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Old 3rd Jan 2012, 6:01 am   #9
Anthony
Hexode
 
Join Date: Sep 2006
Location: Hertford, Herts. UK.
Posts: 338
Default Re: Cossor CDU110 'Scope: Brightness Probs.

Rob - thank you very much for your input.

I'll reply as below:-

Antony, have you checked RV56 for an open circuit track / wiper?

Yes both are good. Track is 2.37 vs. 2.2Meg. Wiper gives full and smooth resistance change. (RV53, Focus, also OK).

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"?

Fully CCW trace is blanked (Grid at -1117 (measured with 100Meg probe)), turning it CW, the trace appears suddenly (corresponding to -975V) and further clockwise rotation of the Brilliance control has little effect. So the Brilliance control is more of an 'on-off' control than continuous although the grid voltage variation is continuous.

As Bill says, be very careful taking voltage measurements around the CRT grids.

Point well-taken

Again, I really appreciate your thoughts. I feel a bit stumped at this point. Comparison voltages with a well-behaved model would be helpful. I do not understand the eVT59 voltage readings.

Anthony
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