Fluke 77 DVM reading HIGH - Page 1
Forum section: electronics / repair

author: Astolat
author_rank: Newbie
timestamp: September 10, 2024, 11:34:03 am
content: I have a Fluke 77 DVM which was originally supplied in 1986. It has been a good servant.I recently took it out of the drawer and found it not working. Dead as a dodo!So I opened it up and found that the connector on the 9 volt battery had fractured. Removed the old battery and fitted a new one. No problem I thought.WRONG. tested it by measuring the output voltage of another 9 volt battery on the DC range. It read 23 volts [ yes twenty three ]Did a continuity test on the two large fuses inside the DVM and continuity was OK [ did not remove them from circuit so could have been measuring smething other than just the fuses. I did not really want to remove the fuses in case I caused other problems. Continuity check was donw with battery removed and rotary switch set to off.I was surprised that instead of reading low or about right that it was reading so high.In the good old days I would have just sent it off for repair and let the boss pick up the bill, but now that I am a 77 year old pensioner I do not have that luxury available.Any ldeas guys and gals? Thank you.Astolat

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2024, 11:53:46 am
content: A reading that far off is more then just normal drift, but more a defect.One could check if the other voltage ranges are also off by the same amont. E.g. check 1.5 V and some 100-190 mV.A test of a few resistors could also help - some defects would not effect the resistor readings, while other would.The fuses are for current readings. Even if open the voltage readings should still be working.A first think woul be to open it up and look for dirt / insects or corrosion from a leaking battery.

author: MathWizard
author_rank: Super Contributor
timestamp: September 11, 2024, 04:20:29 am
content: If you can suffer youtube ads, I bet there's a bunch of video's on there showing people taking Fluke's apart for all reasons.If you had another DMM, you might be able to trace the signal path from the probe, into some voltage divider network. Maybe some solder joint has broke around there. Or some resistor cracked ??But there's probably lot's of SMD sized parts, and that can be tricky to probe on, without shorting stuff by accident. I usually solder on wires in a few places, and then use alligator clips, like for the ground probe.Hopefully, the problem is outside of the chips. But yeah if I get to 77yo, IDK what my soldering skills would be like. Maybe by then I'll make a robotic soldering station.Here's a service manual, for some version of the F77. Ok and you are an EE, I'm just a hobbyist. So that's good.

author: J-R
author_rank: Super Contributor
timestamp: September 11, 2024, 07:50:37 am
content: Take it apart and post high-resolution photos of the innards so we can have a look?

author: daisizhou
author_rank: Super Contributor
timestamp: September 11, 2024, 07:55:54 am
content: Follow the service manual and check it step by step, at least it won't make it worse.Repairing Fluke is something I do often.Or you can try to check if there is any battery fluid leakage near the 9V battery compartment. If you find it, clean it and you will quickly eliminate the problem.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 11, 2024, 12:47:46 pm
content: do a visual inspection .... battery leaked or not ...manually select DC ranges and measure the input impedance(s) of the input "voltage and ground" ... with another trusty metercurrent fuses tests are done between the ground and the ma and amp plugs with another meter10 amps should be around 0 ohms, a 400 ma fuse is around 1.8 - 2 ohms with some Fluke fuses ...and maybe just maybe the rotary dial can or could get crusty dusty, grease has dried if there was ... contacts oxidation ...

author: Paceguy
author_rank: Regular Contributor
timestamp: September 13, 2024, 11:04:07 am
content: Also check if the fusable resisiors are open.

author: bborisov567
author_rank: Regular Contributor
timestamp: September 13, 2024, 11:15:50 am
content: Is there an acid from the battery, by any chance, on the board? If all elements and the reference voltage measure fine i would recommend washing the board with warm water and some kind of degreaser. Residue can be conductive and high input impedance on the ADC doesn't like that.

author: Astolat
author_rank: Newbie
timestamp: September 15, 2024, 02:35:42 pm
content: Thanks for all the good advice some of which I have already done. What I have started doing is making up a test rig so that I can feed it ac and dc current and ac and dc voltages. Plus I will use some resistors that I have had in stock for ages to check the resistance range.I can manage the voltage sources reasonably well using an H & M transformers based model railway controller for the lower voltages and with a few resistors do the current ranges. oh how I wish that I had the variable transformer which I had when I was working for measuring B H curves of current transformers and my electrical fitter to knock up some test circuits from my scratching on the back of a cigarette packet.OK I will get back to you,Many thanks all,Astolat