Hewlett-Packard 33120A function generator fan replacement

Inside the HP-33120A 15MHz function generator

The HP33120A is a wonderful compact function generator that was released waaay back in the early 90s. The students' electronics lab of my university had just upgraded to all-HP equipment in 1995 (?) and the 33120A was part of the gear, together with a digital scope of the 54600 series, a 34401A and a simple HP power supply. Those were the days. Recently I got my hands on a nice unit in great cosmetic condition and with the improved oscillator option. A must IMHO, since that's the only way to sync with your house 10MHz. The only problem is a soon-to-be-dead fan which makes a bit too much noise for my taste. Luckily I have replaced a few fans already, and after my success with the hp-53131a I am now always looking for rubber mounts to replace those pesky and noisy screws. Plus, on the 33120a the screws are huuge:

The HP33120A mounting screws are rather large...

First step is to remove the optional time base board that is in the way. Easy: just unscrew the two BNCs in the back and it pops out (with a little effort due to big PSU caps being in the way). The SMBs have such a satisfying clic when plugging them in that I am tempted to disconnect and reconnect them just for the heck of it, but it's not needed so let's not do it right now.

Putting the optional oscillator board on the side

From here we can see the fan to be replaced, in all it's NFG glory. The heat sinks are a bit in the way but it's not too hard to remove. I replaced it with an equivalent from CUI devices that only makes 12.3dB of racket.

The bad fan, now removable once the oscillator board is out of the way.

In addition and as usual, I'm replacing the screws with rubber mounts as that can make a lot of difference. This time the holes in the chassis are large and the frame is thick, which prevents using the same rubber mounts I used in other instruments. No worries though, Digikey has a suitable rubber thingy that fits perfectly. You can really pull on these things when you install the fan by the way. I was afraid they would snap but they're quite robust. My mounting suggestion is to put the fan in place first, then slip the mounts and pull them until (1) the mounts are nicely fitting the chassis' hole and (2) they are firmly holding the fan. Just in case I have to do this again I have not cut the rubber mounts but you can certainly do so if you wish.

New fan, new rubber mounts

Does it work? You betcha! Completely silent. It's kosher. It's christmas. Great success!