Repair of an HP 3490A

[TSCOLAN]

I received an HP 3490 from ePay.

It is in good cosmetic condition but with a problem with the display, which was clearly shown in the photos of the sale : only one digit is working and several decimal points are present.

While testing it on arrival the working digit is changing as the sample rate light blinks. Better : when shorting the input it oscillate around zero. I was confident that the measuring part was working and the fault was in the display unit.

I attached a logic analyser to the display connector and saw that the scanning of the digits was stuck at low for each of the three lines. The IC generating these signals is a 74161N.

I removed it and replaced it by a 74LS161 and voilà ! It is working again

The only bad thing about this unit is thas it is missing both Ohms and AC Borards, so it can only do straight DC Volts measurements.

Thomas