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Fixing a DMM

I

Ivan Vegvary

Jan 1, 1970
0
Hey, I know this is crazy, but I want to fix a "cheap" DMM. It is a Kelvin 50LE.

This, and similar meters can be purchased for 3 to 5 dollars. Said meter works except the Ohm section does not function. Fuse is fine. Is there an easily identifiable component that has probably been fried?

I have several analog and digital meters to help with this repair. All advice (other than "throw it away") would be greatly appreciated. I could throw away $20 before I could throw away a tool, any tool.

BTW, wife often catches me watching tool porn on the computer (Snap-On, Harbor Freight, etc.).

Ivan Vegvary (has tool fetish)
 
P

Phil Allison

Jan 1, 1970
0
"Ivan Vegvary"
Hey, I know this is crazy, but I want to fix a "cheap" DMM. It is a
Kelvin 50LE.

This, and similar meters can be purchased for 3 to 5 dollars. Said meter
works except the Ohm section does not function. Fuse is fine. Is there
an easily identifiable component that has probably been fried?

I have several analog and digital meters to help with this repair. All
advice (other than "throw it away") would be greatly appreciated. I could
throw away $20 before I could throw away a tool, any tool.


** Look for a low value resistor on the PCB that may be open - or a diode
that might be short.



.... Phil
 
F

fungus

Jan 1, 1970
0
Said meter works except the Ohm section does not function.
Fuse is fine. Is there an easily identifiable component
that has probably been fried?

All of the Ohm section or only particular
ranges?
 
B

Bob Masta

Jan 1, 1970
0
Hey, I know this is crazy, but I want to fix a "cheap" DMM. It is a Kelvin 50LE.

This, and similar meters can be purchased for 3 to 5 dollars. Said meter works except the Ohm section does not function. Fuse is fine. Is there an easily identifiable component that has probably been fried?

I have several analog and digital meters to help with this repair. All advice (other than "throw it away") would be greatly appreciated. I could throw away $20 before I could throw away a tool, any tool.

BTW, wife often catches me watching tool porn on the computer (Snap-On, Harbor Freight, etc.).

Ivan Vegvary (has tool fetish)

Even if you can't fix it, you don't need to toss it. Since
the basic voltmeter circuit is still OK you can always use
it as a dedicated panel meter in some future project.

Best regards,


Bob Masta

DAQARTA v7.00
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusic generator
Science with your sound card!
 
T

Tom Biasi

Jan 1, 1970
0
Hey, I know this is crazy, but I want to fix a "cheap" DMM. It is a Kelvin 50LE.

This, and similar meters can be purchased for 3 to 5 dollars. Said meter works except the Ohm section does not function. Fuse is fine. Is there an easily identifiable component that has probably been fried?

I have several analog and digital meters to help with this repair. All advice (other than "throw it away") would be greatly appreciated. I could throw away $20 before I could throw away a tool, any tool.

BTW, wife often catches me watching tool porn on the computer (Snap-On, Harbor Freight, etc.).

Ivan Vegvary (has tool fetish)
Are you willing to spend $20.00 to fix it?

Tom
 
M

Mark Zenier

Jan 1, 1970
0
Hey, I know this is crazy, but I want to fix a "cheap" DMM. It is a Kelvin 50LE.

This, and similar meters can be purchased for 3 to 5 dollars. Said meter
works except the Ohm section does not function. Fuse is fine. Is there
an easily identifiable component that has probably been fried?

I have several analog and digital meters to help with this repair. All
advice (other than "throw it away") would be greatly appreciated. I
could throw away $20 before I could throw away a tool, any tool.

BTW, wife often catches me watching tool porn on the computer (Snap-On,
Harbor Freight, etc.).

The magic number is 7126, that's the basic part number for (my best
guess for) the chip in them. A low power IC for battery powered DVMs.
It's originally from Intersil, but has had a large (half a dozen)
different second source manufacturers, many of them in Asia. There's also
the original 7106, but one time I checked, more companies in Asia made
the 7126. (Also 7116, and 7136. Too lazy to dig to the bottom of the
pile to find the databook, I might really mean the 7136, but I think
that's the one with autohold or something like that).

There are a number of application notes for these chips that give
schematics for the various functions. And the manual for a similar
meter, from one of the big names like the old Fluke 802x series
(8024, 8026?) or a Beckman of the same era (1980s), might give
you a very close schematic.

The chip is ratiometric, it compares the voltage across the measured
input with the voltage across the reference input.

For voltage measurements, they have a voltage divider for the measured
input and an internal reference for the reference. Really cheap meters
use the on chip reference, more expensive ones use an external chip
(often in a transistor package). The reference is either 100 millivolts
or 1 volt. The chip will display up to +/- 1.999 times that.

For current, they have resistor shunts to turn the current into a voltage.

For ohms, they feed a current through both a reference resistor, across
the reference input, in series with whatever is at the test probes.

If all the ohms ranges are out, first check the range switch, then what
feeds all the reference resistors. If just one (or two) ranges is out,
check the reference resistor for that range.


Mark Zenier [email protected]
Googleproofaddress(account:mzenier provider:eskimo domain:com)
 
T

tuinkabouter

Jan 1, 1970
0
Hey, I know this is crazy, but I want to fix a "cheap" DMM. It is a Kelvin 50LE.

This, and similar meters can be purchased for 3 to 5 dollars. Said meter works except the Ohm section does not function. Fuse is fine. Is there an easily identifiable component that has probably been fried?

I have several analog and digital meters to help with this repair. All advice (other than "throw it away") would be greatly appreciated. I could throw away $20 before I could throw away a tool, any tool.

BTW, wife often catches me watching tool porn on the computer (Snap-On, Harbor Freight, etc.).

Ivan Vegvary (has tool fetish)

Did you replace the battery already?
 
M

Michael Black

Jan 1, 1970
0
Did you replace the battery already?
Or the fuse, though I don't know if those cheap ones have them.

I once had a weird problem with my DMM, and it worked but not right.
Either the battery was too low, or one of the fuses had gained resistance.
SOmething odd. I ended up opening it up, thinking the switch wasn't
making proper contact, and then saw nothing, suddenly realizing it was
something like the fuse or battery (I can't remember which).

Michael
 
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