Hello. I am writing to contribute to repair logs of 34401A.
I
received cheap, broken 34401A with missing mains switch push rod, that plastic part holding front terminal in place and rear plastic frame.
1. problem - not bootingMultimeter was not starting properly. When switched on, it illuminated all display segments, then two beeps and then nothing. Shift was working.
Exact behaviour like this:
Turned out to be
bad connection between front panel and mainboard. Here is what I found. Broken connector. Somebody torn off that plastic part holding ribbon cable in place. Temporary fixed with wires extension. Not very clean solution, but it works until I receive proper replacement.
After this repair, multimeter starts properly.
2. problem - measuring wrong numbers on some rangesAfter ribbon cable fix, multimeter was showing something, but nonsense numbers. When switched to 100V, 1000V range, everything works. All DC amperes OK. 4W Ohms OK. Random errors from selftests.
Then I realized that 2W sometimes work innacurately, sometimes dont work at all.
So I started to trace the signal path when some little voltage applied to input. And then it suddenly started to work.
Turn out to be
bad connection on R104 2512 13K resistor. When I was touching that place, results were changing randomly. Resolved by applying flux and reflowing solder joints on R104.
Therefore I am confirming this:
another thing to check while you got the machine open is the big 2512 resistors close to the big switch. i have seen those develop cracks on the electrodes. simply resolder them.
and indirectly this (from
this thread):
I then started to follow the sense tracks and, hello 24K resistors... why when I press you the sense line goes from open circuit back to good?
So I reflowed all those six 24K bastards. Actually it looks like three of them had a bad joint (they were invisible to me, I could spot nothing with my naked eyes).
After repair, multimeter passes all tests and seems to be working properly. Compared to another two 34401As.
I hope it might help somebody.
Now I should sketch up missing plastic parts for 3D printer. We will see...
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