EHT flashover in Bush ITV8401 repairable ????

  Diemmess 18:02 10 Aug 08
Locked

G'daughter's 18th today and her aging bedroom TV conked this morning and has probably passed the point of no return.

The set will switch on to standby, but any attempt at channel selection, or use of the standby switch on the TV or remote, produces a flash and a loud crack!

Well aware of the extreme dangers of EHT, stored charges and so forth, I removed the back section of the case and using its 'remote,' generated a fruity spark 10-14mm long with a suitable noise.

The spark jumps from the base of the connector to the screen, and what looks like the case (maybe grounded) of a high voltage capacitor.

A long handled artists paintbrush swept away dust but could not see any foreign bodies or surface damage.
I can ask a knowledgeable friend tomorrow but two questions please:

1. I could have expected silence or smells but what is going on?

2. Given the symptoms, is this TV repairable at reasonable cost in time and money?

  amonra 20:24 10 Aug 08

It sounds as though the line-output stage has gone out of control and generating excessive EHT volts to the tube. Can be repaired, but as usual with today's servicing, is it worth it ? This sort of fault could be expensive on parts, so think carefully before proceeding.

  Diemmess 08:56 11 Aug 08

Thank you amonra

Your post sounds very much on the right lines.
I have little grasp of even the theory of the extra high voltage supply to a CRT, it is probably curtains for this set.

Most local businesss will fix aerials and replace components in "young" electronic devices but I'm afraid in this neck of the woods they will not tackle a problem like this.
"Why not replace with this one sir..........?"

  amonra 13:06 11 Aug 08

One minor point while I think about it, while you had the back of the set off, did you notice whether the tube heater was lit ? It's not very bright so you may have to look closely. If the tube isn't lit then there is no current being drawn, hence high volts. If it is not lit, switch the set off, leave it for some hours, then pull the base socket off the tube and refit a few times just in case there's a bad connection there. Good luck...............

  laurie53 19:38 11 Aug 08

Quite apart from the personal dangers of which you're obviously well aware, do remember that an EHT arc is also a very good ignition source, and in a set that old there may well still be a lot of dust and crud you've not been able to get at.

  Stuartli 22:29 13 Aug 08

DON'T mess about in any way with a CRT TV - there are some very high voltages involved.

Even my best mate, who repaired televisions as a TV and appliance retailer, got a nasty belt on more than one occasion and he knew exactly what he was doing.

  Diemmess 10:04 14 Aug 08

Thank you folks.

Closure!

For amonra in particular.
I also had help from a distant friend with considerable knowledge and expertise.

I left it alone for 18hrs, then vac'd and brushed particularly in the EHT section where I did not find either carbonised tracking or any crack at all.

Whatever the fault is, I feel it will involve replacement of a component and that plus the labour and "knowing how to do it" make it silly to take this any further.

Just in case the base socket was making poor contact I did try re-doing the connection.
I was quite unable to see any sign of the CRT's heater working at all. I know the days are long gone from the early times when the cathodes lit the room, but whatever is wrong, the diagnosis is terminal trouble.

  laurie53 21:42 14 Aug 08

One of the problems when you think you know what you're doing is realising when you don't know what you're doing!

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