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Escort ELC-133A Strange Auto-Range malfunction

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Achilles:
Hello Folks,
I snapped a "defective" LCR from ebay. You may have guessed it from the topic: an Escort ELC-133A. The seller stated that it gives you FuSe for defective Fuse at a start up, but the Fuse shall should be ok.
So I got it and tried it....sure...start, a bit of autorange and then FuSe. I opened it up, the Fuse was fine and the specs of the fuse seemed fine, so I checked the solder joints. They looked fine, too.
After a bit of trying and searching on the PCB I saw that there is about 70kOhm as resistance between - and +, So there couldn't/shouldn't be a problem with a disconnected trace.
Then I found it strange that the Autorange went a bit on and on (nothing connected shurely...so I expected OL or 0) and then there was the FuSe message. So I thought, hay. Shorten up the connectors.....started it, no Problem. It was running! Hmmmm, disconnect....autorange....FuSe. Plug in a Resistor and it worked (even though it gave me a quite different reading from my U1253B) The Agilent settled at 9.922k and the Escort at 9.873k (100Hz, Ser).... try my old voltcraft DMM which gave 9.67k. It should be an 10k 5% resistor..... so technically it works.
 Then I switched off the Autorange and it works!....Atorange on,...it begins to FuSe again ;). So it really seems to be depending on the Autorange function.
So maybe sth. in the firmware?! Or could it be that it cranks up the measuring-current when autoranging and somebody inside says goodbye (shortcutting or so). There is also a PTC as secondary fuse as it seems. Maybe that buddy could be making trouble?!....I'll have a look.
Suggestions appreciated ;)

BTW: Who is trusty from the three DMM's :D.....three Meters and three quite different readings ;), but all are within the 5%. I should have a look later with another Resistor with lower tolerances.

PS: If I start the LCR and disable the Autorange it is working.....

Kiriakos-GR:
Oh my that's a very confusing story, even so congrats about "snapped"  part.   :)

amspire:
I guess the fuse is a very low current fuse - probably 100mA or less.

I was thinking that if someone replaced the fuse, and put in a low current fuse that was spiral wound around a former, it would act as an inductor.  The LCR meter might see enough impedance in it to decide the fuse has a problem.  Or perhaps the replaced fuse just has more  dc resistance then the original.

You could try and short out the fuse and see if the error message is still there.

Richard.

Achilles:

--- Quote from: amspire on November 04, 2011, 12:18:33 am ---I guess the fuse is a very low current fuse - probably 100mA or less.

I was thinking that if someone replaced the fuse, and put in a low current fuse that was spiral wound around a former, it would act as an inductor.  The LCR meter might see enough impedance in it to decide the fuse has a problem.  Or perhaps the replaced fuse just has more  dc resistance then the original.

You could try and short out the fuse and see if the error message is still there.

Richard.

--- End quote ---

Hi Richard,
I had tried to shorten the Fuse already, but I tried it again and it didn't help. So the Fuse doesn't seem to be the Problem.
I think if it is a Problem with the resitance of the Fuse or sth. else in the specs of the Fuse it should also give me the error in the Manual Range mode, but it really is just when it is autoranging.

PS: Yeah, it is a 100mA Fuse. They just give you 100mA 250V, nothing else....tiny little Glass-bottle ;)

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