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Old 23rd Nov 2020, 4:01 pm   #1
GreasedMonkey
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Default When is it safe to remove a picture tube?

I plan on completely removing the main PCB from my set. When is it safe to disconnect the tube?
Is it simply a case of leaving the set with no power for a certain amount of time for everything to discharge? If so, how long is safe?

I know this may be a rudimentary question for the experts on here, but I have never before gone near the “bitey bits” beneath the rubber cap on the tube. I get that the EHT will take a good while to completely discharge and don’t want my hair to stand on end. I have precious little of it left as it is.

Also, how does the connection come apart? Will it be easy to figure out upon looking under the rubber cap?

Thanks.
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Old 23rd Nov 2020, 4:11 pm   #2
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Default Re: When is it safe to remove a picture tube?

Earth the final anode (on the crt) to the Aquadag coating.Beware though having done that,you can still get a belt.
ideally wear the correct rubber gloves that used to be available for this job.

Think the gloves were classed as "anti slip".The Anode Cap sometimes can be pressed to one side to release,it has like a prong on the end.Sorry ,not best description.
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Old 23rd Nov 2020, 4:18 pm   #3
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Default Re: When is it safe to remove a picture tube?

I used to use a croc clip lead to link the metal shaft of a big screwdriver to the wire which was connected to the tube bowl's graphite "Aquadag" coating, then carefully slide the tip of the screwdriver under the rubber cup so that it makes contact with the metal electrode. In theory, that should leave both the tube and EHT transformer/tripler discharged (to a greater or lesser extent).

Beware of doing anything which risks you ending up with an EHT shock from arm-to-arm (which could easily affect your heart), so best to always keep one hand behind your back.
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Old 23rd Nov 2020, 4:41 pm   #4
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Default Re: When is it safe to remove a picture tube?

CRT's can retain an EHT charge for ages. As suggested, get a large insulated screwdriver and a length of wire with crock clips. Clip one end of the wire to the screwdriver and the other end of the wire to the aquadag braid earthing/spring clip....whatever method is used for earthing and then slide the tip of the screwdriver blade under the EHT cap. If there is a lot of EHT present, there will be a sharp crack as it discharges....might make you jump the first time. Hold it there for a second or two then repeat the operation again after a few minutes....(the EHT can recover slightly). The second time will either produce no discharge at all or only be very slight. If possible, clip the earthing lead to the EHT connector on the CRT and leave it there for as long as you like...if possible whilst removing the CRT. That way it can't build up again.
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Old 23rd Nov 2020, 4:42 pm   #5
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Default Re: When is it safe to remove a picture tube?

there are plenty of videos on youtube showing how to safely discharge the tube. Nothing elaborate is required. There's always risk of damaging chips if you do it the wrong way, for example someone on a forum (not this one!) recommended using a screwdriver attached by a wire to a radiator.
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Old 23rd Nov 2020, 4:43 pm   #6
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Default Re: When is it safe to remove a picture tube?

Thanks for the tips.

I shall use the crock clip/screwdriver method this evening. I will be sure to keep one hand behind my back as advised, and will probably leave my radiators out of the equation. I am planning to bring the PCB into one of the lab technicians at work, see if he can do anything about the state of the tracking. I have seen this man work miracles with a soldering iron on many occasions. If he cant save it, nobody can.
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Old 23rd Nov 2020, 6:12 pm   #7
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Default Re: When is it safe to remove a picture tube?

You might find that the fault magically vanishes after he goes over the board, It's amazing what a fresh pair of eyes and a strong light can achieve.
As this is a portable mono set it's unlikely there will be too much EHT stored so you shouldn't jump too much! Do be careful removing the CRT base as it's quite fragile - pull it off nice and square. Aplologies if you knew this aleady.
As always with Youtube there are some very instructive videos and some done by people that shouldn't be let near a battery, let alone high voltages.
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Old 23rd Nov 2020, 6:29 pm   #8
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Default Re: When is it safe to remove a picture tube?

I intend to leave the tube base where it is as i think, can't be sure without looking, but I think it is soldered on. Scan coils, main power supply, loudspeaker and volume control simply unplug from the board. I am not sure about the brightness, contrast and tube connections. I think these are soldered to the PCB, in which case I will simply snip the wires off close to the PCB. Just leaving a small amount that I can use to carefully note what colour goes where. I will take plenty photos as well as my memory is not what it once was.

I have come to realise that a persons short term memory is actually in their hair as mine has been in steady decline along with the number of hairs on me head. But that’s a whole other story.
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Old 23rd Nov 2020, 7:25 pm   #9
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Default Re: When is it safe to remove a picture tube?

Don't cut any wires! Tube bases are never soldered onto the tube, they plug in.

There might be a dab of glue on the outside, sometimes.
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Old 23rd Nov 2020, 8:47 pm   #10
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Default Re: When is it safe to remove a picture tube?

