Fluke 87 Series I calibration issue

• Fluke 87 Series I calibration issue
  Posted by Axtman on 17 May, 2019 21:42
• I bought a cheap broken Fluke Series I (1986 Revision H) multimeter and am attempting to repair it. I found that the fusible 1k resistor was open so I replaced it with another 1k fusible resistor.  The meter appears to be working but the readings are off. I downloaded the Fluke 83, 85, 87 Series Multimeters Service Manual and followed the procedure for calibrating volts ac and volts dc. The problem is that I cannot get the readings low enough and the PCB pots don't appear to vary the reading much. For example when I put a 3.5 volt source into the meter I get a reading of about 5 volts.
  
  Any advice would be greatly appreciated.
• #1 Reply
  Posted by Axtman on 18 May, 2019 22:56
• The service manual is located here:
  
  http://assets.fluke.com/manuals/83_85_87smeng0500.pdf
• #2 Reply
  Posted by retiredcaps on 20 May, 2019 02:55
• An open fusible resistor suggests that more parts of the input protection could be bad.  Check the MOVs and PTC.
  
  A good MOV should read "0L" in resistance.  A good PTC should read around 1K ohms.  Both measurements can be done in circuit.
• #3 Reply
  Posted by retiredcaps on 20 May, 2019 02:56
• Quote from: Axtman on 17 May, 2019 21:42

  For example when I put a 3.5 volt source into the meter I get a reading of about 5 volts.
  

  No calibration will ever trim it from 5V down to 3.5V.
• #4 Reply
  Posted by floobydust on 20 May, 2019 04:54
• For voltage readings to be too high - that is worrisome.
  
  If the 1k fusible resistor popped, the 1.5k PTC might be damaged, or the 909k resistor for mV and V. I'd guess it was overloaded on ohms or diode-test.
  The diode/transistor clamps are switched-in for ohms and diode test functions.
  On DCV the Fluke 87 is relying on the 909k input resistor - so leaky clamp diodes/MOV/transistors etc. I don't think can make it read higher?
  
  I would check pin 2 AGND compared to input COM(-), the analog DMM IC has a few grounds. I would check the 1.235V reference near pins 1,3.

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• #5 Reply
  Posted by Axtman on 21 May, 2019 02:45
• The 1.5k PTC (RT1) measures 1.24k.
  
  The 909k input resistor (R2) measures 918k.
  
  The two MOVs (RV1 and RV2) measure OL. Interesting enough one of the MOVs was not soldered to the board. It looks like it was a factory mistake. I soldered the MOV back in.
• #6 Reply
  Posted by retiredcaps on 21 May, 2019 03:00
• I forgot to mention to check the spark gap.  It should also read 0L in resistance mode.
• #7 Reply
  Posted by Axtman on 21 May, 2019 03:05
• Spark gap measures OL.
• #8 Reply
  Posted by retiredcaps on 21 May, 2019 03:05
• Put the Fluke 87 into DCV mode.
  
  Using another meter, what is the input impedance of the 87?  It should be around 11.11 M ohm.
• #9 Reply
  Posted by Axtman on 21 May, 2019 03:42
• I get 10.28 M ohms.
• #10 Reply
  Posted by retiredcaps on 21 May, 2019 03:56
• It should be 11.11 M ohm. See
  
  https://www.eevblog.com/forum/repair/fluke-87-i-repair/msg390462/#msg390462
  
  I suggest a complete IPA bath of the entire PCB.  Remove the lcd.  Let it dry completely and recheck the input impedance.
• #11 Reply
  Posted by Axtman on 21 May, 2019 04:24
• The pcb is very clean as is the rotary switch. There is no evidence of carbon, solder splatter, or other contaminants.
• #12 Reply
  Posted by Axtman on 21 May, 2019 04:26
• I checked the Pin 2 AGND with respect to COM (-) and got a reading of -1.728 v. Floobydust said it should be 1.235 v.
  
• #13 Reply
  Posted by retiredcaps on 21 May, 2019 19:11
• What about all the other readings in table 2-1?  Please report them all.