Tenma 72-2040 Repair
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Every so often one uses a multimeter with the
wrong switch setting then after a quick check (and maybe a new
fuse) all seems well.
Over the years I've fitted a new fuse in the mA circuit
lots of times because this range wasn't working and with a new
fuse all was well. Recently I was measuring current drawn by
a circuit board in my Plessey receiver and wondered why the value
was only half that expected so did a quick check with a second
multimeter and found an indicated 50mA on my Tenma was really
100mA.
Today (14th March 2024) I decided to take the Tenma
apart and see what was wrong. |
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Opposite is what my Tenma 72-2040 looks
like without its outer shell.
As the meter was working in all respects other than
its mA range I looked for something close to the mA socket and
quickly noticed a pair of transistors labelled Q4 and Q5 where
the latter was short circuit. The markings are clear with both
being labelled "8050" and "5F C". The circuit
board is clearly marked "E", "C", "B"
and these letters are also printed on the transistors so no mistaking
their pinning and codes.
A search declared that these transistors were "SS8050"
but, when I checked, this type has pinning E-B-C so the SS8050
is not valid. I tried lots of times to identify the thing but
every search threw up the same result, but finally I found an
on-line datasheet whose search gave a whole page of "8050"
matches including one whose TO92 form gave me E-C-B pinning.
This was a "3DG8050A" and proved to be a fairly standard
spec NPN device (circa 40V and 800mA). I hunted in my spares
box and turned up a pair of "2SC1815" transistors.
I couldn't find a circuit diagram for the meter so just in case
these were a matched pair I fitted both the 2SC1815s which had
a similar voltage spec to the 3DG8050A and whose gains were identical. |
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I removed Q4 and fitted a pair of 2SC1815
transistors then checked the mA range. With a test current of
200mA the reading was only 100mA so whatever function those transistors
had didn't seem to be related to the mA range.
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By peering into the side of the meter I could
see several 1N4007 diodes and a quick test revealed two were
short-circuit (D4 detached). The good news was that these were
connected to the mA fuse.To access these meant removing the top
circuit board held in place by six screws and a pair of in-line
connectors.
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Diodes D4 and D5 were short-circuit so I removed these
and fitted a pair of replacement 1N4007s. Below, a test against
my Digimess HM200, reveals that pair of diodes was the culprit.
The discrepancy might be due to the type of fuse fitted in the
Tenma. |
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