Re: 7904 - Very Dim Trace

 

Diagnostics on DC restorers are not much fun. Measurements require a
high voltage probe and component tests require a high voltage power
supply.

The most common failure is leakage in the high voltage capacitors and
diodes but since testing would require their removal anyway, it is
usually easiest to just replace them if a fault is suspected. New
parts are not expensive so if I had to order them, I would rebuild
both the z-axis and focus DC restorers.

The grid bias control and the output of the z-axis amplifier together
clip the peak-to-peak value of the high voltage drive from the
transformer. The clipped peak-to-peak voltage drives the charge pump
to produce the high negative DC output voltage. High frequency AC
from the z-axis amplifier it capacitively coupled to the high voltage
negative output.

Before messing with the DC restorer, I would verify that the output
from the z-axis amplifier is correct. This can be done with just a
voltmeter if necessary.

On 09 Feb 2016 21:23:08 -0800, you wrote:

OK - reading p 3-65 (Control Grid DC restorer) from the 7904 manual...

Starting to make sense...

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