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Old 3rd Jan 2012, 12:47 am   #8
glowinganode
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Location: Aylesbury, Buckinghamshire, UK.
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Quote:
Originally Posted by WME_bill View Post
I suspect the extra resistor chain R85-R86 is a feedback mechanism to correct for main voltage variation, or to filter the grid supply, and shouldn't have much effect on your problem.

Bill m0wpn
Hi Bill, I've a feeling that these resistors set the time constant to give a similar decay of voltage on the grid as the cathode ie. to minimise the grid- cathode voltage, at power off.

Antony, have you checked RV56 for an open circuit track / wiper?
You say you have around +90V on eVT59, as long as this does not vary, then it's probably ok. Even if it isn't, this would cause a different (bright-up) problem.
In fact it should not vary, but the negative end of the "variable 1000V battery" should with adjustment of the brightness control.
Can you clarify what you mean about the operation of the brightness control "coming on suddenly and then no effect"?
As Bill says, be very careful taking voltage measurements around the CRT grids.
Rob.
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