Re: 475 Probe Calibration problem with x10 probes


Oct 23 #151898

I have two 475 scopes. Both have developed a similar probe calibration problem on >one channel. I am using the scope calibration signal. I have verified results with >multiple probes

First scope - Channel 1 probe calibration tests fine with x1 and x10 probles. Channel 2 >calibration tests fine with a x1 probe but the waveform has a sloped rise and fall time >when testing with a x10 probe on a vertical sensitivity setting of .5v/div and above.
Second scope - Channel 1 probe calibration tests fine with x1 probes but the >waveform is a spike when testing with a x10 probe with vertical sensitivity setting of >.5v/div and above. Channel 2 probe calibration works fine with x1 and x10 probes.

Has anyone seen this problem?

Mike N2MS
(quote doesn't seem to work so I put the little arrows in like Usenet, hope it is clear enough)

Unless those switches are dirty at many ranges or something it is not really likely they need to be cleaned and that is it. Therefore, troubleshooting is in order.

A look at the front end on the print tells that in .5V/div and up the X100 attenuator is switched in the circuit. It is bypassed at the more sensitive rages. I assume here that when you wrote "and up" you meant higher in V/div. because "sensitivity" could be expressed the other way, more sensitive being <V/div.

Also note that the X100 is the first in line so is most prone to damage by ridiculously high voltages. (that can happen when they DO NOT LISTEN ! - KEEP IT IN 10X AT ALL TIMES UNLESS YOU REALLY NEED THE GAIN. Better to fry the 9 meg or whatever resistance in the probe than the front end of the scope.

The print does not give details on the internal of the attenuators, and with further examination I find that they are a unit, encapsulated and everything. I'm sure you'll find plenty in the hen's teeth department.

As such, I say most likely there is a resistor in there that is blown open. I also say that most likely it simply goes from the input to the output. Its value should be calculable using the resistance of the output of the attenuator to common. (ground) Maybe 99X that measured value ?

What it is not saying is if all these attenuators are in one package. All but the X10 show no connection to common. (ground) This is more than unlikely so I suspect there is simply an attenuation "block" of sorts. You'll have to determine which pins need the resistor.

Either that or replace it/them. If there are separate ones for each channel at least you can make one good scope out of the pair. If there is only one for both channels you are up the creek.

Actually it occurs to me now that you HAVE a good channel, a precision ohmmeter will tell you the value of resistor you would need to jump it out in the bad channel. You'll have to build that no doubt, unless you know where to buy like a 9.04256 megohm or whatever it is. Be mindful of RF/EMI, keep the leads short and straight as possible.

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