Hi all,

I have had a question on my mind for a while, and finally decided to ask about it. What is the story with the aquadag coating on the exterior of a cathode ray tube? I thought it was suppose to be running at HV, acting as a part of a capacitor of sorts with the EHT anode/faceplate voltage inside the tube.

I originally noticed that with my AWA P1Q, there were small bolts holding the CRT to the bezel of the set, and which were in contact with the aquadag. I then noticed that those bolts are mounted from the outside of the set, leaving them exposed for 'wandering fingers'.

Then this afternoon, whilst working on my newly found Rank Arena C-1421, I discovered a ground cable running from the implosion protection band on the CRT to the shielding case behind the coaxial socket. I had been picking up the coaxial unit (which detaches from the case) to clean it whilst the set was running, unaware that it was connected to the CRT. Isn't it the shield supposed to be grounded?

If anyone can shed some light on this, I'd be most grateful.

Chris

Hi Chris
If there is an Aquadag coating on the outside of the tube, it does indeed form a capacitor with an identical coating on the inside. The inner coating is connected to EHT and the outer coating should be connected to ground.

The inner coating at EHT potential is to trap any secondary emission electrons and by forming a capacitor with the outer coating, you get a bit more smoothing of the HV supply.

The coatings are a water based graphite material as they must be conductive.

Harold

So the aquadag coating on the exterior shouldn't be carrying any voltage, since it is grounded?

In theory, what would happen if you touched the aquadag on a CRT whilst in operation?

Chris

Correct, there should be zero volts on the outer coating, but as I'm not an expert on TV circuits, I wouldn't guarantee this is always the case.

In theory, you could touch the outer coating BUT, if the ground connection is faulty, you are connected to the EHT voltage via the aquadag capacitance and that is not good!!

Perhaps someone with more TV experience than me could respond and advise in the safety aspects of this question.

Harold

In theory, you could touch the outer coating BUT, if the ground connection is faulty, you are connected to the EHT voltage via the aquadag capacitance and that is not good!!

This is no different to the 'hot neutral' fault in a mains electrical circuit and traditionally has been the source of most electric shocks received by electricians, including myself.

If the return path of the circuit is not correctly bonded to the negative rail of its power supply then it is running at the same potential as the feed path and thus the serviceman would receive a shock should he come into contact with it.

Similar faults could exist on single wire earth return (SWER) rural electricity supplies. If the earth bonding of the neutral wire corrodes then the voltage drop of the circuit will eventually fall across the corroded terminal rather than the electrical loads inside the house.

Another example is electric trains. Power is taken from an overhead cable. The wheels of the train have to sit on perfectly clean rails to complete the circuit for the motors, especially so since steel is not the best electrical conductor, although good enough for this application. Electric trains have been known to lose power on sections of rail where steam trains have dumped sand on the tracks so their wheels can maintain good grip.

I worked for a plumber years ago in the days before RCD's that always wore rubber gloves when cutting through outside water pipes. Older houses used the water pipes for the earth return and he told me more than one plumber had paid the ultimate price for a faulty appliance when the end of the pipe closest to the house became live.

Yes, I've heard a few stories like that. I don't know if the SAA Wiring Rules specify it, but I think EP bonding of the water pipe should take place as close as possible to the meter though I've seen countless installations where the EP bond is attached to pipes around the back, usually along runs that have been installed when the house has been extended or remodelled.

Gas pipes don't get bonded and this is one of the reasons why. Even cutting through old lead pipe with a hacksaw would throw a spark when the two lengths of pipe were parted if such a fault existed.

The aquadag coating on CRT's is usually tied to chassis ground in most sets but some models (eg Philips) have the aquadag floating above chassis ground potential by around 50 volts or so. Grounding CRT's that are not meant to be grounded will result in circuit damage.

The point of the aquadag is to form a smoothing capacitor for the CRT EHT supply. The tube will work fine without it but picture quality will suffer due to fluctuations in the voltage. Since the internal coating is at EHT potential the outer coating must be near ground potential for it to work.

The aquadag coating does not have any significant voltage on it because the EHT supply is DC and no current will flow through a capacitor (the CRT glass) with a steady voltage across it. If for some reason the earthing fails, then a substantial electrostatic voltage will build up on the outer aquadag coating, it won't normally be dangerous but it will hurt to touch it.

As far as touching the coating while the set is on, it all depends on the circuit configuration, but is generally something to avoid.
On sets with the aquadag tied directly to chassis ground and with an earthed chassis (via a 3 core power lead) then it is safe to touch.
On sets with a floating (double insulated chassis) with only a two core power lead you should not be touching any part of the TV chassis with the power on!

Too summarise, the coating is not the danger, it is what it is grounded too that is the source of the danger.

Thanks TV Collector, for explaining the TV aspects of the aquadag coating. My experience with CRTs was in military radar, so covered many types of tubes, but in a different environment than a TV set.

I'm sure the younger members, as I do, appreciate your comments.

Harold

"On sets with a floating (double insulated chassis) with only a two core power lead you should not be touching any part of the TV chassis with the power on!"

A long time ago I thought I would add a headphone socket to a double-insulated TV set, only to find that the shield of anything plugged into that socket became live and delivered quite a punch to anyone who touched it. I think it was only 12volts, but plenty of amps to make the experience worth remembering. That was in the days before I read manuals or asked for advice!

Maven

I think it was only 12volts, but plenty of amps to make the experience worth remembering. That was in the days before I read manuals or asked for advice!

It will have been more than 12 volts. 12 volts will only belt you if your skin is waterlogged although if that happens, 50mA is still strong enough to kill.

The volts are what punctures your skin but the amps is what does the damage to your heartbeat. The resistance of the top layer of skin is usually around 600 ohms and that's why we are usually immune to anything below 110 volts DC or 32 volts AC.

Really? I've never felt myself to be immune to shocks from a 12v car battery!

Maven

I can't say I've ever received a shock from any battery, including radio B batteries though, again, it would depend on the state of the skin on your fingers.

One probable exception was 'tasting' 9 volt transistor batteries as a kid, though this isn't a standard handling procedure.

though this isn't a standard handling procedure.

It isn't?? I've been known to do it on occasion if I don't have a meter within easy reach. Can't say I enjoy it though

You mean you get a shock touching each terminal of a 12 Volt vehicle battery?
I've never tried it, but never thought this would happen.