HP 34401A Repair


This post documents the repair of a HP 34401A Multimeter, I’ll be updating this post as I work on it.

The meter powered up and appeared to be fine for DC voltage/current and resistance measurements. However AC and frequency measurements were showing 0 and the self test failed. Still pretty happy with it for 50USD.

First obvious problem was the following blown cap, sorry I didn’t take a pic before removing it:


But wow it left it’s mark:


You can also see a blown 3.3V regulator next to it. The LM317 which drives this appeared to be outputting 30V rather than 18 as it should. I’ve ordered replacements and will update when I’ve refitted the components.

The replaced regulator, not a great job I’m afraid, it was hard to get to:


4V p-p input, 1KHz, square:



The voltage is a bit off, but I’d probably blame my function generator (300USD Siglent).

And some resistance measurements for fun, first a 0.01% 100Ohm resistor:

And then a 10K:

I measured the cables as 0.05Ohm, so this looks pretty good!