EHT rectifier change question.

[lloydwells]

This will likely sound stupid to most of you but I'm currently working on my first vintage tv set and I'm basically only used to radios. I now have the set up and running however due to various picture faults the state of the EHT rectifier has been called in to question. As TV valves are generally cheap i've shelled out three quid on a NOS one. Now my dear departed grandad was a TV fettler and used to terrify the hell out of me as a child by discharging the anodes on CRT.s. It occurs to me this valve is in some way attached to the anode cable therefore do I need to discharge the CRT before changing it?

[ENGLISH VICTOR]

Lloydwells, where to start. I would strongly recommend you discharge the CRT Anode to chassis after first removing the mains lead from the wall socket. Having done that I will take the liberty of suggesting you find an early book on basic TV repairs, circa 1960 would be a good start, I am presuming you have some radio repair literature as you say you are familiar with radio repairing. If the latter is not correct buy a suitable radio repair guide, there are plenty around and will possibly help with some basic but essential theory and safety techniques.
It might also be a good idea to find a local retired radio/TV service man who is prepared to part with some of his hard gained experience. The new breed of repairers may be quite unfamiliar with component level servicing let alone valves.
Good luck.
Victor.

[dtvmcdonald]

To start, be careful. The "usual" procedure for EHT rectifier removal is first to inspect the EHT area itself to see if you can get your hand in without coming close to anything (especially if a colo(u)r TV!). If so, the plate cap of that tube is not at EHT, so remove and and discharge the cap to ground (it is at HT). Then wave your grounded discharge probe around the rectifier base, and if easily reachable, all the base pins. If you touch all the base pins ... and they are not potted ... that will discharge the CRT. If not, carefully remove the tube, holding onto just the part near the plate cap. Then you can discharge the CRT by grounding all the contacts on the now exposed tube socket.

Of course, you can try discharging at the CRT connector. If the wire is exposed (no rubber cup) this is easy and safe, and is the way to go. But at least in the US, the wire is exposed only on small sets with EHT under 9 kV. The real killers use the rubber cup. Trying to get the cup off of an undischarged tube is a problem! I personally,unless there are exposed EHT points in the EHT cage (which there frequently are not in 1940s-1950's B&W sets in the US), discharge by removing the EHT rectifier.

If you HAVE to discharge in a war with a rubber cup, ground a screwdriver with a long plastic handle, then use one of those long plastic screwdriver tools you use on IF can ferrite slugs with slots on them to lift up the edge of the cup, and slide in the grounded screwdriver. My strong advice is DON'T try lifting the cup with something metal ... I've scratched too many (like three) cups or tubes with a screwdriver ... and had corona problems forever after because of the scratch.

[lloydwells]

Thank you for replying guys only 200 other people had bothered looking at it LOL. I'm quite capable of discharging the tube safely and will now do so. What I find quite astounding is the amount of people who apparently read this query and didn't reply. Things like this may seem like a stupidly easy question to someone that's ex trade and been repairing valve sets for fifty years but some of us weren't around in that era and if nobody tells us we're going to struggle keeping these things going.

[Nuvistor]

The question was answered in the second post with more advice on various books to help. 200 extra replies saying the same thing seems a bit over the top.
Good luck with the repair.
Frank

Valve rectifiers and circuits can stay charged for days, semiconductor ones discharge in a few minutes. Even then, discharge before touching. Worst case a bang and flash, best bit YOU survive. When working on anything live, left hand IN POCKET (so it can't grab anything else). A bit dramatic and I hardly ever use capitals in a post, in this case I think it is justifiable. It must be 20 odd years since TVs in service had valve rectifiers leaving the newcomers to restoration possibly unaware of their dangers.

I am not a heath and safety rules freak by any means, think and stay safe, thought has saved more lives than any rules. I am sure that every member here would have a 'daft' question asked than loose a member.

[Lloyd 1985]

Originally Posted by lloydwells

What I find quite astounding is the amount of people who apparently read this query and didn't reply.

Sorry, one of those was me My excuse is my laptop is being moronic at the moment with Firefox crashing excessively and even Windows is having a go at it now... By the time I was back up and running, there was a good reply to your question.

One thing I would add from a little experience I had with an Ekco TV is to look closely at how the EHT rectifier valve is fitted, the Ekco in question had the valve perched on top of the LOPT housing, with a little plastic cap over the top of the valve, and a metal cap over the end where the base of the valve fitted. It turned out that the metal cap, which is in free air when the back of the TV is off, not insulated in any way at all, is at EHT potential when the TV is on! The connection to the anode of the CRT is just the end of the wire stuffed into this metal cap. It did make it really easy to discharge the CRT though, just bridge that metal cap to chassis.

Whenever I'm going to be touching anything that may be connected to the anode of a CRT, I always discharge it first, just to be sure it's dead. I've had one too many nasty nips from it, one whilst actually handling a CRT I'd removed from a set, surprisingly I managed not to launch the tube across the room!

Regards,
Lloyd.

[lloydwells]

Thanks very much for the replies it could possibly be an idea to have a sticky how to thread at the top of this part of the forum. I'm not daft enough to poke a finger in anything until I'm absolutely sure it's dead, others might be.

[Tractorfan]

Hi,
Bear in mind that a a CRT, capacitor,or other other similar device can build up another charge after being 'discharged' the first time. I'm told it's a good plan to put a shorting link across such devices if they're being left lying idle.
Cheers, Pete.

[TonyDuell]

The CRT _is_ a capacitor here. The final anode (the electrode to which the EHT is applied) is an aquadag coating on the inside of the glass flare. Another coating on the outside is connected to the chassis. They form the EHT smoothing capacitor with the CRT glass acting as the dielectric.

My experience (albeit mostly with more modern devices) suggests that if the set is working correctly, the beam current will mostly discharge the CRT at switch-off. But if for some reason there is no beam current, then the CRT will stay charged.

More modern/high end TVs, and especially monitors, may have a bleeder resistor across the EHT supply. It is never a good idea to rely on that, though.

Often a semiconductor rectifier will have enough reverse leakage to discharge a CRT if left for a few days (at most). But a valve rectifier, as here, is a very good insulator, and won't. As others have said, the CRT is connected to the base pins (cathode) of the EHT rectifier valve, the top cap of the valve goes to the line output transformer, and is not 'live' when the set is switched off. You can often 'get away' with plugging in a new valve without discharging the CRT, but I wouldn't risk it. And of course if you have a soldered-in rectifier valve like an EY51, the filament leads _are_ connected to the CRT and thus 'live' until the CRT is discharged.

I don't like discharging CRTs by drawing a spark. I was told this can damage the connection between the final anode aquadag and the connector. On semiconductor-based sets it can blow devices all over the chassis if you are unlucky. I use my EHT meter (an old Heathkit one, just a high voltage 800M resistor and a 50uA meter in series) for this. It draws enough current to discharge any reasonable-size CRT in a minute or so.

[Maarten]

Originally Posted by merlinmaxwell

Valve rectifiers and circuits can stay charged for days, semiconductor ones discharge in a few minutes.

There is some correlation, but please don't use this as a rule of thumb as you will get shocked sooner or later if you rely on it!

Discharge rate mostly depends on the presence of a bleeder resistor (and whether it hasn't gone open circuit!). Incidentally, triplers and DSTs often have one built in and valve rectifiers mostly haven't which is why this rule often works.