Electronics > Repair
Fluke 87-1 Constant beeping, stuck in V AC and no measurement possible
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Wildsau96:
Hey everyone,

first of all. Thanks for all the suggestions. I got a little bit of time to work on the board.

First i wanted to check R42. I measured the resistance over it in circuit and got a reading of 6.75 kOhm.

Then i desoldered R42 and measured the component itself. This time i got 1 MOhm. So the resistor is ok.
Resistance over the pads from R42 (Vss to AP6) was still at 6.75 kOhm.

After turning the meter on, i measured the voltage between ground (COM) and AP6 and still got -2.85 V. The behavior of the meter was also still the same.
Unfortunately i don‘t have a scope the check if AP6 oscillates. But in V AC i got no reading when checking AP6.
I got an OL reading when measuring from R70 to Vss/Vdd/Vbt+/Vbt-. So i think there are no stray connections.


So i thought, there might be a leakage somewhere in the trace of AP6. I decided to cut the trace one via away from U4 (Pretty nervous, because i‘ve never done this before and lifting the pin is way out of my skillset).
The voltage of U4 pin 87 now measures -3.182 V, which matches Vss exactly.
AP6 now measures 0 V (with R42 desoldered)

Then i took some other measurements (with the meter powered off):
- Resistance U4 pin 87 to Vss: 7.1 kOhm (Must be internally to U4)
- Resistance AP6 to Vss (over the pads of R42): Kind of all over the place. It starts at about 500 kOhm, then suddenly drops to about 80 kOhm and then slowly rises to about 200 kOhm (takes about 20 seconds).
- Resistance COM to AP6: 60.3 kOhm (which matches R65-R70. So everything still good there).


In a last effort i decided to also desolder C34, just to be absolutely sure and in circuit i got weird measurements.
For itself the cap measured OL for resistance and a capacitance of 10.7 nF, which matches the spec. So the cap is ok as well.

So to my understanding, this means, that U4 pin 87 got somehow internally (in U4) shorted to Vss.
Or does someone have any other ideas?
alanep:

--- Quote ---I decided to cut the trace one via away from U4 (Pretty nervous, because i‘ve never done this before and lifting the pin is way out of my skillset).
The voltage of AP6 now measures -3.182 V, which matches Vss exactly.
The rest of the AP6 trace now measures 0 V.
--- End quote ---

I'm a little confused with the double use of the term "AP6". Now that you've isolated U4 pin 87 from the AP6 line, it would be best to distinguish between them.

I assume you mean "The voltage of U4 pin 87 now measures -3.182 V, which matches Vss exactly" and "Resistance U4 pin 87 to Vss: 7.1 kOhm (Must be internally to U4)".

If so, it does sound like U4 pin 87 is internally leaking to Vss (when I believe, it should be a high impedance input). I'm not sure how you measured the 7.1K ohm i.e. powered or depowered. If depowered, then the 7.1K could be incorrect. Another way to roughly check the pin impedance/resistance is to power the meter up & connect a deliberate load on the (isolated) pin & see how the voltage changes (without dragging too much current). Could you temporarily connect a 68K leaded resistor (similar to R65-R70) between "COM" & the now isolated U4 pin 87? I would use an alligator clip lead between the "COM" terminal to one side of the 68K resistor & probe/touch U4 pin 87 with the resistor other side. In theory, the U4 pin 87 voltage should drop to zero since it's an input....

As well, with U4 pin 87 isolated, the AP6 line should be -180mV when powered.
Shock:
Do you have details about the onset (how it came to be not working)?

Please do look at (test resistance) and clean the input jacks. They leak across to the lead detection or become intermittent, high resistance. If you are using the jacks to take measurements you could get erroneous measurements. Better idea to solder or hook a wire on the pcb somewhere. I've cleaned up a similar problem on a 179 and it was input jacks and dirt on the pcb. If you ever see dirt while cleaning the input jacks high chance a contamination issue existed. In the past I've used an IPA soaked cotton bud with some of the cotton unwound. You have to clean until it comes out spotless after several attempts.
 
AC will bounce around a little until you short the leads together then it should zero.

Edit:
Oh and to be triple obvious the battery, always swap batteries twice or run it up (double checking polarity) on a bench supply. Contamination can sit under the lead housing which is another reason you want to troubleshoot the lead detection circuit, dirt inside the case plastics is another dead giveaway.  You can isolate the fuses and see if the meter reacts normally without leads (try switching through the various functions).
daisizhou:
Have you cleaned the 4 internal jacks of this outlet?It looks like there is a leakage in the "A" and "ma" jacks.
Wildsau96:

--- Quote from: alanep on May 14, 2024, 11:25:39 pm ---I'm a little confused with the double use of the term "AP6". Now that you've isolated U4 pin 87 from the AP6 line, it would be best to distinguish between them.

--- End quote ---

You‘re right, sorry. I‘ll edit my post.


--- Quote from: alanep on May 14, 2024, 11:25:39 pm ---If so, it does sound like U4 pin 87 is internally leaking to Vss (when I believe, it should be a high impedance input). I'm not sure how you measured the 7.1K ohm i.e. powered or depowered. If depowered, then the 7.1K could be incorrect. Another way to roughly check the pin impedance/resistance is to power the meter up & connect a deliberate load on the (isolated) pin & see how the voltage changes (without dragging too much current). Could you temporarily connect a 68K leaded resistor (similar to R65-R70) between "COM" & the now isolated U4 pin 87? I would use an alligator clip lead between the "COM" terminal to one side of the 68K resistor & probe/touch U4 pin 87 with the resistor other side. In theory, the U4 pin 87 voltage should drop to zero since it's an input....

--- End quote ---

I measured the resistance with the meter powered off.
I‘ll see, what i can do about the 68K resistor. I have to check if i have a fitting one.


--- Quote from: alanep on May 14, 2024, 11:25:39 pm ---As well, with U4 pin 87 isolated, the AP6 line should be -180mV when powered.

--- End quote ---

I measured the 0V on the AP6 line with R42 an C34 desoldered (so completely isolated, except for the voltage divider, which connects to ground). Seems to be a good idea, to resolder them and measure again. I‘ll try it.
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