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Author Topic: HP 3478A OVLD 2 wire ohm test  (Read 4101 times)

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Offline cswilsnTopic starter

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HP 3478A OVLD 2 wire ohm test
« on: August 13, 2013, 10:04:24 pm »
Hey Guys,

I'm having a problem with my old volt meter...seems a few issues are starting to arise with it at once :).

2 wire ohm test is showing OVLD MOHM....Anyone have any idea what might be causing this? i tried a reset.

Hoping its a setting, though it just started so i'm concerned....hopefully its repairable. Has anyone ever ran into this with these old meters?

Thanks again in advance!

CW
« Last Edit: August 13, 2013, 10:08:36 pm by cswilsn »
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Offline Dave

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Re: HP 3478A OVLD 2 wire ohm test
« Reply #1 on: August 13, 2013, 10:08:35 pm »
Are your test leads, by any chance, disconnected?
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Offline cswilsnTopic starter

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Re: HP 3478A OVLD 2 wire ohm test
« Reply #2 on: August 13, 2013, 10:09:03 pm »
connected or disconnected I get the same error.
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Offline Dave

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Re: HP 3478A OVLD 2 wire ohm test
« Reply #3 on: August 13, 2013, 10:10:14 pm »
And you also get that when shorting the leads?
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<fellbuendel> it's arduino, you're not supposed to know anything about what you're doing
<fellbuendel> if you knew, you wouldn't be using it
 

Offline cswilsnTopic starter

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Re: HP 3478A OVLD 2 wire ohm test
« Reply #4 on: August 13, 2013, 10:13:37 pm »
Same thing :/ No change.
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Offline cswilsnTopic starter

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Re: HP 3478A OVLD 2 wire ohm test
« Reply #5 on: August 13, 2013, 10:15:59 pm »
4 wire seems to be OK, though I have never had to use that function before.
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Offline Dave

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Re: HP 3478A OVLD 2 wire ohm test
« Reply #6 on: August 13, 2013, 10:56:51 pm »
I suggest you crack it open and start poking around with a multimeter.
Here is the service manual for this unit. You can find the troubleshooting guide for resistance measurement on page 107.
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<fellbuendel> it's arduino, you're not supposed to know anything about what you're doing
<fellbuendel> if you knew, you wouldn't be using it
 

alm

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Re: HP 3478A OVLD 2 wire ohm test
« Reply #7 on: August 14, 2013, 01:25:10 am »
In 2W mode the current source and voltage input should both be connected to the same terminals, so it can source the 1 mA current and measure the voltage drop. My guess is that something goes wrong in the switching of this, since this is probably the only difference between 2W and 4W mode.

I'm not sure if this is switched by discrete relays/transistors or by the input switching ASIC.
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Offline cswilsnTopic starter

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Re: HP 3478A OVLD 2 wire ohm test
« Reply #8 on: August 14, 2013, 05:35:23 am »
Welp, Thank you! I fixed it.

Module U102 had some mild corrosion on one of the pins, either than that the whole thing looks fine. I spent the past few hours cleaning everything just in case.

The attached image is the meter with a 1/4 watt 49.9 ohm resistor...that works for me!

Thanks again Dave!
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