Re: 475 Grid Bias problem

Jeff Davis

Bill, Fabio, David,

I swapped out the CRT tonight with a known good one, and the problem went away. So much so that with the grid bias at minimum, the intensity control could not turn the trace off completely! With the grid bias cranked back a bit, intensity control and the beam finder work just fine.

Fabio, before I swapped CRTs I checked whether I could get a dot in X-Y. I could, with suitable settings of grid bias and intensity.

Bill, I lifted the 100 ohm resistor R1396 to remove +50V from the 2nd grid that I call the gun anode. I didn't really see much if any difference from doing so. The beam finder still worked, and the trace wasn't visibly dimmer. Not sure I understand why - need to read up more on CRT guns. But I suspect that +50 V sandwiched between two roughly -2.5KV grids doesn't add much acceleration. It's probably there for some other purpose - but I'm only guessing at this point.

So I'm tentatively concluding that there was a CRT problem. One possibility I haven't accounted for is a bad connection between the tube socket and tube pins. To check that, I'd have to swap CRTs back again. I'm definitely not up for that tonight - we'll see if tomorrow I get a gumption refill. Bill, maybe you might want to wiggle / deox that connection and see if it helps - it's an easy thing to try.

Thanks for all your help, suggestions, and analyses. I've been learning a lot in the process.


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