Electronics > Repair

Agilent 34401A repair help request (showing voltage without input) (FIXED !)

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HighVoltage:
Hello eevblog community,

I recently bought an Agilent 34401A on ebay and it arrived in good physical condition but has a big problem.
This is the newest version, with round knobs.
Serial Number is MY47012232

No input wires connected:
After turning it on, it shows a DC Voltage of 2V to 5V and is changing slowly to different values.
After a few minutes, it settles to 4.9593 Volts and it sits there very stable.

If I shorten the input terminals, the instruments reboots sometimes.
But sometimes the value shown, goes to almost perfect 0 mV, as it should be.

If I hook up a small battery with 0.886 Volts, it shows the value correctly or the instrument reboots.

The 2W Ohm setting first shows a 100Ohm resistor to be 130 Ohms and then slowly settles to 100 Ohms
Shorten the input in a 2W setting shows either 20 Ohm, 2 Ohm or 0.3 Ohm

The Self Test is passed without any problems.

Normally I do not repair test gear and this will be my first attempt to repair an Agilent instrument.
May be someone can walk me through a repair procedure on this 34401A
I would really appreciate any help.

Where to start?
Thanks for any help

KJDS:
There's an old Irish tale, of a man who stopped and asked for directions to a small town, and the reply was, "If I were you, I wouldn't start from here."

I similarly believe, that if you want to start out fixing test equipment, a precision 6.5 digit meter isn't the place to start.

Having said that, can you do a full set of tests, with various AC and DC voltages, currents and resistor measurements and let us know what it says. I'm not sure if the service manual is available for this, and if it is whether it has schematics, but check the power supply rails, both with a meter and with a scope to make sure they are ok and not noisy. If that all checks out, undo and reseat every connector in the instrument, check electrolytics for swelling and let us know how you get on.

tszaboo:

--- Quote from: KJDS on May 12, 2014, 07:50:05 pm ---There's an old Irish tale, of a man who stopped and asked for directions to a small town, and the reply was, "If I were you, I wouldn't start from here."

I similarly believe, that if you want to start out fixing test equipment, a precision 6.5 digit meter isn't the place to start.

Having said that, can you do a full set of tests, with various AC and DC voltages, currents and resistor measurements and let us know what it says. I'm not sure if the service manual is available for this, and if it is whether it has schematics, but check the power supply rails, both with a meter and with a scope to make sure they are ok and not noisy. If that all checks out, undo and reseat every connector in the instrument, check electrolytics for swelling and let us know how you get on.

--- End quote ---
The 34401 has a full schematic flying around on the internet.
I think I would first open it up, look at it for any obvious issues. This includes components without the smoke still inside. Then, you can start poking around the meter on the analog side, with an other multimeter in one hand, and the schematic in the other, looking for a not conducting transistor or a wrong value resistor etc... I think this instrument is actually easy to repair, well if the ASIC still works.

plesa:
Measure the current flow in the 2wire Ohm measuremet on all ranges, and check for changes in time, and check the current measurement if it works correctly.

linux-works:
check the caps.  always check the caps first.  to restore a box, re-capping is one of the first things to be done.

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