How to disipate current in TV

It's probably not practical to discharge every capacitor on the PCB. By far the most dangerous part of a TV is the mains rectifier smoothing capacitor. By dangerous I mean lethal. It can be easily found- it's usually the largest, fattest capacitor in the TV, and is often rated at 400V or 450V.

Check the voltage across this cap before you work on the TV, it should be very low, eg less than 15V to be safe. If it shows no signs of dropping after the TV has been unplugged for a few minutes, it means there is no bleeder resistor fitted across it. You can safely discharge it by using a normal household bulb across the terminals, (at least here in Europe with

230/240V bulbs!) or a large wattage resistor of suitable resistance. Make sure the voltage has indeed dropped to a safe value afterwards, never assume anything.

You should also discharge the CRT to remove its residual charge if you intend on removing the chassis. Electrically, a CRT is amonsgt other things a HV capacitor, and can pack enough of a wallop to cause very painful and potentially injurous muscle spasms.

To discharge the CRT, just take a screwdriver, attach one end of a croc lead to it and the other end to the metal ground of the CRT (*NOT* a circuit board ground!) The metal grounding braid stretched over the CRT is a good place. Slide the screwdriver under the anode cap until it touches the metal terminal. You may hear a sharp crack as the charge is grounded. Take care not to scratch the CRT with the screwdriver.

Once these staeps are taken, the TV should be safe. However, still be cautious and be prepared for a tingle off the CRT if you touch the bare anode terminal- the charge has a habit of creeping back a little! I always try to keep a grounded screwdriver in contact with the anode as I'm removing the cap just in case, and avoid touching the anode wherever possible.

You might want to check the voltage across some of the higher voltage electrolytics if you want total peace of mind, but I find these are usually shunted by the circuitry and do not pose much of a hazard on modern TVs. No guarantees though, YMMV as they say! Just don't touch anything you absolutely don't have to, and certainly avoid touching the PCB or its components with both hands at once. That way there can be no electrical path across your body. IOW, if you must put your hand near potentially charged caps, keep the other hand in you pocket!

Dave

Dave

I used to work for Rediffusion years ago, and they were always really into safety research and training for engineers. One thing that they came up with was that it was very dangerous to zero-ohm a CRT cavity connector, as momentarily, a huge current flows. This, they said, caused micro-cracks in the bowl glass, and could lead to substantial reductions in the intrinsic strength of the CRT structure. I seem to remember a microscopic photograph of this appearing in the company magazine.

Soon after this, all workshops around the country, were issued with pukka discharging wands, which were made of a sort of hard cardboard tube, with some resistors inside, a filed down threaded rod at the business end, and a croc lead for grounding at the other - a bit like a typical EHT multiplier probe for your AVO back in the 70's.

I was never sure that I really believed it, and continued to discharge tubes in the ' traditional ' way by sticking a pair of crossed screwdrivers across the cavity to the 'dag. To counter the charge backwash effect, where the substantial charge retained by the CRT runs away from the discharge point to the corners, then quietly creeps back to have you just as you lift the tube off its mountings, I had an old EHT lead with poppy connector on one end, and croc clip on the other. You could clip this to the end of the 'dag grounding spring, to keep the tube tamed ...

Arfa

That's very interesting, and I suppose it makes sense if you think about it. You'd never discharge the mains smoothing cap by putting a screwdriver across it- it'd damage the capacitor, as well as send chunks of white hot metal flying around, so I suppose theoretically a CRT could be damaged by directly shorting the anode.

However, one has to look at the sheer number of techs who use a direct short to discharge CRTs, and the lack of CRTs suffering any visible effects. Perhaps Rediffusion were being a little over cautious! I suppose it would be a bad thing to do daily, but I expect the few times a TV usually gets its CRT discharged is unlikely to have that much of an effect.

Dave

That was pretty much my feeling also. However, I did one day get caught kicking off the neck of a B+W tube that I'd just replaced. I wasn't wearing any of the workshop safety equipment provided - facemask, apron, dustman's gloves. Worse, the person who caught me was the regional chief engineer, a person about 2 points to the left of God himself ... With him was my branch chief engineer. Worst of all, I was the branch safety officer .... !!

To say that I got the bollocking to end all bollockings, is the epitomy of understatement.

Oh happy days ! How did we ever survive without all the safety legislation that there is in the workplace now ??

Arfa