Re: 475A : excessive jitter on delayed sweep


Reed Dickinson
 

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.

As to the 'hot jitters', with the scope hot, carefully measure each power supply with a good DC VOM, if you see no variation use another known good scope to view the various supplies, if you don't have another scope switch the VOM to AC and, observing polarity, connect a 10uF, 200VDC cap in series with the positive lead of the VOM, you are looking for low frequency variations in the supplies and the cap is used to block the DC component.  Once you find a supply that is varying the VOM reading, troubleshoot it and continue.  I have found that the 741 op-amp IC in the power supply is frequently at fault, a cheap fix!

Good luck!

Reed Dickinson

press the

On 4/15/2019 3:06 PM, Alberto I2PHD wrote:
I fixed my 475A by replacing five tantalum caps with quality electrolytics. So far so good.
But now a new problem has surfaced. Let's see if I am able to explain it correctly, with the aid of a couple photos...

I send a 1 kHz square wave signal to the 'scope, set the A sweep to 50 us/div, triggering on the negative slope of the square signal.
Doing so, I see on the screen only the lower half of the cycle (50 us/div * 10 div = 500 us, half cycle).
Then I set the B sweep at 1 us/div, "Start after the delay" (important...)

Pressing "A intensified" I see this, which is what is to be expected :

http://i2phd.org/public/A-intensified.jpg

Now I press "B delayed", and what I obtain is this, BUT ONLY AFTER A COLD START OF THE 'SCOPE :

http://i2phd.org/public/B-delayed.jpg

As long as the 'scope is cold, the image is very stable. After about ten minutes, when the 'scope has had time to warm up, the display is jittering badly, going left and right, even beyond the borders of the screen...

It is clearly a problem related to temperature... Has anybody words of wisdom about which components could cause, when warning up, this problem ?

TIA

Alberto







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