Re: 475 Grid Bias problem

Jeff Davis

Ugh. Yahoo Groups did a number on my careful table formatting, compressing all the spaces and reducing it to marginal readability. The essence is that the predicted grid voltage and measured grid voltage were within 3V for both the scope with the good working beam finder and the faulty one.

From: <> on behalf of Jeff Davis [TekScopes] <>
Sent: Sunday, August 20, 2017 3:07 PM
Subject: Re: [TekScopes] Re: 475 Grid Bias problem

Something about my analysis yesterday hasn't been sitting right with me, and I think I've figured out what isn't right.

I was basically hypothesizing that the peak to peak voltage applied to the grid bias clipping circuit was not high enough to be clipped at the highest setting of the grid bias control. This came from looking at the voltage drop across C1326. The problem with this analysis is that even if that were true, fixing it would not solve the beam finder problem. It would allow a larger value of the grid bias voltage to be set. The grid voltage out of the DC Restorer can be calculated as CATHODE_VOLTAGE - GRID_BIAS_VOLTAGE + Z_AXIS_VOLTAGE, so a larger bias voltage makes the grid MORE negative, which would have the effect of allowing one to make the trace even DIMMER than before - not brighter (given the same setting of Intensity).

To shed some additional light on this, I took the same measurements as I had previously on a 475 with a perfectly good working beam finder. I found that the voltage going in to C1326 was 310 V p-p (versus 300 on the 'bad' scope), and coming out was 233V (versus 231 on the 'bad' scope). So essentially the same. So much for my "bad C1326" hypothesis.

I then realized that I could test for correct operation of the DC Restorer by measuring the grid voltage directly with the beam finder activated. When beam finder is pressed, the horizontal amplifier is disconnected and the Z axis output is a DC voltage of about 24.5V. Since it's not varying (the blanking pulse is inhibited), one can use a DMM with a HV probe to measure the grid voltage directly.

I set the grid bias to the same value on both the good scope and the faulty one, and measured the voltage levels with the beam finder on. Here's what I came up with:


475 475

------------- ------------

CATHODE -2439 -2431 measured with DMM and HV probe

GRID BIAS 107 107 measured as peak voltage at anode of CR1373 (scope, 10X probe)

Z_AXIS 24.5 24.5 measured as minimum voltage at anode CR1373 with beam finder on

----------- -----------

PREDICTED GRID -2521 -2513

MEASURED GRID -2518 -2510 measured with DMM and HV probe

From this data, I'm concluding that the DC Restorer is the suspect 475 is operating correctly, that the grid voltage is where it should be, and that the cause of the beam finder problem is somewhere other than in the Z-axis / CRT grid circuitry.

Can you guys with more practical experience check my reasoning on this? If it's not completely faulty, I'm back to square one in terms of a hypothesis that explains the beam finder behavior. Any ideas?


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