Fluke 8502A acting weird - diyAudio
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Old 12th June 2010, 09:53 PM   #1
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Location: Sarasota, Fl.
Default Fluke 8502A acting weird

I picked up a Fluke 8502A which seemed to work fine for DC voltage measurement, it read the same as another 8502A.

After moving it and connecting a DC input to it the read out would oscillate between the actual level (confirmed with several other tools) and twice the level.

Interestingly the decay seemed like exponential decay, so I suspect a capacitor issue, like something randomly dumps charge on it and then it decays down to the measured value, then for some reason something will randomly dump charge on it again and the cycle repeats.

I've opened it up to look at it and looked at the DC signal conditioner module and nothing looked particularly out of place.

Anyone seen something like this before or got any tips? Thanks

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Mike
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Old 12th June 2010, 09:58 PM   #2
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also I just noticed when there's no signal attached it will oscillate and reset where the display goes blank and it seems to find a new range. When it does this I can hear a clicking which appears to come from DC signal controller.
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Old 12th June 2010, 11:04 PM   #3
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after opening up the DC signal conditioner and closing it up and re-seating it few times it seems to work fine. I'm going to guess contact cleaner is in order.
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Old 14th June 2010, 05:29 AM   #4
1audio is offline 1audio  United States
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Be very careful with cleaners and that instrument. Its input resistance is in the gigOhms. And contamination can degrade it.

The clicking is the range relays.

If the input is open is normal for it to drift up or down until its connected to a voltage. There are adjustments but a 6 1/2 digit instrument is beyond most calibration resources short of the vendor or a National Lab.

You can download the service book most likely and it may have some suggestions for testing. A loose module would cause many odd issues.

What options does it have?
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Old 16th June 2010, 09:35 PM   #5
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I know it has the current option, it will take me a bit to get it back out to open it up and see what modules are installed.

The contacts on the module card I looked at looked pretty bad. If it were to be cleaned how would you recommend doing it? My thoughts were either fine steel wool or contact cleaner.

Also, this issue happened intermittently on my other unit but stopped.

And yea, I don't have anything that does that resolution that I trust. In fact, when the Flukes appear to be working right I generally trust them.
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Old 16th June 2010, 10:15 PM   #6
1audio is offline 1audio  United States
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Don't use steel wool on the contacts. The gold plate is between 10 and 50 microinches thick and would get removed easily. Also any flakes or bits from the steel wool will be a real problem. However Deoxit would be a good start. If you need more a pencil eraser is probably the most abrasive thing to use.

If there is much corrosion then they may have been in a "bad" environment for a long time. That can make them perpetually unreliable. It seems unlikely that anyone would buy a $2500 meter (you could have bought a new car for that then) and leave it in a humid or otherwise lousy place, but?? Most were in cal labs or standards labs or research or academics so they would have been well cared for.

I have 4 standards I compare among. All are more (way more) than 10 years from calibration. Two from Fluke, one for HP and one from Prem. At 1 V they all agree to 50 ppm or better. Its unlikely that they would all drift identically so I have some confidence in them. In truth closer than 100 ppm is more than I could meaningfully do anyway 9or really care about)
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