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Old 21st Feb 2021, 3:21 pm   #1
Niechcial,Steve
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Default Self Igniting Valve Holder!

This was a new one on me. I was working on an HMV 1816 from 1952 which started making a loud intermittent clicking noise with a bright white spot on the picture. Pretty quickly, the clicks and spots became frequent and rapid, looking and sounding very much like the old car ignition interference on 405. Finally all was revealed when the valve holder for the frame o/p valve (ECL80) spectacularly burst into flame! Like most of the valve holders on this set, this one consisted of two pieces of paxolin rivetted together with the valve pins sandwiched between them. I presume the paxolin carbonised over time and eventually became conductive. Interestingly, the fire was on the inside, between the two layers, rather than on the outside. When you look at the outer sides there is not much sign of carbonisation. I attach two photos, but I don't think the effect shoes up very well. As I say, nearly all the valve holders fitted are of this type save the LOP valve and boost diode. They are probably the hottest running valves and they sit in more substantial plastic valve holders. I wonder if that is because HMV were aware of the limits with paxolin holders? I have known carbonisation of paxolin printed circuit boards cause many strange problems, but I've not seen it on free-standing valve holders. Replacing the valve holder was a real pain because as usual the pins were in a different relationship to the mounting holes. The set itself produces a lovely picture.
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Old 21st Feb 2021, 4:02 pm   #2
mark pirate
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Default Re: Self Igniting Valve Holder!

These are very low quality valve holders, I have had connection problems with my HMV 1870 chassis sets, though luckily no burnups!
I have had to clean & re-tension several holders in the RF strip, great care is needed, as I have had the tags snap off with little provocation.


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Old 21st Feb 2021, 4:21 pm   #3
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Default Re: Self Igniting Valve Holder!

Yes seen it before. Bit surprised at the humble ECL80 holder though. Normally higher power valves could cause this but I suppose the pulse voltages could have been a lot higher than the DC voltages. We always used to view these holders with suspicion for intermittent flickering, crackles and things like that.....changed many of them.

They might be low quality but they've done pretty well to last this long....
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Old 21st Feb 2021, 6:02 pm   #4
Niechcial,Steve
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Default Re: Self Igniting Valve Holder!

Yes when you pull them apart, the actual surface area that is in contact with the valve pin is tiny. As you say, I reckon it's the frame flyback pulse wot done it.
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Old 21st Feb 2021, 6:56 pm   #5
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Default Re: Self Igniting Valve Holder!

I’ve seen this type of burnt valve holder several times during my 60 years of service work. Often caused by switch cleaner which leaves a residue that gets between the layers and gradually tracks across. Back in the days when they were common it was usually the valve holder that was choice of many manufacturers because of price.
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Old 22nd Feb 2021, 8:59 am   #6
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Default Re: Self Igniting Valve Holder!

Yes, it certainly got worse after the application of switch cleaner. Does it happen just with high voltage pulses or more generally?
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Old 22nd Feb 2021, 9:08 am   #7
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Default Re: Self Igniting Valve Holder!

Mainly pulse voltages and heat. I don't think you'll find these holders used for line output valves or boost diodes......asking for trouble!
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Old 23rd Feb 2021, 1:40 pm   #8
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Default Re: Self Igniting Valve Holder!

An O/C in the frame linearity feedback loop can produce very high pulse voltages between the anode pin and heater of the frame output valve.

Paxolin was a great invention for the radio trade but could break down with spectacular effects. The GEC IF power supply plug in the 300 series the early symptoms of which was a low level crackling on sound followed a few weeks later by a massive crack of H.T. to the earth/grid pins of the B9A plug. The radio/chassis plug in the Ekco TS105 series suffered a similar fate.

EMI employed valve holders that they probably picked up on the street corner. They were very low quality and suffered heavy losses in TRF and Superhet I.F. strips. The HMV 'Highlight' series is a much better chassis than the 1807 series. They work quite well once you sort them out. Regards, John.
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Old 23rd Feb 2021, 6:41 pm   #9
Niechcial,Steve
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Default Re: Self Igniting Valve Holder!

Thanks John. Yes this set has a regun tube and the picture is pretty good. Having changed all the IF decoupling caps it now needs some re-alignment to maximise the definition, but I think it will perform as well as anything of the era. I'll post some pics when I have done it.
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Old 23rd Feb 2021, 6:52 pm   #10
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Default Re: Self Igniting Valve Holder!

Those cheap-and-nasty Paxolin 'wafer' valveholders are always a liabilty: in the past I've had 'creep' leakage in their carbonising parts cause strange fizzly background noises in SW receivers, and a major burnout when one of them used to house a power-rectifier with 700V between the anodes decided to fail.
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