Author Topic: TDS 644A probe gain compensation fails  (Read 1290 times)

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Offline resistor

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TDS 644A probe gain compensation fails
« on: September 29, 2014, 03:02:41 pm »
I recently finished recapping a TDS 644A.  The startup self-check now passes, as does the signal path compensation.  But when I try to perform a probe gain compensation cal operation, it fails.  Interestingly, it only fails when my probe is in 10x mode.  1x mode appears to work.  I'm using cheapie 100MHz probes from eBay.

Question: Is this some lingering issues with the scope, or is it just that my probes are too crappy?  If the latter, is there any way to set the attenuation factor in the scope's software manually?
 

Online nctnico

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Re: TDS 644A probe gain compensation fails
« Reply #1 on: September 30, 2014, 09:14:03 am »
Maybe you missed the caps in the calibrator circuitry. IMHO I would skip the gain calibration.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline macboy

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Re: TDS 644A probe gain compensation fails
« Reply #2 on: October 01, 2014, 10:34:13 pm »
I recently finished recapping a TDS 644A.  The startup self-check now passes, as does the signal path compensation.  But when I try to perform a probe gain compensation cal operation, it fails.  Interestingly, it only fails when my probe is in 10x mode.  1x mode appears to work.  I'm using cheapie 100MHz probes from eBay.

Question: Is this some lingering issues with the scope, or is it just that my probes are too crappy?  If the latter, is there any way to set the attenuation factor in the scope's software manually?
It fails in 10x because the scope does not know that you have a 10x probe. See all those contacts surrounding the BNC jack on the scope? Even the simplest passive 10x Tek probe will contact one of those (probably grounding the biggest contact to ground) telling the scope that it is a 10x probe. You wan-hung-lo probe doesn't have those contacts.

Do yourself a favor and get some faster probes. Even if all you can afford is cheapie generic 250 MHz passive probes, get them. Then replace those with better as money and opportunity arises. Those 100 MHz probes are crippling a very nice fast scope.
 


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