Hello folks,

Another noob question here but I need some advice. I'm trying to discharge this set so that I can do some work on it. I've earthed my alligator clip to the chassis, with the other end of the clip on my screwdriver, then touched my screwdriver tip onto the anode but I don't see a spark/pop of any sort. Am I discharging this correctly? How else can I tell if it's been discharged?

Any advice would be appreciated!

Thanks folks,
Nick.

The only parts in a radio that can store an electric charge are the condensers.

Once a radio has been disconnected from the mains supply for a few minutes the condensers are usually discharged by then.

Thanks Brad. I've heard it's different for TV sets though. I'm listening out for a pop sound but not hearing anything.

Ahhhh sorry, I should have paid more attention to the subject.

The large condenser located near the tripler will cause the tripler to bite even when the power has been off for a while. I think there is a tool you can get to discharge televisions safely, most likely involving a resistor to allow the condensers to discharge slowly.

It is possible to ruin the condensers just by shorting them.

No worries, Brad, I think I should have been clearer about it in my post!

Any idea what this tool is called? I'd like to try and find one if I could find out what it's called!

Thanks,
Nick.

Hi Mangonick.

Here is a youtube clip with that tool Brad was talking about.

http://www.youtube.com/watch?v=H_4qc9Jyt5A.

Cheers.

I was given to believe that the old CRT's in these, being glass can retain considerable charge.

Marc

Me too Marc, that's why I'm worried as it's not popping when I try to discharge it.

Thanks Monochrome, I had come across the same clip when I was researching discharging, but I think I'm gonna have to contact that user & see what that tool was called.

I believe about 33pF for the tube is why it takes a long time to bleed down (hours).

The tool may have actually be .. the idiot who put their uneducated fingers in the wrong place?

A HV resistor say 10 to 20K on a clip lead across the cap will bleed it down.

The Ultor cap / cathode area should not be an issue if you are not working on its side of the EHT transformer or near it.

Years since I fiddled with TV

Marc

I was given to believe that the old CRT's in these, being glass can retain considerable charge.

Now that you mention it...

Back in 1995 when I was shifting house I knew I'd be travelling over some very rough gravel roads and thus had cause to dismantle the Thorn 63cm telly I own.

Anyway, guess who didn't discharge the static from the picture tube? I copped a big wallop right through the chest and wondered what the hell had hit me. Thankfully I am still here to tell the tale but at the time I assumed that if I discharged all the electros on the motherboard then all'd be well - it wasn't.

Anyway, the tube fell out of my hands and onto the floor. The picture tubes AWA was using at the time were made by Mitsubishi and they must be tough inside because there were no problems at all when I reassembled the set and powered up. Linearity, convergence and all the other usual tests along with colour were mickey mouse.

I know we are always warned about the kick from a CRT anode, but doesn't the charge naturally bleed out after a period of disuse? If Mangonick has not had the thing plugged in recently, perhaps he is doing the right thing but there is just no charge left to give him that "pop" he is listening for.

Certainly I have often found computer monitor CRTs to have no charge left after a period of being switched off. Many have circuits that will bleed to earth in a couple of hours via a resistor, providing the set is left plugged in and earthed after switching off. Even without that, the charge just leaks away over time.

Maven

Greetings,
You are game using a screwdriver, a good one will afford you 1000V of protection on a potential of 30 000 Volts. There is no capacitor as such, the tube is coated on the inside and outside with an inch of glass separating them.
Modern televisions (all colour sets as far as I know) have a bleeder resistor inside the line transformer, so if the set is working fine the voltage should bleed out over a few minutes. If there is a fault with the line transformer who knows if it will bleed at all. I think that some of the older B&W sets do not have this bleeder.
I use a high voltage probe, it has a flat 'U' shaped end that slides under the ultor cap.
As with all high voltage equipment you should earth it before working on it. After the tube is fully discharged it can regain it's charge as the minutes go by.
Also make sure you use the earth that goes across the back of the tube and not an earth near an expensive silicon device.
As marcc said, if you use a resistor make sure it is a high voltage one, or use many, many resistors in series. If the cap is disconnected it is the tube connection you want to earth. I don't know if the doubler/trippler will hold a charge in these valve sets.

Exactly how insulated should you be when discharging a CRT? Particularly for a 13.4" tube?
I handle lots of tubes, and use a chrome vandium screwdriver (with a thick rubber and plastic handle), with an alligator clip. I'm thinking about constructing a special device with a well insulated PVC pipe handle and a couple of sections of the EHT anode wiring from a television.

Also, where is the best place to ground the tube? Using the basic principle of a capacitor, I ground the anode to the aquadag via the tensioned implosion protection wires. Is this a good method?

These tubes can bite! the smaller 5" tubes have 8kv at the anode, so just imagine the voltage in one of those 40" tubes (used in a Sony XBR)!

Being a Trinitron, imagine how heavy that tube would be! Wow!

Chris

P.s. There's an image of a Sony XBR CRT here:
http://hardforum.com/showthread.php?t=947232

The best answer to that I think is very insulated. This is because the tube holds the postive and negative points that store the charge without a reference to earth so decent shoes and floor covering is not the answer. The tube should be discharged before it is removed from the chassis/cabinet.

Once the tube is discharged then there is nothing to worry about except to remember that the weakest part of the tube is the bit at the back with the electron guns inside. This part of the tube is no stronger than any normal radio valve or light globe and only supports the vacuum inside the tube because it is small.

Speaking of the neck, it always strikes me as funny. Whenever I salvaging components from a scrap TV, and try to break the tip of the neck to release the vacuum, it never works. Even throwing a block of hardwood at the neck doesn't work, yet sometimes when I don't want to break the tube, the neck breaks as I remove the back case!

Mind you, nowadays I just punch a hole through the metal anode connection with a screwdriver.

Cheers,

Chris