Re: [Tek 485] No intensity control


Ozan
 

Hi José,

I definitely cannot control intensity below 20ms time base.
Voltage at collector of Q1548 is -15.01V in Normal mode and -61mV in X-Y mode
I have a guess of what is happening: The circuit around Q1544/Q1546/Q1548 limit beam current to ~ 5uA in XY mode and in slower than 20ms sweep time, otherwise limit is ~ 25uA. In your case it looks like even with minimum intensity beam current is at ~ 5uA. Then intensity control doesn't work because it can't go above 5uA. However, in other modes limit is 25uA so intensity control can make a change.

This could simply be an issue with wrong "GRID BIAS" setting. Please confirm whether you see a dot in XY mode at minimum intensity and you can't turn off even with "GRID BIAS". If you can't turn off the dot the issue is in DC restorer of grid bias. If you are able to turn off the dot with "GRID BIAS" you should gain control of the intensity and collector of Q1548 go to ~ -15V.

To debug grid bias (if needed): When you set "GRID BIAS" adjustment to max (schematic doesn't say which direction) do you see +120V at cathode of CR1660? With +120V at cathode of CR1660 and +8V at "Z OUT DC", grid bias DC restorer should put enough negative grid bias (~ -100V) to turn off the beam. If you have a high voltage probe you should measure grid voltage (top of DS1669) ~ 100V more negative than the cathode voltage (bottom of DS1668) with max "GRID BIAS" setting and min intensity.

I read that you debugged an issue with grid bias DC restorer. There could still be an issue remaining.
Ozan

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