Unusual TV fault - UK Vintage Radio Repair and Restoration Discussion Forum

Peter.N. · 26th Nov 2009, 1:30 am

As many members seem to like a puzzle to solve, I thought you might like this one. I concerns a Pye VT2 12" bakalite TV, not recently I must add, this was when the set was fairly new. The customer complained the the picture was fading out after ten or fifteen minutes, but of course it didn't do it when I was there when I was there, eventually though it did, the picture did indeed fade out slowly. I'm sure many of you will immediatly sus the cause, but bear this in mind, it has a series heater chain and the rest of the set remained working. I have only ever had this precise fault once and would be interested to see if anyone else has come across it.

Peter

ENGLISH VICTOR · 26th Nov 2009, 8:00 am

V7 sync sep intermittent heater to cathode short, or intermittent crt heater s/c. or perhaps a nasty 1000pF hunts tubular de-coupling cap from the heater line to the crt base intermittent s/c. My circuit does not show one but I had a similar fault with a different early Pye. Also there is a point where the crt heater lead attaches to the heater chain on the main chassis there might have been a wisker of wire shorting to earth there.
Victor.

Peter.N. · 26th Nov 2009, 10:05 am

All very logical but no, you'r getting warm with the last sentence though.

Peter

ENGLISH VICTOR · 26th Nov 2009, 10:09 am

Peter,
well I can only imagine that the heater wire to the crt, brown I suspect must have been trapped under the chassis or a piece of metal from a tag or similar and the plastic insulation damaged causing a s/c to earth when the set warmed up expansion or something!!
Victor.

Peter.N. · 26th Nov 2009, 10:24 am

No, but I will give you a clue, when I took the CRT base connector off the rest of the set continued to work! Temperature change was the cause but as I say this is the only time I have seen this fault and it may never ever have happened again. Have one more try.

Peter

ENGLISH VICTOR · 26th Nov 2009, 10:51 am

The metal clamp that holds the wiring going to the crt base to the chassis may have pinched the heater wire. If there is one. The clamp I expect would have been anchored by a single s/t/ screw. As the chassis warmed up the wire's insulation may have softened permitting a s/c This you realise is just a pure guess. I doubt the fault was at the crt base because removing the base I would imagine would have mechanicaly moved the wire position and removed the s/c and I doubt the base was faulty but anything is possible.
Victor
P.S. Thanks for entertaining me.

Peter.N. · 26th Nov 2009, 12:46 pm

Nope. I will put you out of your misery. To find the fault I had to peel back the top layer of paxolin on the base to reveal that where the pin sockets had been pressed into the second layer and the tops splayed out, rivet fashion, the two heater pin sockets were virtually touching, probably with no more than a thou between them, once warm they were touching hence the the short remaining when the base was removed.. I suspect that was a once in a lifetime fault - unless anyone knows different.

I will post another one if you feel energetic

Peter

ENGLISH VICTOR · 26th Nov 2009, 12:57 pm

Please do.
Victor.
P.S. Bedtime here now.

26th Nov 2009, 1:30 am	#1
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Unusual TV fault As many members seem to like a puzzle to solve, I thought you might like this one. I concerns a Pye VT2 12" bakalite TV, not recently I must add, this was when the set was fairly new. The customer complained the the picture was fading out after ten or fifteen minutes, but of course it didn't do it when I was there when I was there, eventually though it did, the picture did indeed fade out slowly. I'm sure many of you will immediatly sus the cause, but bear this in mind, it has a series heater chain and the rest of the set remained working. I have only ever had this precise fault once and would be interested to see if anyone else has come across it. Peter

26th Nov 2009, 8:00 am	#2
ENGLISH VICTOR Hexode Join Date: Feb 2008 Location: Sydney, Australia Posts: 356	Re: Unusual TV fault V7 sync sep intermittent heater to cathode short, or intermittent crt heater s/c. or perhaps a nasty 1000pF hunts tubular de-coupling cap from the heater line to the crt base intermittent s/c. My circuit does not show one but I had a similar fault with a different early Pye. Also there is a point where the crt heater lead attaches to the heater chain on the main chassis there might have been a wisker of wire shorting to earth there. Victor. Last edited by ENGLISH VICTOR; 26th Nov 2009 at 8:18 am.

26th Nov 2009, 10:05 am	#3
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Re: Unusual TV fault All very logical but no, you'r getting warm with the last sentence though. Peter

26th Nov 2009, 10:09 am	#4
ENGLISH VICTOR Hexode Join Date: Feb 2008 Location: Sydney, Australia Posts: 356	Re: Unusual TV fault Peter, well I can only imagine that the heater wire to the crt, brown I suspect must have been trapped under the chassis or a piece of metal from a tag or similar and the plastic insulation damaged causing a s/c to earth when the set warmed up expansion or something!! Victor.

26th Nov 2009, 10:24 am	#5
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Re: Unusual TV fault No, but I will give you a clue, when I took the CRT base connector off the rest of the set continued to work! Temperature change was the cause but as I say this is the only time I have seen this fault and it may never ever have happened again. Have one more try. Peter

26th Nov 2009, 10:51 am	#6
ENGLISH VICTOR Hexode Join Date: Feb 2008 Location: Sydney, Australia Posts: 356	Re: Unusual TV fault The metal clamp that holds the wiring going to the crt base to the chassis may have pinched the heater wire. If there is one. The clamp I expect would have been anchored by a single s/t/ screw. As the chassis warmed up the wire's insulation may have softened permitting a s/c This you realise is just a pure guess. I doubt the fault was at the crt base because removing the base I would imagine would have mechanicaly moved the wire position and removed the s/c and I doubt the base was faulty but anything is possible. Victor P.S. Thanks for entertaining me. Last edited by ENGLISH VICTOR; 26th Nov 2009 at 11:06 am.

26th Nov 2009, 12:46 pm	#7
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Re: Unusual TV fault Nope. I will put you out of your misery. To find the fault I had to peel back the top layer of paxolin on the base to reveal that where the pin sockets had been pressed into the second layer and the tops splayed out, rivet fashion, the two heater pin sockets were virtually touching, probably with no more than a thou between them, once warm they were touching hence the the short remaining when the base was removed.. I suspect that was a once in a lifetime fault - unless anyone knows different. I will post another one if you feel energetic Peter

26th Nov 2009, 12:57 pm	#8
ENGLISH VICTOR Hexode Join Date: Feb 2008 Location: Sydney, Australia Posts: 356	Re: Unusual TV fault Please do. Victor. P.S. Bedtime here now.