Fluke 77 fluctuates on resistance readings - Page 1
Forum section: electronics / testgear

author: Aurora900
author_rank: Contributor
timestamp: July 20, 2015, 12:28:28 am
content: I just got a hold of a couple old fluke meters, an 8020B and a 77 (the original series) and I was testing them out on voltages and they seem to read pretty well... within 0.01v of eachother, which is more than good enough for me... But when I tried to measure the resistance of a 4.7k 2% resistor (its the highest quality one I had around) it keeps fluctuating between 4.63k and 4.72k. Those are both within spec, but even though I was making good contact with the leads it never settled, it kept bouncing between the 2 and a couple other numbers inbetween. Is this normal for this meter? The 8020 reads a solid 4.69k ohms. I also tested with a cheap $30 meter I have and it read 4.69k with no fluctuation, so it kind of concerns me.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 20, 2015, 12:59:04 am
content: Quote from: Aurora900 on July 20, 2015, 12:28:28 amNo.Give the Fluke 77 a generous IPA bath and clean the input terminals with a qtip dipped in IPA. Allow plenty of time for things to dry and/or use a fan.If the IPA cleaning does not help, check the PTC on the Fluke 77. I purchased an used/abused one and found that it was out of tolerance and fluctuating slightly on the resistance readings. The PTC which should have measured around 1.1k ohm was reading 100k ohm. After I replaced that, the resistance readings were within spec and rock solid.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 20, 2015, 01:03:19 am
content: I forgot to mention to check the solder quality on the Fluke 77 input terminals. Wiggle them and see if they move or if the resistance measurements stay steady. I'm told by modemhead that he has had to resolder/reflow a handful of them.http://mrmodemhead.com/blog/all-posts/edit: Added a pictorial example from modemhead's website. While this picture shows a Fluke 179, the input terminals on the 77 are virtually identical to each other.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 20, 2015, 01:24:55 am
content: Quote from: Aurora900 on July 20, 2015, 12:28:28 amIf you want more info on the 8020B, seehttps://www.eevblog.com/forum/testgear/fluke-8020b-teardown-30yo-dmm-technology/http://mrmodemhead.com/blog/fluke-8020b-teardown/BTW, the 8020B is one of modemhead's favourite meter for continuity testing. It isblazing blazingfast.Since the 8020B meter is probably 30 to 35 years old, I suggest you consider replacing the electrolytic capacitors. In the 8060A, they usually leak (underneath the bung) so it would not surprise me if the same happens on the 8020B.

author: Aurora900
author_rank: Contributor
timestamp: July 20, 2015, 01:43:41 am
content: Oh wow, some good info here. Thanks! I'll pop it open and give it a good cleaning and check the solder joints. I have to admit I'm not familiar with the term PTC actually. And good to know about the 8020... it does impress me that it's still accurate and working after all this time, considering its one of their very first DMMs. Just goes to show you why Fluke is amazing, haha. The only thing I dislike about the 8020 is the strange mechanical switches that you have to combine to get to certain modes... but that does kind of add to its charm in a way.

author: Aurora900
author_rank: Contributor
timestamp: July 20, 2015, 02:18:41 am
content: Update: The solder joints looked great, and over all the meter was super clean inside. I just used some alcohol to clean out the banana plugs on the meter and the test leads, and now I get a solid 4.66k ohm reading, no fluctuation. This is actually a different resistor than before... I dropped it on the floor and cant find it haha. This new resistor measures 4.68k ohm on the 8020b. So heres the real question... which one is the one that's off, since both are within the expected +-94 ohms that this resistor should be? Or should I not really care about 20 ohms... lol

author: retiredcaps
author_rank: Super Contributor
timestamp: July 20, 2015, 05:14:02 am
content: Quote from: Aurora900 on July 20, 2015, 01:43:41 amPTC = positive temperature coefficienthttps://en.wikipedia.org/wiki/Resettable_fuseThe 77 manual/schematics will show you where the PTC is located.Dave talks about it as part of the input protection.

author: lowimpedance
author_rank: Super Contributor
timestamp: July 20, 2015, 05:25:46 am
content: Quote from: retiredcaps on July 20, 2015, 01:24:55 amAnd since the only electro caps in the 8020B are near the sensitive A/D area a 'very' good IPA clean is needed if a leak is suspected. Otherwise apart from any obvious corosion, you will get unstable readings.Clean the area any way when changing the caps.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 20, 2015, 05:26:38 am
content: Quote from: Aurora900 on July 20, 2015, 02:18:41 amThe 77 is 0.5% + 1 spec for 32k ohm. The 8020B is 0.1% + 1 spec for 20k ohm.Thus, on paper, the 8020B is likely to be more accurate. I would say both are within their respective specifications.If you try different value resistors, they will likely be within 1 or 2 least significant digits of each other. For most purposes, this is "good enough".PS. The 77 continuity is dreadfully slow compared to the 8020B. The 77, however has 2000 (two thousand) battery life vs 200 (two hundred) on the 8020B.

author: tautech
author_rank: Super Contributor
timestamp: July 20, 2015, 06:09:07 am
content: We should also point the OP to this tread:https://www.eevblog.com/forum/testgear/old-fluke-multimeters/