2204A can't measure through resistance

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dbctronic
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2204A can't measure through resistance

Post by dbctronic »

I've had a 2204A for a few years now. Suddenly, both probes show a significant drop in apparent signal voltage through a series resistance of 1K. At 2.7K, it's down to half the correct value, and at 4.7K or higher it shows nothing - not even a few millivolts when looking at a 4V peak sine wave. I've tried a half dozen different resistors--behavior is rigidly consistent. This means I can only measure very low impedance points in the circuit, such as supply rails or an op amp output. What could be wrong?

Gerry
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Re: 2204A can't measure through resistance

Post by Gerry »

Hi dbctronic,

It appears that your PicoScope may have developed a fault. Is the behaviour the same on both Channels?

Could you respond by sending an email into our Help Desk at support@picotech.com, with an example psdata file showing the problem, as it would appear that you may need to consider getting the PicoScope repaired. We can then take you through our returns procedure, if you decide you want to go ahead.

Regards,

Gerry
Gerry
Technical Specialist

dbctronic
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Joined: Wed Sep 15, 2021 4:02 pm

Re: 2204A can't measure through resistance

Post by dbctronic »

Gerry,

Thanks for the response. I have recently discovered that:

1. Both channels detect DC signals correctly in DC coupling mode, even through 50K resistors. They seem to have resistances of 1 to 2 meg, based on attenuation seen through several high value resistors, which is about spec for them isn't it?

2. If I test a low impedance AC signal source (only tested at 370 kHz so far) with one probe through a 10K resistor, it shows a very low amplitude waveform. If I connect the other channel to the same point (it must be active for this to occur), the erroneous waveform jumps to correct amplitude and agrees with the directly connected channel.

So they behave as though they have correct DC resistances but very low AC impedances, with the added behavior of suddenly jumping to extreme attenuation as input voltage goes down.

I'm doing a set of various DC, AC, and combined DC/AC tests with each channel separately and together, at different frequencies, to better characterize the behaviors. Smoke me a kipper.

Any idea what repair cost, shipping, and turnaround time might be from the American Midwest? It might not be worthwhile.

dbctronic
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Joined: Wed Sep 15, 2021 4:02 pm

Re: 2204A can't measure through resistance

Post by dbctronic »

Quick update:

My 2204A appears to be dying. Channel A is now right dead using any of three probes that work on Channel B. I was hoping I could at least get by using just Channel B, but it now cannot stand series resistance to a low impedance point in excess of 100 ohms without seriously dropping apparent voltage, and at 400 ohms it goes wacky and drops out the signal completely. Decrease the resistance to 350 ohms, the signal pops right back, but at half amplitude. Time for another unit!

Sure was a fun little machine while it lasted.

Gerry
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Re: 2204A can't measure through resistance

Post by Gerry »

Hi dbctronic,

To answer your 1st question, yes the input impedance of a channel on your PicoScope is 1MΩ.

If you contact our North American Office (see here: https://www.picotech.com/about/usa-address, or send an email in to support@picotech.com, if you haven't already done so) then they will be able to give you a repair charge in US$, along with a turnaround time and shipping cost (as you will be sending it in to the office in Texas).

Regards,

Gerry
Gerry
Technical Specialist

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