Fluke 87 repair help ...

[liquias]

Hi,
I need some help repairing a Fluke 87 (I) multimeter.
I bought a second hand Fluke 87 on E-Bay a few days ago.

15A Fuse was blown.

Fluke works fine in general, But when I turn on AC measurements (all - Volts, Amps/milliAmps , uAmps) I get a "ghost" reading of three right digits that start to go down and the rate of value drop is declining:

For example:
If I just switched the meter from DC Amps to AC Amps - The display shows about 180 which drops down after one second to about 70 and after one more second it drops down to 15, after about 10 more seconds drops to 4, after about half a minute I get a reading of 3, a minute later I get 2 and so on... A reading of 1 seems to be permanent (stuck) and it won't go away.

Does someone have any experience in how to go about fixing these meters ?
What could have gone wrong ? (My guess is that it's whatever blown the fuse...)
How do I fix it ? What do I look for ?

Any help will be wonderful ...

p.s.
1) No warranty on this device ...
2) I got the 83 85 87 service manuals and schematics...

[ingenieus]

Sounds like a capacitor that is discharging.

[liquias]

It does look like that.
But I have searched for this cap. all over and can not find any defective cap. (or resistor)...
What is even stranger, is that it happens only in AC measurements. DC is fine.

Any ideas ?

[mrmodemhead]

Sounds like it could have something to do with the RMS converter, which would be involved in all the AC settings. Check everything around U2 (AD737J), especially C7 and C6. In fact, I'd just replace them, if possible, to see if it helps.

This is just guessing on my part, most of the Fluke meters I've fixed involved cleaning the rotary switch and replacing the fusible resistor. The 87 is a nice tool, worth fixing.

[liquias]

As a matter of fact I did. Since posting the previous reply This is exactly what I did. I replaced C6 and C7. Sadly I can say, there is no change.

another strange thing is that this meter has a special power-on self test for the dial, in it seems to fail the test. I get different values than I should.

The rotary switch is on the same position when measuring DC Amps and AC Amps (push-button changes the function) so I never suspected it to be bad ...

Any Idea what should I check ?
Could the whole problem be the rotary switch ?

[mrmodemhead]

Originally Posted by liquias

another strange thing is that this meter has a special power-on self test for the dial, in it seems to fail the test. I get different values than I should.

Are your values inside the +/-12 count acceptable tolerance? It appears the function is encoded with a voltage divider, and this is what the self-test checks. I doubt it could check all the other contacts. For reference though, the 87 I have here reads spot-on in all switch positions with the multiples of 32 listed in the chart.

Originally Posted by liquias

The rotary switch is on the same position when measuring DC Amps and AC Amps (push-button changes the function) so I never suspected it to be bad ...

I follow your logic there, and it seems sound.

Originally Posted by liquias

Any Idea what should I check ?
Could the whole problem be the rotary switch ?

I struggle to make complete sense of the "signal flow" diagrams in the service manual, but, it still seems that the fault lies in something that is unique to the AC-related functions, which would indicate it is something connected to the processor pins 80-84 and thereabouts. Let's hope it's not internal to the processor.

In any case it certainly wouldn't hurt to inspect and clean the switch.

[thaumaturge]

Originally Posted by liquias

Hi,
I need some help repairing a Fluke 87 (I) multimeter.
I bought a second hand Fluke 87 on E-Bay a few days ago.

15A Fuse was blown.

Fluke works fine in general, But when I turn on AC measurements (all - Volts, Amps/milliAmps , uAmps) I get a "ghost" reading of three right digits that start to go down and the rate of value drop is declining:

Sounds like someone plugged the amp scale into the mains. Zwatt!

I'd go straight to the RMS converter. (AD737). Also take a look at RV1 & RV2. Probably MOV protection devices that may be leaky now. If you can get a pin probe from another meter into it check the output of the RMS converter on pin 6 while feeding an AC source into meter. Reference to input com. Output of RMS converter should be stable DC. After that I'd clean the silicon strip between the board and the display with alcohol. Finger oils or other conductive substance between segments could cause display ghosting.

Doc

[davada]

Does the Fluke 87 use the same shunt for dc and ac measurements?
15 amps is a big fuse to blow for a small multimeter. I would look for a burned trace at or near the shunt. Especially a ground trace.

Cheers,

David.