EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Stormchaser on December 04, 2014, 01:13:36 am
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So I snagged a Hitachi V-212 20MHz scope from eBay for $30 USD and $20 shipping (hey Dave was right, good old scope for $50! Thanks Dave!!!). It came with 2 original AT-10AK probes and even the manual. Wonderful condition, the back panel and the little label below the screen still have the protective plastic sheet on them. I was messing around with it today, hooking it up to my phone and playing some music when I noticed something: one channel needed to be set to 5mV/div while the other was set to 50mV/div to get waveforms with the same amplitude. Both probes were set to 1x mode, so I started messing around with them hooked up to the .5V calibration point. Channels checked out OK, it was when I switched probes I noticed the problem. Here's the suspect probe's wave in 10x mode with the scope set to .1ms/div and 50mV/div (after correcting the compensation):
(http://i.imgur.com/K8i6Lkn.jpg?1)
Now when I switch the probe to 1x (time and voltage divisions still the same):
(http://i.imgur.com/mOlvIOn.jpg?1)
I can't get it to lose that overcompensation peak no matter how I mess with the adjustment, and then there's the glaring fact that it should be 10 times larger than the first picture but it is not. It's almost like it is stuck in 10x mode, but not quite since I can correctly compensate it when it really is set to 10x.
Is this a fixable situation or should I just mark it as "10x ONLY" and go look online for a new probe? If that's the case, any recommendations on probes?
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There is NO COMPENSATION adjustment for 1x probes, so don't even try. Given that the usefulness of 1x probes is generally pretty limited, due to the excessive capacitive loading and low frequency response, I'd say simply forget about the 1x mode.
If you want to check the 1x position, measure the series resistance of the probe. It should read a few hundred ohms, vs. 9Mohms in the 10x position.
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Yup something went screwy in one probe. I wasn't worried about the compensation adjustment, sorry if you took that to be the main question I had. My main concern was how the wave did not change between 1x and 10x modes, that's what tipped me that something was really wrong. Correct me if I am wrong but if the probe was OK, with my scope set to 50mV/div, 10x mode should go up just 1 division when reading the .5V calibration pin and go way off the screen when in 1x mode, yet they remained the same.
Anyway, all that aside, yes something is wrong. One probe (the good one) shows roughly 165 ohms in 1x, and 9Mohms in 10x. The other probe (the bad one) is 9Mohms in both 1x and 10x.
I don't see any easy way to open these things up to service them, so I suppose I should look for a new probe! Anyone have any recommendations? Are the less expensive eBay ones OK for hobby use?
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Try to find the P6000 series probes, they are known to be good for hobbyist use.
Likely you will find a pair for less than US$15 with shipping.
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Or just continue using the probe as-is, only in 10x mode.
Is there a reason you need 1x mode? If not (and if you don't know, there isn't), don't worry about it.
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http://www.ece.vt.edu/cel/docs/Probe_Fundamentals_Tektronix.pdf (http://www.ece.vt.edu/cel/docs/Probe_Fundamentals_Tektronix.pdf)
Short answer is 1x probing sucks. Don't use it unless you understand its limitations and actually need it.