Unstable ohmeter

[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 acircuit with dry cell, resistor, zeroing potentiometer, leads and a subject resistance.

Scans of the relevant pages from the manual are here.
http://carnot.yi.org/SanwaBX505Manual.jpg

What is the cause of this behavior?

Thanks, ... Peter E.

 

[Phil Allison]

"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:

http://www.radiomuseum.org/r/sanwaelec_multimeter_n_501n50.html

It featured a 2uA, taut band movement.


..... Phil

 

[Mark Zacharias]

In order of probability:

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

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

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

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

--
Jeff Liebermann [email protected]
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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.

 

[Michael Black]

In order of probability:

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

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

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

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

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

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

 

[Phil Allison]

"Michael Black"

I didn't think analog multimeters had fuses.

** Some did, to protect the current ranges.

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.

** 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

 

[Peter Easthope]

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

A freshly charged AA rechargeable didn't help.

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

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.

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

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.

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

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.