Fluke 77 ohms reading cal off

On Jun 16, 7:34 am, Bob Urz wrote: > One of my handheld flukes (77) now is reading about 2 ohms when its > leads are shorted on ohms scale. I tried another set of leads off of > another fluke, the same. I used test clips and bypassed the internal > protection fuses, the same. There was one pot by the display, did not > change the Zero ohms cal. Anybody got a clue on how to adjust this or > what parts to look at? Seems to work OK otherwise. Its the original, > not a version II or III. >

Don't know about the Fluke 77 specifically as I have 2 8060s. The sockets on the 8060 are directly on the PCB and those joints sometimes crack and give bad ohms readings. I just remove the old solder and apply new solder which is good for years of daily use.

GG

I have a 77 about 22 years old? Isn't there a protect device on the minus lead inside? Too late to take it apart tonight :) Take it apart and try shorting across minus and ohms right on the board Look around if you don't see anything I'll pull mine apart when I wake up tomorrow.

Do the DC voltage ranges show a good stable zero reading with the leads shorted together? If not, you might have a damaged A/D converter, or autozero capacitor. If the DC zero is good, you can get a pretty good idea whether the problem lies in the front end or the A/D circuitry by applying an accurately known DC voltage to the lowest voltage range. If the meter reads correctly, then the problem is likely in the ohms circuitry or in the protection circuitry. If the DC voltage reading is off, then you might have a damaged A/D. If the A/D is damaged, you might be better off replacing the meter rather than spending a lot of time hunting parts that might be more expensive than a new meter.

How much different is the basic model 77 from the series II? I have a copy of the service manual for the 77 series II that I can email to you if you think it might help.

Cheers!!!

shorted

circuitry.

A/D

it

If you have the schematic, email it to me. I am going to check it against my other meters and see what's up.

Bob

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You may have something causing the offset (bad solder connection). The unit may have been damaged. The pot is for calibrating the 300 mv dc range. As you have found it is the only adjustment. You either put in an accurate 300mv or 3Volts and adjust the pot. Everything else falls in to place (unless it has been damaged). Good Luck.

I took it apart and resoldered the studs for the input leads. I also took both fuses out and used emery cloth on the contacts. It now reads about .8 to 1 ohm leads shorted. My other 75 reads about .2 ohms leads shorted. And yes i swapped the leads to see if it made a difference. (it did not).

Voltage readings was about .1 volt difference between the two on DC volts. The meter is not useless, i just cannot use it to reliably test very low value resistors.

I wonder what could affect the ohms scale and not the DC volts?

Bob

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Does it have a software low ohms calibration function? I have a Fluke 79 that has this. You short the probes together, then press the button in the middle of the selector knob and it'll display CAL and moments later 0.00.

Not on my 77 (just checked.) My meter reads 00.1 with the original leads BTW.

For a digital meter, that 0.1V difference could be significant (unless you're measuring an unstable source). What voltage range did you check on the meter? If the lowest range, then I'll bet that's your problem.

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Well Bob i don't know if your problem is fixed or not...but with those meter the rotary switch is sometimes the problem to try cleaning it and if that does not seem to work i would try to replace.

I would also check if that 2 ohms is consistent through out. try to find a standard 100 ohm resistor and check if the reading is off by 2 ohms again.

tech.