Re: 475A : excessive jitter on delayed sweep


Alberto I2PHD
 

On 2019-04-16 1:08, Reed Dickinson wrote:
Hi Alberto:

What you see on the display is probably correct.  With your scope cold,
press the A push button and turn the A TIME/DIV knob such that two or
more cycles are shown.  Rotate the B TIME/DIV control two or three
positions past the A setting and press the A INTEN push button.  Adjust
the intensity to show the highlighted portion of the A trace, rotate the
DELAY TIME POSITION (DTP) control and watch the intensified portion
move.  Using the DTP control position the starting portion of the
highlighted A trace right on the rising edge, now press the ALT push
button, if all is working OK you should see the A trace start then
continue at the rising edge of the A trace but at the B rate.  Now press
the B DEL'YD button, you should see the portion of the highlighted A
trace swept at the B rate, you can move the display using the DTP control.
Hi Reed,

   thanks for your answer. Yes, that is exactly what I did to obtain what is shown in my photo Nr. 2.
All normal and all well understood.  But what I expected to see was a stable rising edge of the signal, as in the photo, taken with the 'scope cold, just switched on.
The problem is that when the instruments warms up, that rising edge is no more stable, the trace moves left and right, in a random fashion.
Clearly a problem related to the warming of the 'scope...

I will check, as you suggested, the voltages. There is a suspect tantalum capacitor, which I did not replace, namely C933, which is in the path that brings working voltage to the Delay Time Position control.... just as good measure, I will replace that also...

Thanks and regards

Alberto

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