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Old 30th Dec 2011, 6:02 pm   #1
Anthony
Hexode
 
Join Date: Sep 2006
Location: Hertford, Herts. UK.
Posts: 338
Default Cossor CDU110 'Scope: Brightness Probs.

Is any one familiar with this instrument? The trace on my work-horse (15 years with me) has been progressively more out of focus for the last few weeks. I have cured it by replacing a couple of 2.7Meg resistors which had gone high adjacent to the focus control.

Although there is now a nice sharp trace, the trace intensity is now very poor. The 'Brightness' control kicks in suddenly from nothing and then has little effect on the brightness. Cathode voltage appears fine at -1kV but I cannot achieve the manual's spec. of 0.45-0.65 volts across R50 by tweaking the 'Set Beam Current' 5K wire-wound pot (RV50). R50 and RV50 are spot on. +100V at one end of RV50 is also spot on. Nothing else obviously wrong except burning on the board (long-standing) around one joint of R51 which itself reads fine. Have rewetted that joint. Also recapped C50 to no effect.

I must confess to not understanding how the 'Brightness' control works on this insturment. Have not (yet) attempted to measure the +3kV EHT.

Any ideas? Will be thankfully received.

Anthony
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