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