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An experience fixing the ch1 vertical input of a Tektronix 475 oscilloscope

This page logs an attempt to fix a problem on my tek 475 scope.

The symptom applied to ch1 only and was the following:

Applying the calibrator signal to ch1 with a proven good probe, just rotating the Volts/div knob, I got the following displays:

50 mV/div	0,1 V/div	0,2 V/div

The middle one is clearly bad. Moving the knob or just moving the scope the shape of the wave changed sometime to something better, sometime to something worse.

Looking at the shape I though that the problem should have be related to some capacitor connected to the input, but only when in the 0,1 Volts/div. As I had the scope already open because I had just fixed the vpots problem, I noticed that an array of trim capacitors is really present for each of the vertical input selection frames. So I turned on my scope and started to lightly tick the trim caps of the ch1 array. I got it! Ticking on the first cap of the array (circled in the picture above) I was able to get the display instable. So I concluded that cap to be the cause of the problem.

When thinking on how to get the vertical sensitivity frame dismounted to get the cap out of the PCB, I fortunately (experience??) tried to pull the cap as if it was inserted instead of soldered. BINGO! Those caps are not soldered! Their leads are inserted in tiny bushes in the PCB; think of them like dual-in-line integrated circuit inserted in sockets. You can remove them just levering with a small screw driver: be careful, their leads are thinner that hair!

Just removing the cap, dropping some tiny drops of isoprophilic alcohol, reinserting the cap, solved the problem.

I don’t know if what I made is a definitive surgery. Until you don’t find any update here, is means it is still working. Today is 7/4/2001.

Ciao

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