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Electronics => Repair => Topic started by: beanhauler on January 24, 2021, 09:37:42 pm

Title: Tektronix 2213 Intensity control
Post by: beanhauler on January 24, 2021, 09:37:42 pm
Would be very grateful if anyone has a visual of a waveform on test point 19 of the circuit of the intensity control of a Tektronix 2213 scope.
Originally the intensity control did not work and it was maxxed out and looked around the z driver stage to see if anything was wrong there.
Noticed that there was no incoming intensity control square wave that was supposed to modulate the z drive.
I have definitely the 20khz from the power transformer that is around 100V pk to pk but no incoming square wave from the x driver or timebase as was missing.
Checking further i found CR868 was short so replaced it with a fast diode that will handle an amp and suitable reverse voltage.
Now when i powered it up it had a beam that just gets fainter and fainter and disappears.
My guess is that the grid is now charging via the changed diode and pinches off the beam current. It would seem that the missing waveform could be the key.
The duty waveform arrives on pin 11 of U825B and is also the control switch which switches between pin 12 @gnd and the intensity control setting.
This exits on pin 14 and after being integrated by the RC combination should according to the waveform be seen on pin 3 going back into the switch[mux] which i do not see. Neither do i see it on pin 4 which is hard switched by the logic around the 14503.
Is it likely the 14503 is dead? Anyone had any experience of the intensity controls on the 2213?
Funny thing is that text in the manual makes mention of the "average" voltage but no direct referrals to the waveforms itself. does the switch waveform
with its fall to ground get replicated in the following Op Amp.
Any info or suggestions on this appreciated.