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Old 3rd Jan 2012, 11:59 am   #10
glowinganode
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Join Date: Apr 2009
Location: Aylesbury, Buckinghamshire, UK.
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Hi Anthony,
thanks for the reply.
If eVT59 is sitting at +85V, this seems about right for bright-up condition. You say you do not understand the eVT59 readings, do you mean you don't understand how the circuit works, or why you are getting the reading you do?
Once you get to the point where the trace does not get any brighter, this is probably due to MR44 + 45 being forward biased and clamping g1 at around +1V wrt cathode.
Can you connect your dmm between g1 and cathode to confirm this? Connect the meter whilst switched off and use good insulated test leads, do not handle the meter or leads whilst carrying out this test.
If you're getting a normal reading, say -100 to +1v as you adjust the brightness control, then something else is limiting the brightness.
Is the +3kV supply holding up?
Regarding the trace you can get, try displaying a high amplitude, high frequency sine wave to try to fill the screen up as much as possible. How does the brightness compare to just displaying the straight line with no input?
Rob.
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