I think GM means that the wires from the tube base PCB are soldered to the main PCB rather than plug-in, but there should not be any need to separate the two, just tape them together.
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Old 24th Nov 2020, 8:30 am   #11
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Default Re: When is it safe to remove a picture tube?

kyle_B was correct in that I had thought the tube base PCB was soldered directly on to the tube. Kyle was also correct in that it was not and I did manage to very gingerly separate the two. In the end, the only wires I had to cut were from the switch I added to select the sets original tuner or the digital tuner. There was no big scary flash when I discharged everything using the crock clip lead method suggested here. (I doubled up on the crock clips lead just to make sure I hadn’t selected a dud from the box).

All in all it was nice and easy to removed. Had I known how easy it was, I would have removed the thing long ago and given myself so much more room for component changes.

Thanks all for the help once again. PCB will be delivered to the lab this morning.
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Old 24th Nov 2020, 10:16 am   #12
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Default Re: When is it safe to remove a picture tube?

I used to use a spray bottle of water when handling recently discharged CRT's.

Once the final anode has been shorted to the 'dag' and any charge removed then I would remove the anode cap and lightly spray the glass around the connection such that it was damp. That would ensure no charge could build up.

The glass itself and an effect called 'dielectric absorption' means that the charge can build up again once a simple short has been removed in exactly the same way it does with capacitors. Often this occurs over hours and even a day or two. So a quick discharge and then a spray while handling and and it was always good to go. No nasty surprises.
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Old 25th Nov 2020, 4:29 pm   #13
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Default Re: When is it safe to remove a picture tube?

Never heard of that - especially when we spent ages making sure customers' sets were as dry as they could possibly be to prevent corona discharge! Mind you, it would work.

Apparently a large rental firm reported larger than expected numbers of the then new ITT Digivision TVs being dropped by installers. Repairers you could understand (!) but installers? Turned out there was a plastic banner on the front of the set extolling the set's virtues. Static would build up as you describe which then found its way to earth via the hapless installer who dropped the TV. Later they removed the label prior to installation and no more casualties.
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Old 26th Nov 2020, 12:32 pm   #14
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Default Re: When is it safe to remove a picture tube?

The water was used on the old CRT's coming out of the sets, new ones never seemed to have any charge whatsoever although I always checked first. I didn't wet the new ones.

The charge on an old CRT can really build up over a few hours/days and the unwary get caught.

Same with old reservoir caps that have been in service for years. You remove them, discharge totally and pop them out of the way, then come back a few days later and measure the terminal volts and it can be back in the high numbers again. Ouch!
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Old 26th Nov 2020, 1:21 pm   #15
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Default Re: When is it safe to remove a picture tube?

I am sure it was ITT but when I was at Granada we had to change the handle on a portable due to risk of a belt from the final anode.
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Old 28th Nov 2020, 11:17 am   #16
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Default Re: When is it safe to remove a picture tube?

I didn't imagine you'd go into a customer's house armed with a spray bottle of water, though it might have come in handy to scare off inquisitive pets!
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Old 28th Nov 2020, 11:33 am   #17
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Default Re: When is it safe to remove a picture tube?

Hi.
Never used water on a CRT. A couple of discharges are sufficient. To be fair you'd need to have your hands and fingers quite far around to bulb to get in contact with the anode cap and really wouldn't be holding the tube safely.
On a big majority of sets the EHT will be somewhat discharged after switch off as beam current doesn't decay immediately. Then discharging manually a couple of times is sufficient.
If you are worried then this can cause paranoia, which can lead to mistakes as your too worried about the anode cap then you don't see the angle the tube is sitting at when removing from fixed scan coils etc, then implosions are a bigger risk.
One thing that is a good idea is nitrile gloves as they can help with grip and also help with sweaty hands if you are scared of the tube.

My advice is if your not comfortable replacing a CRT don't do it, leave it to someone else that has no problem doing the job.
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Old 28th Nov 2020, 11:34 am   #18
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Default Re: When is it safe to remove a picture tube?

Some years back I had to remove the tube base from a Philips K35 or similar.
After gently prising off - I dislike bent crt pins - I find that one of the pins is trapped in
the socket and no longer in the glass. I extracted the pin and gently tapped back in to
the glass with a small hammer with a rubber tip. Although it made good contact and
crt was undamaged since then I have always sprayed some cleaner/lubricant on crt
sockets prior to removal.
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Old 3rd Dec 2020, 6:39 pm   #19
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Default Re: When is it safe to remove a picture tube?

The best way to discharge a CRT is simply to remove the CRT EHT connector, insulate it away from the chassis so that it cannot arc over then turn on. The beam current completely discharges the tube and you can often see a vanishing picture as it warms up.
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Old 3rd Dec 2020, 6:42 pm   #20
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Default Re: When is it safe to remove a picture tube?

Well I never! I'm learning a lot from this thread. Still, crossed screwdrivers for me...
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