Fluke 77 AC current measurement slow to settle down

[Jamieson]

I've got an early original Fluke 77 (series 1) that seems to work fine in all measurement modes. However if I switch from resistance/diode/continuity modes to AC current mode, I see an initial high reading > 1A which then slowly falls to nearly zero over the next minute. This slow settling out only affects AC current mode, and only immediately after switching from resistance/continuity modes. If I power off, then immediately switch to AC current mode, quickly skipping past the resistance/continuity settings, then AC current mode works normally.

Any suggestions? I've tried all the usual stuff like cleaning the switch and scrubbing the PCB really well with IPA.

[mqsaharan]

I also have an old 77 series 1 w/o 10A fuse. Please forgive my laziness, I didn't document the changes I have made to this meter. But I like this meter because of non fused 10A socket and the fact that I use in this meter 9v batteries that are rejected by other electronic equipment.
I, too, saw about the same behavior in my meter. I don't remember the initial reading and its settling time now. But I installed a few (perhaps 3) capacitors (if I remember correctly, C0G type, C14, C15, C16) in it as per the schematic of 77 series III service manual and after that it is much better in operation in general. Now it jumps to <0.35 AAC and settles to 0 in about 15 seconds, usually less than that.
If there are any -ve effects from installing the capacitors then I am unaware of these.

[Jamieson]

Thanks for the info. I'll look into those caps. I also see the same slow settling behavior in AC volts mode, but it's not as dramatic as when switching to AC current mode.

[mqsaharan]

Truthfully, I don't remember its original behavior on ACV. Its been a few years since I installed these caps.
I just checked mine. If I short the probes, it takes a few (2 to 3) seconds to show zero in ACV function. If I connect it to AC source like the wall socket or the output of a stepdown transformer, it settles quickly like in <2 seconds.

[mqsaharan]

Sorry, I forgot to mention in my previous posts that if you are seeing this slow settling behavior now and it wasn't there before then you need to resolder or preferably reflow the IC U1. And if you are seeing some display related problem like missing segments, for example if you clean the zebra strips and they still disappear/flicker after some time, then you need to reflow U2 as well.
Some minor problems of this meter disappear after reheating the IC(s) for a few minutes with hot air gun/station. Some people have successfully revived their dead meters by just reheating the IC(s). There is thread about it on EEVblog forum.

[Jamieson]

This Fluke 77 had a mangled rotary switch that I repaired. I think all contacts are working properly now, but if something is resistive that might account for some strange behavior.

• DCV, DCA, DCmv, ohms, diode/continuity are all good.
  
• ACV takes ~20 seconds to settle to zero. Shorting input leads does not speed up settling time. After settling out the reading is maybe 5% high.
  
• ACA takes ~90 seconds to settle to zero, especially after switching from ohms/continuity. After settling out the reading is pretty accurate.
  

I'll double check the solder connections around the analog IC and maybe heat it up a bit.

[mqsaharan]

I thought you have this meter for some time. I didn't know you recently acquired it and are unfamiliar with its history. You also failed to mention the real problem of the meter.
Anyway, if the switch contacts were twisted then the first thing to do is to desolder and remove the switch off the board. Because other side also has contacts. After repairing and making sure it is working all right then focus on the rest of the board. And my suggestion is to start with the cold and cracked solder joints.

[Jamieson]

I should have mentioned that this Fluke 77 was given to me with a mangled rotary switch. It is the original model 77 series 1 (7X-3001, Rev. K).

First thing I did was repair and clean those rotary switch contacts. The switch is working fine.

Turns out the issue was that tantalum capacitor C9 was installed backwards. On the PCB there is a + indicator for C9 and this marker is in the wrong position. And due to this mistake the factory installed C9 backwards with the + lead connected to ground.

I removed C9 and it tested OK, so I rotated it and installed it so that + went to R18 and - went to ground, as the schematic indicates.

Problem solved! Now switching to AC mode current from any other mode quickly settles down to zero as expected.

I also have a Fluke 75 series 1 (7X-3021 Rev. A) and I confirm that cap C9 is installed properly and the PCB marker is correct.

So it turns out that at least some of the very early 7x models had this manufacturing defect that was fixed in later revisions. Usually when a tant cap is installed backwards there's some fireworks, but in this case I think the currents were quite small and the cap was not damaged but rather just acted a bit strange. Kind of a.... fluke!

I've had many Fluke meters come through my shop but this one is the first time I've seen a very early model 7x series 1. Kind of an oddball meter with some questionable safety features, or lack of safety features I should say. For example, the high current input is completely UNFUSED. Um, okay.... Also, R1 which is the 1k protection resistor, is not fuseable/flameproof. As you can see in the picture it looks like a big old 2W carbon resistor. Glad that this stuff was fixed in later revisions.