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

--- Quote ---I need the help of someone with a working 87-1.
I need the values of P03 and AP6 in the different switch positions. I attached a picture of the backside of the PCB. I marked the positions, where AP6 can be measured red and for P03 they’re marked yellow.
AP6 should be constant for all ON positions and P03 should have different values depending on the switch positions.
I would be very thankful if someone could measure them
--- End quote ---

I opened up my working 87-1 & measured the following voltages on AP6 (end of cap), P03 (solder pad near the top) as per your photo. All voltages are with respect to the "COM" terminal.
PosAP6 (Red)P03 (Yellow)Off0V0VV~-180mV0VV-180mV-30mVmV-180mV-60mVR-180mV-90mVDiode-180mV-120mVmA/A-180mV-150mVuA-180mV-180mVmid pos-180mV-4.46VThese are certainly different to your measurements:

--- Quote ---P03 (U4P66) starts at -0.468 V in the OFF position and 0V in the V AC position and then goes down to -2.8 V (AP6) in about -0.45 V increments with every switch position. Between the switch positions it blips down to -4.4 V (Vbt- is -5.35 V).
May that be a indicator of the problem?
--- End quote ---
Wildsau96:
Thanks so much for the effort  :-+

Now i have a baseline for what i am looking for. But slowly i am suspecting a cooked U4. Very sad :(

I‘ll keep you updated, if i make new discoveries
forrestc:
So here's my shot at understanding this - it might be completely wrong.

AP6 is normally at -180mV.   The other end of the voltage divider is at zero.  So your switch ranges should go from zero to -180mV at P03.  Looks like alanep has verified this is correct.

In your case, AP6 is -2.8.   As a result, the voltage divider would range from zero to -2.8V.   This means that the divider seems to be working correctly based on your incorrect AP6 voltage and your readings.

AP6 is apparently an output from U4.   So, either U4 is outputting the incorrect voltage, OR something external to the part is preventing AP6 from reaching the correct voltage.   It looks like C34 is a filter cap to ground - that isn't likely to cause this issue (unless AP6 is a PWM signal).  However, R42 is supposed to be a 1M resistor to Vss, which happens to be -3.2V.   If this resistor has shorted or otherwise changed it's value, the voltage on AP6 could be dragged toward Vss.  This is true of any other place on the board with contamination or some subtle conductive defect between Vss (or Vbat-, etc) and the AP6 output.

If I was troubleshooting this, I would think about what steps I could do to further troubleshoot this based on my skillset.  Not in any order:

I might attempt to measure and/or lift R42 - if voltage changes to normal and operation is restored, replace.  Note that occasionally one will find contamination *under* a resistor which will cause this type of issue, which means that you might want to just remove, clean, and put a new 1M00 resistor in place.

I might put a scope on the AP6 output (or switching a DMM to AC range) to see if it's unfiltered PWM I'm reading.  Doubtful, but possible.  If it seems to be oscillating or otherwise, replace C34.  Or replace C34 for good measure.

I'd be 100% certain that the microamp contact on the switch doesn't have any stray connection to -vbt that you missed.   I might measure between that side of R70 to Vbt or Vss to make sure that you don't have a short there.

I might also be tempted to cut the AP6 trace at an easily repairable spot near U4, or lifting the AP6 pin on U4 (both somewhat dangerous to do), to determine if the voltage out without any circuit attached is around the -180mV we expect.

I'm pretty comfortable that your problem is somewhere on the net between R70 and the AP6 output on U4 (or U4 itself).  Either something is "shorting" that net/trace to a negative voltage (like through a damaged resistor, or contamination on a board, or something like that), or U4 is cooked.

One other thing I might try just for fun is to lift R70 and inject -150mV into the divider between R70 and R69 and see if that restores operation.   Not sure how that helps to actually fix anything, but it would be interesting to prove that this is the problem.  Note that -150mV is the expected voltage at that point in the divider.
alanep:
From the schematic, I think the AP6 line to U4 pin 87 is an analog voltage input (not an output) which contradicts other opinions (sorry). The service manual mentions in section 2.5 using "a voltage ratio measurement" to read the switch/potentiometer position. The AP6 pin on U4 should be reading the voltage feeding the top of R65-R70 as the constant part of the voltage ratio i.e. -180mV.

The -180mV is solely being generated by the voltage divider of R42 (1M) to Vss (-3.2V) & the sum of R65-R70 (6 x 10K = 60K) to digital GND.

It sounds like the PTF (R65-R70) is OK (i.e. not open) because it still has voltage steps but larger (-0.45V in lieu of -0.030V). If so, somewhere there's leakage onto the AP6 line. Maybe U4 pin 87 is now acting as output... Hopefully, it's something else.



forrestc:

--- Quote from: alanep on May 14, 2024, 12:13:47 pm ---From the schematic, I think the AP6 line to U4 pin 87 is an analog voltage input (not an output) which contradicts other opinions (sorry).

--- End quote ---

I think you're correct, now I do the math. I was basing my opinion on other descriptions of the circuit which probably are wrong as well.

I'll modify my last post accordingly when I get to a real computer later.
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