Agilent 34401A front inputs reading 0.5 Ohms with shorted test leads

[alligatorblues]

Hi, I recently purchased a used HP34401A. When I put the device in 2W resistance mode and plug test leads into the front inputs I get about 0.5 Ohms when the leads are shorted together. The same test leads measured through the rear inputs give me a more reasonable ~0.02 Ohms. Does this indicate a fault or is it possibly just a calibration issue?

Thanks,
Charlie.

Try working the froont/rear switch, Clean out terminals. Get a 4-wire short and try that on the DCV setting. Calibrate zero volts. Try again. If necessary, Reflow the solder joints on front terminals.

[martinr33]

K101 can also be a problem. It is a reed relay that sits in the high input line. If it has a problem, it only shows up on the 2W resistance range.

You can try tapping it. You might also try CAL 0 front and rear, becasue a bad 2W 0 cal can also cause problems.

Use a good short.

[JohanH]

0.5 ohm is way too high. Even cheap leads shouldn't have higher resistance than 0.1 ohm. Typical leads that I have are around 0.02 to 0.05 ohm.

With a proper short, as in the picture below, the values are much lower. The values aren't stable, they toggle a few milliohm up and down. The front and rear terminals are equal, though.

There was an issue with the 4W measurement with this meter when I bought it. The front panel was cracked and the plastic clip that supports the front panel and front terminal assembly was missing. Due to this the terminal assembly easily bent inwards when you inserted leads into the jacks. This had led to pressure on the PCB, repeatedly bending it and cracking the solder connections of the 4W resistors at the front, near the terminal. Resoldering the resistors and securing the front panel fixed the issues.

[cskilbeck]

Just to close this out - I cleaned the front/rear contactor as best I could and then did the zero ohm calibration using this jerry rigged short. Everything looks good now, the readings look very reasonable and agree with my other DMMs.