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Old 12th April 2013, 08:27 AM   #1
jpr_pt is offline jpr_pt  Portugal
diyAudio Member
Join Date: Dec 2008
Default Repair Fluke 45

I have a Fluke 45, bought some years ago on ebay.
It worked ok until last month I found that the DC reading were ok if the voltage under measurement is positive.
If it's negative, it display the correct value if the input is limited to the 0 to -3V range.
For values bellow 3V the ralays comute and display "OL".
Of course I can just switch the probes...
I looked at the service manual and proceed with the tests defined (limited to the 3V range) and everything seems ok.
Since I use it only for hobby activities and have no plans to buy a new one, does anyone have a clue about what can be the problem?
In my opinion this is a fine piece of equipment and it's a pitty if I have to put it away.
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Old 12th April 2013, 09:03 AM   #2
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JonSnell Electronic's Avatar
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
U1 is probably faulty. The usual cause is an overvoltage or just old age. I would return it to your local Fluke Service Centre.
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Old 12th April 2013, 09:17 AM   #3
euro21 is offline euro21  Hungary
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Join Date: Sep 2004
Location: Budapest
Default Fluke 45 Service Manual
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Old 12th April 2013, 10:22 AM   #4
euro21 is offline euro21  Hungary
diyAudio Member
Join Date: Sep 2004
Location: Budapest
Missing my post. :-)

Looking at 167. page (A1 schematic), it seems that above 3V K1 switches alternates. If R8, or R3 damaged, "OL" error displayed correctly.

Try 10A probe (for example known 0.5-1A DC current mearurement).
If result is correct, this parts are OK.
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Old 13th April 2013, 01:15 PM   #5
jpr_pt is offline jpr_pt  Portugal
diyAudio Member
Join Date: Dec 2008
Thanks for your posts.
I checked the current measurement test and works ok.
Now I discovered a situation that I hadn't identifyed before.
It works ok (both positive and negative - amp or voltage) if the rate is slow.
If the rate is fast or medium the problem arises.
I suspect that the K1 may be the fault.
Anyway, I'm already looking for replacement of the relay and probably U1. Since the equipment is rather old I'm also planning the replacement of the electrolytic caps.
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