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 ?


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

 

[BradtheRad]

1.

Sounds typical of a capacitor discharging somewhere inside the meter. The curve is a close match to your values.

Since the '1' never goes away it suggests there may be a leak somewhere. Maybe the same leak which charges the capacitor, either before you switch to an AC range, or at the moment you do so.

Measuring AC is not as straightforward as measuring DC. AC has to go through some kind of rectification and averaging process, before it's applied to the ADC.

So that's a lead that you'll want to check the AC conditioning circuitry.

2.

Does the meter have a dial? It may be worth it to unscrew the dial to lift it away and examine for any foreign material, worn tracks, melted or misaligned contacts, etc. And to clean electrical surfaces while you're at it.

This is a mistake-prone part of the job. You must be careful not to lose springs, clips, ball bearings, etc.

When re-installing the dial, you must turn things the right way so those same items don't fall where you don't want them to.

3.

Can you verify that it's supposed to use a 15A fuse? The digital meters I've used have a 200mA or 250mA. Big difference.

It's easy to blow the 200mA fuse by running overcurrent into the 200mA DC range. THen it's tempting to use a much larger value fuse 'so it won't happen again'.

But then the overcurrent might be applied again, except next time the fuse doesn't blow, meaning there could be excess voltage/current, which might damage something in the meter circuitry.

 

[liquias]

Hi,
First of all thank you !
Regarding (1):
It does sound (and look) like a cap is discharging somewhere in the meter. It happens only on the AC features. If There is a leak somewhere I have no idea where to find it.
Measuring AC is different than DC. In addition to what you already said, This meter has a "True RMS" feature. There is an additional circuit for RMS to DC conversion in the Fluke 87. (I suspect this circuit may be related to the failure...)
Regarding (2):
The meter has a dial. This is strange because the dial has a special power-on self test feature which it does not pass!
But, going from DC Amps to AC Amps does not require to change the (only press a push-key). Going from DC volts to AC volts does require moving the dial. Every time I go into any AC mode the problem occurs. Since all other features of the meter seem to work fine I assumed it is not dial related.
Regarding (3):
I am sure the fuse is (and should be) 15Amp fuse. It's a 15A 600V Midget type fuse interrupt rating of 100kA. Fuse part number - KTK-15. I have seen it in the service manual. and I have
replaced it with an original.
BTW:
The meter has two fuses, each guarding a different probe-input, the second fuse is rated 1A and is (and was) OK...

How do I proceed ?
any suggestions will be blessed !

Thanks,
Liquias.

 

[BradtheRad]

1.

You describe a ghost reading which is the same order of magnitude whatever the AC range.

This suggests it doesn't occur prior to any kind of scaling network.

Because if it occured before a scaling network then it would be:
180 on the mV setting.
.180 on the V setting. Etc.

This may not help much. It's just using the Sherlock Holmes method to narrow down the possibilities.

2.

My experience with Ebay purchases has been positive overall. In a few cases however I get the idea that the item started showing some problem. So the owner unloaded it on Ebay.

Sometimes the problem is mentioned in the auction listing. Sometimes it's not.

Regardless, if you can fix it, then you've got a bargain.

As for Fluke, it's a top brand in meters. Considered reliable. Can you conjecture what a meter might be subjected to that would cause your problem? Without destroying the meter?

Just one mistake might do it. For instance feeding hi V into the AC mA range? (If it has an AC mA range.)

Etc.

3.

Can you measure the rogue reading directly on the circuit board?

Can you apply a voltage to reproduce the rogue reading? Or to counteract it?

Don't go over 0.2 V right away. Just to be safe.

 

[kak111]

This is helpful....................

Fluke -- 87 -- Service Manual
**broken link removed**