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Tech Chat [Sticky] Determining/Understanding Why a B&K Reads B&W Tubes as Low.

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Cathovisor
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Posted by: @crustytv

especially remembering that the B&K is not the best device for reading old B&W tubes.

Without me finding mine and dismantling it, does anyone know what the FSD of the meters are in a 467? In older CRT testers (e.g. Radar) they were typically 250µA. I'm just wondering if this is what we are dealing with in a B&K - from memory, the meter in a Leader has compression applied towards the upper end of the scale for high-current cathodes which may explain why it's a better bet for later mono tubes, where they're more sensitive before the compression kicks in.

 
Posted : 03/05/2022 11:24 am
crustytv
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I have some spare meters, so hooked up a test to determine the FSD.

FSD, as good as 200µA.

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Posted : 03/05/2022 11:46 am
Cathovisor
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@crustytv Thanks for that 👍 So the next question is - are they shunted in a 467? The point I'm making (badly) is that the older Radar (and similar) testers used the meter without a shunt to measure the current between grid and cathode; modern colour CRTs were capable of higher cathode currents as we know so I can't believe there weren't shunts involved.

 
Posted : 03/05/2022 12:10 pm
crustytv
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I can't be sure as I'm having trouble deciphering the cct, but I believe when switched to setting 4, (emission test) 100k is brought into cct for the cathodes.

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Posted : 03/05/2022 12:30 pm
Cathovisor
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Reading the service manual that's available on the Web, it suggests that the meters are calibrated to read 0-2mA on emission. The beam current on even a good older mono tube is but a fraction of that. Maybe that's the issue?

 
Posted : 03/05/2022 3:56 pm
crustytv
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Interesting Mike, I sort of feel a little vindicated now for all my bangings on about not using a B&K to test B&W tubes. I had nothing technical to offer other than hearsay to back it up, even though it was relayed to me by the TV engineer I bought the tester from. He stated at the time not to use it to test old B&W tubes and to stick with using my leader LCT-910 for them.

Maybe all those that dumped their B&K 😉 , might now reflect upon these findings.

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Posted : 03/05/2022 4:42 pm
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