Tag Archives: HP3478A Not Reading AC Voltage

HP 3478A with no AC Voltage or AC Current Reading


I found this nice unit on eBay at a discounted price as a part unit, because according to the seller, the unit doesn’t read AC voltage or current. Otherwise everything else seem to work perfectly.

After some researching and digging through the schematic. There are few components like K104, U102 and U301, U302 and U303 ICs possibly failed.

Before proceeding further, if you are going to make a similar repair make sure it is disconnected from the AC line and take extreme care not to short battery. Otherwise all the calibration settings in CMOS RAM will be lost and need recalibration.

Once, I received the unit, I began tracing AC signal through K104, U102 and U301, U302 and U303 input. The signal made it through and range switching seem to work just fine. However, the output of U303 (AD536A True RMS-to-DC converter) is stuck at near ground regardless of the input signal.

To confirm that U303 is dead, I cut the JM302 to DAC section and reconfirmed that AD536A is faulty. Then to confirm that meter is still able to read the AC input from the DAC. I placed a .5V at DAC side of JM302 and the meter is responding, so the DAC is able to read the converted AC signal and it is a good sign.

After some research, I finally ordered Analog Devices AD536AKH replacement for U303 from eBay. When the unit arrive, I began surgically removed the old unit by clipping the legs and cleanly removed the old solder and pins before installing the AD536 replacement unit and bridge the JM302 jumper.

The AC volt and current is working now and the reading is slightly off and need recalibration.

Next improvement is to replace the lithium battery.

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