2236 Focus range.


Dave Peterson
 

Hi All,

I'm running through calibration on a 2236. Everything's going great. But something I noticed is that the focus is at the extreme CCW end of the FOCUS knob/pot.

I'm getting a fairly good trace line. Not as good as my bench 465 (I can just about hide the trace behind a graticule line on that), and not quite as good as my 2236A. But even the 2236A isn't as good as the 465. I can't help but wonder if the focus could be/would be better if the adjustment weren't at the end of its range.

I've gone through the power supply and CRT sections several times making sure voltage levels are staying good, setting a proper grid bias, geometries, and astigmatism. The cathode voltage is within spec at -1933v. It started at -1934v this morning, and the scope's been on all day. So pretty stable. All the other PS values are within mVs from this morning too. When I circled back to the grid bias adjustment this evening it needed a tiny tweak, but hardly anything that suggests a larger problem with the grid circuit.

I got through the vertical section of the calibration today, and all is well. The measured BW is around 130MHz. Even the 2mV/div BW was about 118MHz (spec is 90MHz). The focus is reasonably good - again, I don't know that it would get better with more range on the knob. But I can't help but think that the adjustment range should be closer to center, and that it might be indicative of a problem. Something marginal on its way to failure?

When I look at the schematic nothing really jumps out as me as a likely problem, but the focus circuit appears powered by the HV multiplier output. Could the anode voltage be sagging? How do I check that? I don't mean go buy an HV probe - I've done that. I mean, how do you safely access and measure the anode voltage? It's all sealed up, and I'm not keen on the idea of piercing insulation or jamming sharp things into the anode connector. Do 'yall just unplug it and use a wire or something to measure the socket inside the female side of the connector?

And I haven't seen what it should be. What should the anode voltage be for a 2236? What are it's specs? I have a physical manual, but I'm not completely familiar with it - I don't know where it would tell me what the anode should be, and what its limits are. It's not part of the calibration procedure.

Any help is appreciated. I need to call it a day and get some sleep. Thanks all.
Dave

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