Fluke 87-1 Constant beeping, stuck in V AC and no measurement possible

[Wildsau96]

Hi everyone,

i got a problem with my Fluke 87-1. When i turn it on, it just beeps and the meter is stuck on V AC. No matter what i select on the rotary switch. In the mA/A position the meter crashes. And there are no measurements possible.

At first i suspected the rotary switch. I cleaned it and the whole board with IPA. The ptf section seems to be in good condition (I attached some pictures of the board). There are no shorts in the banana jacks.
I also inspected the board for damaged traces or components. I couldn‘t find anything.
It fails the rotary switch test (0000 in every position and crashes at the mA/A positions) (The switch shaft is not reversed).

So i dug deeper into the whole thing. Modemheads guide on his blog already helped me understand the workings behind the mode selecting.
All my values for the power supply section of it are correct, except for AP6 (U4P87). When the meter is turned on, it reads -2.8 V.

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?

I really hope U4 is not cooked, because otherwise the meter is in good condition.

I also checked various other aspects of it:
- The varistors RV1 and RV2 both measure OL.
- The fusible resistor R1 measures 1.011 kOhm
- The thermistor RT1 measures 1.469 kOhm
- The spark gap E1 measures OL
- The bridge rectifier U1 shows good results in a diode test
- The fuses F1 and F2 are still good
- The input impedance measures OL in the V AC position and 11.10 MOhm in the V DC position with a battery installed. Without a battery installed it measures about 22 MOhm in V DC
- The meter draws 9.3 μA from the battery, when the rotary switch is in the OFF position and 1.2 mA in the ON position
- I resoldered some pads that looked suspicious

[Wildsau96]

It also seems like i get a ghost reading in the V AC mode. The reading seems to be unaffected by the range setting.

Here is a link to a video, where you can se the problem with the meter:
https://youtube.com/shorts/WHH5sR39Y2c?si=ev4wmNNqIr8c1dWH

[Paceguy]

Hi, I would check the 4 input jack block. One appears to have cracked plastic. The 87 beeps when leads are installed in wrong jacks in respect to the rotary switch settings. Also check the spark gaps.

[Wildsau96]

At first i suspected the rotary switch. I cleaned it and the whole board with IPA. The ptf section seems to be in good condition (I attached some pictures of the board). There are no shorts in the banana jacks.

- The spark gap E1 measures OL

[Louis_C]

Have you checked the resistor network chip. It's the large white SIP on the side of the board. Cold solder or broken connection can cause this to occur.

[Wildsau96]

All the resistances in the network check out with the schematics. I reflowed all the connections to the board for good measure.
I also checked the traces from the network to the components and they are good.

I am slowly running out of ideas   

[Wildsau96]

I need to correct myself.
P03 (U4P66) starts at -0.468 V in the OFF position. I edited it in the original post.

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

[bostonman]

Have you looked at the service manual?

https://xdevs.com/doc/Fluke/83_85_87smeng0500.pdf

[Wildsau96]

Yes, i already read it completely.
Unfortunally there is no mention of AP6 except for the typical values. P03 is not mentioned altogether.

[bostonman]

Sorry, thought maybe having the service manual and schematics would have provided an answer.

Unfortunately I don't have an 87; else I would remove the cover and measure the value for you.

[alanep]

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

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.

Pos	AP6 (Red)	P03 (Yellow)
Off	0V	0V
V~	-180mV	0V
V	-180mV	-30mV
mV	-180mV	-60mV
R	-180mV	-90mV
Diode	-180mV	-120mV
mA/A	-180mV	-150mV
uA	-180mV	-180mV
mid pos	-180mV	-4.46V

These are certainly different to your measurements:

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?

[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]

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).

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.

[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]

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.

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]

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.

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

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....

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.

As well, with U4 pin 87 isolated, the AP6 line should be -180mV when powered.

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.

[Wildsau96]

Do you have details about the onset (how it came to be not working)?

I got it from a colleague. „One day i turned it on and it started beeping and didn‘t work anymore“. The meter didn‘t have a hard life. More or less just stationary use in a laboratory.

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.

I already cleaned the jacks several times. Cotton buds stay spotless, when i insert them soaked in IPA. I also resoldered the pins of the jacks. There are no shorts in the lead detection.
I also bathed the whole board in IPA for several hours and scrubbed it down with a toothbrush.
I do all my measurements connected to the COM jack of the 87. But i can repeat the last ones and take the ground from somewhere on the board.

Edit: All the measurements are the same when taking ground from the board

[Wildsau96]

I resoldered R42 and C34 to the board (The trace near U4 pin 87 is still cut)
Now i get a voltage from AP6 to COM of -178,9 mV. So everything seems fine there.

[daisizhou]

The focus of cleaning is the yellow area

[alanep]

I resoldered R42 and C34 to the board (The trace near U4 pin 87 is still cut)
Now i get a voltage from AP6 to COM of -178,9 mV. So everything seems fine there.

That indicates the only place the extraneous leakage can be coming from is U4 pin 87 or whatever remaining pcb trace that's connected to it. If the trace is short, it's starting to look awfully like U4 might be stuffed given I believe, U4 pin 87 is an input. There's probably no point doing the 68K load test.

Interestingly, U4 pin 87 is very close to the U4 pins used as lead detection/sense for uA/mA/A i.e. pin 88 (AP5), pin 89 (AP4). Perhaps there was some kind of flash over due to crud between the terminals etc at some stage...

[Wildsau96]

I cleaned and inspected the area around the remaining trace of pin 87 extra carefully again. I can‘t find anything that indicates a leakage.
It‘s hard for me to say that, but i declare U4 for dead.
Since i‘m not able to lift U4 from the board (and the prices for a working chip are pretty high), i declare the whole meter as a parts donor.
Thanks to everyone who helped me narrow down and find the problem! Without you, i would still stare at the schematics and the board, trying to understand the workings of it 

If someone needs a parts donor Fluke 87-1 in or around germany, he can pm me.