Re: Tek 485 - Intensity at Max, Cannot Adjust Down


Tom Gardner
 

Nothing below contradicts your presumption that the DC restorer circuit is faulty.

Note that a month ago I hadn't looked at a CRT since school (not university) physics lessons.

On 21/03/16 16:36, otrfan01@... [TekScopes] wrote:

and the meter moves a very slight amount higher when measuring the grid.
That phrase makes me suspect all your measurements are w.r.t. ground. The critical voltage w.r.t. intensity is the difference between the cathode and grid, not the absolute voltage above ground. I put my handheld DVM directly across the cathode and grid, so I measured Vkg and did not have to compute Vk-Vg.

Note carefully I am not saying going anywhere near -2450V is safe; use your judgement and exercise extreme caution. Not being near the DVM/scope/leads when the scope was on made me feel less apprehensive :)

I see in your tests that the grid was driven -50V wrt the cathode, which pointed towards the bad CRT. My results seem to lead me back to the DC Restorer circuit.
I am led to understand that is a traditional problem.

Since the caps are expensive and hard to find, I replaced the cheap easy HV diodes. When that didn't improve anything, I started looking at changes in Vkg (average measured with DVM not a scope!) with changed intensity control and changed timebase/blanking waveforms. Proportionate changes in Vkg lead me to suspect the DC restorer was working, and hence the problem had to be in the CRT itself.

Thanks again for your help!
This group was very helpful to me; I try to return the favour.

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