Focus Pin Corrosion

[toshiba tony]

Just a quickie please, why did the focus pin on colour crt's used to corrode, going back to the tv days in my past I can remember this happening. Many thanks.
Tony Walker

[Nuvistor]

A guess, I wonder if there was some corona around the pin under damp (condensation) conditions and once started the the ozone corroded the pin/socket.

It is an interesting question.
Frank

[Nicklyons2]

I would think largely an effect of damp combined with the high voltage. Sets with low voltage focus and sets used in dry conditions didn't, in my experience, suffer any problem. I used to look after many sets where the kitchen and living room were either combined or adjacent and, even with hybrid sets, when a dinner was being prepared - steaming pans etc, condensation could be enough to cause eht flashover; I've used many a tube of eht grease to that end.

[Nuvistor]

The older CRT's with larger bases did not, for me at any rate, cause too much of a problem. It seemed the smaller necked tubes, with very often smaller screens suffered more. Of course the smaller sets where probably in the Kitchen or bedroom where condensation would probably be more in evidence.
I only saw a few sets with the problem though, perhaps because I was out of the trade before portable colour sets became popular.

[Heatercathodeshort]

I remember this. I had quite a few do just this and it was difficult if not impossible to salvage the tube. I think it was the small black base used in the 110 degree in line tubes. J.

[Chris55000]

Hi!

Another point to note was that the original delta-gun CRT usually needed only about 3500V from what was a relatively medium-impedance supply, whereas the more modern in-line CRT often required 5500 to 7500V, (particularly towards the end of the CRT era) and the introduction of diode-split LOPTs did tend to mask the fact that the actual operating voltage was often much higher!

I suspect there's a threshold of about 4000-4500V where corona between the focus-anode connection socket and the actual CRT pin starts to become a problem - below that voltage, as in older CRTs, it rarely showed up unless the house was actually reeking with damp!

Chris Williams

[Welsh Anorak]

Hi there
Another reason might be the demise of the spark-gap. Once upon a time, if the house was damp then the gap would start to fizz happily before the pin corroded. I've had many of those little red polo-mint gaps disintegrate before the owner reluctantly called me in - often after a visit to the opticians!
Glyn

[HamishBoxer]

I certainly remember and the spark gap problems.