 firebird caught fire 3 years ago - IN THE
CULVERS DRIVE THROUGH!
It was actually sort of funny in retrospect
Drive through guy: "Welcome to Culvers, what can I get for you today?"
Me: "A ****ING FIRE EXTINGUISHER!"
I can't say I miss that car too much with all the trouble it caused.
Anyway, back on topic...
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Gordon JC Pearce
Sent: Thursday, November 03, 2005 2:27 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Discharging a VT100 CRT
Julian Wolfe wrote:
It's funny, my particular life's experiences
have pretty much told me that
if I have some sort of safety net with me, I never need it ;)
Which is why I have about 100kg of tools in the boot of my car *all the
time*. And spare spare spare sound cards and the like...
Gordon.
8:49 a.m.
You got it, I spent several years servicing CRT devices both TVs and monitors
Don't bother with the resistor, you want to hear the crack of discharge !
It is the sound of success, If I don't hear a crack, I hold the short until
I have completely removed the anode to be sure.
You are only discharging the capacitor formed by the metallic
coating on the inside and outside of the glass. A very primitive glass
capacitor charged to around 20kv.
The anode is isolated from the working end of the tube
and will not heart anything but you if you don't discharge it.
BTW: one hand in the back pocket was what Uncle Sam they taught
us in radar school in the late 60's early 70's.
Now there was some big tube with a lot of crack :)
I wonder how much crack my 38" wide screen CRT TV will make
when I have to work on it some day :)
Have fun, but respect the electrons .... they can bite :)
Bob
On Wed, 2 Nov 2005 21:52:28 -0600, Wolfe, Julian wrote:
Okay, I'm going to try and take another crack at
replacing the flyback on my
VT100. A few months ago I almost did, and chickened out. I'm very very
afraid to do so, as the idea of trying to discharge the CRT scares me very
much. The terminal has been off and disconnected from power for at least
2-3 months now.
This is what I've learned so far from reading:
Put one hand in your back pocket. Use an insulated
screwdriver with
alligator clips on either end, one to the screwdriver blade, one to the
frame (VT100 service manual says the frame right above the CRT is where you
should ground to). Place the screwdriver under the anode cap and touch the
metal connection underneath. Wait till the crackling stops. Remove the
anode cap.
Can anyone reassure me that this is a semi-safe
procedure? Has anyone done
this sort of thing before?
Thanks
Julian
1:36 p.m.
Okay, I'm going to try and take another crack at replacing the flyback on my
VT100. A few months ago I almost did, and chickened out. I'm very very
afraid to do so, as the idea of trying to discharge the CRT scares me very
much. The terminal has been off and disconnected from power for at least
2-3 months now.
Well, glass [1] is an excellent insulator and could hold a charge
for 3 months _but_ it is very likely the CRT has discharged either
through leakage in the EHT rectifier diode (built into the flyback in
this unit), or through the beam current when the thing was last used.
The worst case for the CRT to hold a charge is if the EHT side (flyback,
etc) keeps working and there is no beam current due to the grid being
very -eve or the heater going out, or... There is then nothing to
discharge the CRT.
[1] The capacitor you are discharging consists of aquadag layers on the
inside aod outside of the CRT flare. The CRT glass envelope is the
dielectric.
This is what I've learned so far from reading:
Put one hand in your back pocket. Use an insulated screwdriver with
More particularly, work one haned, keep the other hand well away from the
unit and anything that's earthed. The most dangerous current path is up
one arm and own the other.
It is normal to keep your left hand in your pocket. Some say it's because
your heart is on the ledt side, I personally do it because I am right-handed.
alligator clips on either end, one to the screwdriver
blade, one to the
frame (VT100 service manual says the frame right above the CRT is where you
should ground to). Place the screwdriver under the anode cap and touch the
You should connect to the CRT earth. I seem to rmemeber on a VT100,
that's a spring contact on the yoke with a white wire/faston clip. The
idea is to ensure that the discharge current doesn't flow through
anything it shouldn't
metal connection underneath. Wait till the crackling
stops. Remove the
anode cap.
Can anyone reassure me that this is a semi-safe procedure? Has anyone done
this sort of thing before?
It's safe for _you_, but I wouldn't do it that way. The high initial
current could damage the CRT (burn the conenction between the anode
conneector and the internal aquadag layer), and more particularly could
damage IC's all over the terminal.
The first thing I'd do is take out the logic board(s) and PSU. That gets
all the delicate ICs out of the way.
I would also use a discharge resistor. Take a 1M resistor (or if you
prefer 10 100k resistors in series if you're worried about the voltage
rating), connect one end to the CRT earth, the other to the screwdriver
blade [2]. Carry on as above, but hold the screwdriver in contact with
the anode conenctor for a minute or so. If you're worried, repeat the
process with no resistor just to be sure.
[2] Don't do what I did once when inebriated [3] and connect both the
chassis and screwdriver to the same end of the resistor, thus not having
it in-circuit
[3] Why was I working on a monitor when drunk? Well, it was my 21st
birthday, one of the presents was a non-working FTS-88 computer. Of
course I started to repair it ASAP.
The charge stored in a normal-size CRT -- particularly a monochrome one
-- is not likely to be dangerous in itself to anyone without heart
problems. But it will make you jump, you might then drop the CRT with
very unpleasant results.
-tony
6836
days inactive
6836
days old
14 comments
7 participants
participants (7)
-
ard＠p850ug1.demon.co.uk -
Bob＠BRADLEE.ORG -
fireflyst＠earthlink.net -
gordonjcp＠gjcp.net -
ISC277＠CLCILLINOIS.EDU -
pat＠computer-refuge.org -
zmerch＠30below.com