Unstable ohmeter

In order of probability:

1. Dirty battery contacts. Clean the contacts and possibly replace the battery.

1. Dirty test probe contacts, broken test probe wires, corroded test probe tip. Clean or replace.

2. Dirty rotary switch. Spray with a small amount of contact cleaner.

1. Dirty fuse contacts or bad fuse. Clean and/or replace fuse.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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"Peter Easthope"

I have a Sanwa BX-505 analogue multimeter where the needle wavers and can not be zeroed on the x1, x10 and x1k scales. Zeroing is possible on the x10k scale. The ohmeter is really an electromagnetic voltmeter connected in a circuit with dry cell, resistor, zeroing potentiometer, leads and a subject resistance.

** I take it that analogue multimeters are new to you.

The ohms ranges depend on AA and a 9v batteries - they must have sufficient voltage or the needle will not zero.

Try some new AAs and make sure all the contacts are clean and bright.

A little WD40 sprayed on a rag is good for this.

BTW:

I used to own your meter's bigger brother:

It featured a 2uA, taut band movement.

.... Phil

Bad meter leads, bad connectors, dirty switches, dirty battery contacts.

What he said.

I would only add that pushing, poking, tapping around etc while you are looking around inside might help reveal the one or more bad connections that might be there.

Also, since these problems tend to be pretty intermittent, you might think it is fixed, only to recur later, so don't give up after one or two failed tries, you'll fix it soon enough if you keep trying.

I have a Simpson 635 and a 270 series 2 that have had similar problems, and a Fluke 8840A which also had issues with the low range ohms.

When somewhat more current is required, as in a Rx1 or Rx10 low ohms function, poor connections become a much larger percentage of the total circuit resistance.

Speaking of which, for troubleshooting, one trick would be to remove the battery, short across those terminals, and measure the total resistance through your lead wires and through the meter itself using another multimeter. Meanwhile, the aforementioned pushing, poking, etc might show up the culprit on the other meter.

Have fun!

Mark Z.

Mark Z.

Sometimes it's just a matter of pulling the leads out of the jacks, and putting them back in.

I didn't think analog multimeters had fuses.

But, one time one DMM was reading odd, and I ended up opening it up, expecting the contacts on the switch to need cleaning. NO. It turned out eh fuse was bad, it didn't open but had a higher than normal resistance. I can't remember what was "high", but it wasn't a large amount, just way more than it should have been.

Michael

What about

1. Dodgy zeroing pot (internal, external, both) ?

It's a minor nuisance on one battery/pocket analogue meter of mine, but then I don't try and guess resistances off it! :)

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--------------------------------------+------------------------------------ 
Mike Brown: mjb[-at-]signal11.org.uk  |    http://www.signal11.org.uk 

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"Michael Black"

** Some did, to protect the current ranges.

** A 0.5amp fuse is often used in the ohms and 200mA ranges of many DMMs.

Some 0.5amp fuses have 2 to 5 ohms of resistance while others have only 0.3 ohms.

Depends on the fuse wire being copper or tin alloy.

.... Phil

A freshly charged AA rechargeable didn't help.

Testing the probes and leads was easy. Remove the black lead. While holding the red probe on the black socket move the lead in various ways. No correlation between needle movement and lead movement evident. Likewise for the black lead.

The wiper-leaf-spring is held on the back of the switch with one nut. That was removed and all the contacts were cleaned thoroughly with a cotton tipped applicator. The dirt was only some blackness left on the applicator. With the switch reassembled the ohmmeter was dead stable again. Appears the problem was just the barely visible trace of dirt on the contacts.

There is a spare fuse in plastic clips beside the working fuse. The two fuses have about the same resistance. Swapping fuses made no improvement.

Thanks to everyone for the tips, ... Peter E.