HP-8112A pulse generator onboarding

The HP-8112A front panel at boot time

The Hewlett-Packard 8112A is the last affordable pulse generator made by Hewlett-Packard. Of course this being HP we're still talking about many thousands of dollars. But after the 8112A pulse generators became much more expensive with the lineup consisting only of high-end models. Times had changed. These days the 8112A is a great addition to the hobbyist workshop as it can be obtained for under 50 yuros and doesn't take too much bench space. The problem is that its design is very old: we're talking late '70s here. Still, the model was refreshed a few times and the later examples feature the same king of rear panel bracket as the venerable 34401A. As is tradition with HP, this pulse generator was also made in Germany like its older siblings. The unit I picked up at auction ended up costing me 40 euros because 1) the 'error' indicator was ON and 2) I sold the feet of the instrument for 30 bucks. This error indicator will be ON whenever a bad setting combination is selected (e.g. high time larger than the period) so it's not an immediate cause for concern, especially from reputable sellers that would show any error codes anyway. A good deal for a unit of the latest revision, around the early 90s.

Inevitably there are some issues that should be resolved, and the 'don't turn it on, take it apaaaart' is a good motto to apply here. Still, I did power it up first and the error was not visible: the person who checked the instrument before selling it must have pressed buttons randomly, engaging the 'error' indicator, and he probably thought that the equipment was damaged. A simple reboot fixed it! (no CTRL-ALT-DELETE on these bad boys)

Still it's a must to have a look inside as the equipment is 30 years old and 'ya know, caps and all...

Everything inside looked good at first sight except for the dreaded RIFA mains filter cap which was promptly replaced of course:

 
The bad RIFA cap is promptly replaced...

Other than that the inside of the instrument is pretty clean. The 8112A is one instrument that is nice to work with: the boards have special cut-outs to lock them in supports so you can easily work in open-heart-surgery mode. Well done HP! Some board pics follow too just in case.

The HP-8112A opened up and ready for testing

The HP-8112A boards

A cute coax junction that looks like a bodge but isn't (check out that little wire support in its center...)

One weird issue was the power switch which was quite sticky: pressing was a bit hard, and it would come back in a very goooey way. The source of this problem ended up being another typical HP problem: they use some rubber-ish stand-offs on PCBs as well as other places, and those completely disintegrate with time, becoming very sticky and even oozing a bit. One of those stand-offs is used to 'guide' the power button 'bar' and the fix simply consisted in removing it and cleaning the mess. Great success!

Sweaty inner foot caught in the middle of its decomposition stage

What remains after a bad foot was removed

A very ugly foot rubbing against the power rod

Rubber has even transferred on the rod itself!

The last little thing to do was to check the fan. After 30 years replacing it is probably a good idea, even if it's a very good PABST unit. I went for a quieter but a little less powerful fan; this should be fine as I don't plan to run this pulse generator 24/7 anyway. To further reduce the noise I used shock mounts similar to the ones used in my HP 53131A silencing operation.

New fan on rubber mounts

The rear panel with the blue rubber mounts visible

Last note for potential buyer of a 8112A: the front panel has a plastic film over it. It's not the kind that you very satisfyingly peel off. Just don't do it. It's not supposed to come off. But many instruments of that series (8111A, 8116A) will show signs of the plastic film peeling off, especially between buttons. As far as I can tell this is not repairable (not easily at least) so if you're a bit OCD watch for those signs. Protecting plastics often stay on for the lifetime of any device in Asia where I live: you can see car navigation screens with patterned plastic over them, and given the state of the film it's not a new car anymore. Thus I thought I was getting a really pristine example with still the film on, but it turns out the little peeling off is not a good sign. Not a big problem on my unit though.