Re: 475 Grid Bias problem


Jeff Davis
 

Hi Bill,


I'd be happy to make that check for you. I think I will do the CRT swap first, though. That will either confirm or deny the CRT in general as the problem source. However that works out, I'll lift the gun anode voltage (R1396) on the good unit and let you know.


I haven't replaced the 47pf cap. The parts haven't arrived yet, but more importantly, I think I've ruled out problems in the

Z-axis/grid circuit by virtue of comparison with a correctly working 475. That's why I'm going down the path of swapping CRTs. Feels like I've ruled everything else out.


Jeff


________________________________
From: TekScopes@yahoogroups.com <TekScopes@yahoogroups.com> on behalf of bill.spratt@verizon.net [TekScopes] <TekScopes@yahoogroups.com>
Sent: Tuesday, August 22, 2017 12:48 PM
To: TekScopes@yahoogroups.com
Subject: Re: [TekScopes] Re: 475 Grid Bias problem



Jeff, I believe that I may have found a problem with my scope but I'd like to know if it repeats with yours. Although I have a solid 50VDC on CRT pin 8 I don't believe that it's connected internally. I lifted R1396 which connects to the 50V supply and my problem didn't change at all. Additionally, while displaying a normal signal on the scope I tried connecting and disconnecting that resistor to that path without any visible change to the signal intensity (or any change at all). I was expecting to see the intensity decrease or disappear entirely. Of course it may be that that CRT pin is not intended to accelerate the beam so it may be normal for the tube to run fine without it

On your other good scope, would you try the same experiment to see if it also has no impact to the intensity? If it does change the intensity, would you verify that you can create the grid bias problem we're working on?


Also, did you ever replace the 47pf cap - C1326?


Thanks,
Bill

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