Re: Pease help diagnosing my 2235


Kevin Wood G7BCS
 

3. The DC Restorer circuitry is not working properly. Kevin, I'm
trying to find reference to the blanking pulse during retrace--Is this
excerpt from pg. 3-17 it?

* "Fast-rise and fast-fall transitions of the Z-axis output signal
are coupled to the crt control grid through C855 to start the crt
writing-beam current toward the new intensity level. The Dc Restorer
output level then follows the Z-axis output voltage level to set new
bias voltage for the crt control grid."
Reading this stuff is really making my head smoke!
Yep, that's it. You can regard it as a network that turns the Z axis
signal (let's say it varies from 0-100V for the sake of argument. I'm not
sure on the exact range) into an AC signal with an amplitude of 0-100v.
That signal is then coupled into the cathode circuit where it's rectified
again, so it appears as a -2000V to -2100V signal. C855 bypasses it to
improve the transient response.

So here's my dilemma: The manual lists a bunch of equipment needed to
make adjustments, none of which I can really afford. My entire
test-bench consists of a Fluke 77 DMM and a broken Tektronix 2235.
Based on the above assumptions I've ordered a Fluke 80k-6 probe to check
the CRT voltages and now I'm planning to get an ESR meter to check the
caps in and around the PS.

Is there any other piece of equipment I absolutely need to have to make
this diagnosis?
I wouldn't say so at this stage. The DC restorer just needs the diodes and
resistor values around it checked. That can be done with the fluke.

Only thing that might be handy is another scope to check for ripple on the
power supply circuitry although if the ripple is not present when
measuring the calibrator signal it makes me wonder if it's the ground
connection in your probe that's the problem.

Kevin

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