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B&W TV RE: KB Royal Star; PVP20

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sideband
(@sideband)
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OK after a week of sweltering temperatures in the loft/workshop, I ventured up there again to a much more reasonable 24 degrees. The TV is working well but the tube is another matter. It's reasonable in the dark....actually very watchable although any text is quite blurred but any increase in brightness causes the classic silvery effect of a duff tube....in fact it almost goes into complete negative on really high-brightness parts. Not much more I can do really. I can borrow a BK booster/analyser but I get the feeling that anything I do will be very short-lived. The tube has probably already been boosted to death but you never know. It's not as though it will get daily use and I might even be able to find a better tube. I haven't actually checked CRT voltages yet so it might be worth checking A1 volts although I get the feeling that it will just prove what I already know.....!

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Topic starter Posted : 25/07/2021 8:42 am
PYE625
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If the CRT won't come good after a boost, then I am sure another one will become available at some point. At least this type is not as rare as some.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 25/07/2021 9:31 am
Katie Bush
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If the tube is fully goosed, is there any harm in contriving a means by which the heater volts can be increased? I'm not talking extreme overrunning since there is a reasonable picture in the dark, but say, half a volt or so above the baseline heater volts. Or, to find a reasonable level, take the heater out of the chain (temporarily) and feed it from a variable bench power supply so that with the set powered up, you can play around with various voltages until you can find a happy medium, then contrive to run the heater at that level? Unless of course, it needs a ridiculously high voltage to make a worthwhile difference. In which case, leave it as it is, I'd say.

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Posted : 25/07/2021 5:18 pm
sideband
(@sideband)
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Well I had already thought about that. You should really use a proper 'boost' transformer which has a low capacitance but for a test, I could use a 7.5 volt transformer that I have.

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Topic starter Posted : 25/07/2021 7:26 pm
Katie Bush
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Good point - I hadn't actually thought about the capacitance issue. I did initially think of batteries to power the heater for testing purposes. I just though the bench power supply gave infinite control.

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Posted : 25/07/2021 8:56 pm
Nuvistor
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@katie-bush 
Specially made boost transformers had low capacity and had normal taps and 25% boost. The normal tap was useful for CRT’s with heater  cathode shorts.

Frank

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Posted : 25/07/2021 9:44 pm
PYE625
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Posted by: @sideband

I haven't actually checked CRT voltages yet so it might be worth checking A1 volts although I get the feeling that it will just prove what I already know.....!

A wise move before finally deciding the CRT is worn out, you never know.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 26/07/2021 9:57 pm
Katie Bush
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Posted by: @pye625
Posted by: @sideband

I haven't actually checked CRT voltages yet so it might be worth checking A1 volts although I get the feeling that it will just prove what I already know.....!

A wise move before finally deciding the CRT is worn out, you never know.

Speaking of voltages, are we certain that the heater is receiving its fair share?

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Posted : 28/07/2021 5:04 pm
sideband
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Well the heater looks OK so I don't think there is an an issue but I will check the relevant area's when I next get back to it.

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Topic starter Posted : 28/07/2021 8:33 pm
sideband
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Oh well....as suspected, all voltages around the CRT are good, so in a nutshell, the CRT is knackered. I'm going to see if an old trade mate of mine still has his BK CRT tester/booster. If that doesn't work then hopefully I can find a good/better CRT from somewhere.

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Topic starter Posted : 01/08/2021 6:35 pm
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