HP 5328A Universal Counter Teardown
Why I specifically chose the HP 5328A model was because it has a BNC connector on the back panel which allows you to feed in an external time base. Even with a "good" crystal oscillator as the counter's time reference, you can still build a "better" oscillator by stabilizing a crystal oscillator in an oven, or even using an atomic oscillator like my Rubidium reference, which I managed to kill yesterday.
Unfortunately, while bidding on a few of these on eBay, I accidentally bought two of them.
So this simple fix to pop open the box, desolder the switch, and replace it. Turns out, by virtue of having no idea what I was doing, I approached the problem from the entirely wrong direction.
Now looking at my other counter, which I did pay a bit more for, we have the amazing luck of seeing the only three options that didn't happen to be install in the first counter; the oven-compensated oscillator, the HPIB IEEE-488 network card, and the option C 500MHz channel.
AdaFruit GPS receiver. The lower one with the oven-compensated oscillator (which is marked as being calibrated in 1999) is spot on, while the top counter using an unprotected crystal oscillator drifts up and down 0.1-0.2ppm, and is almost 5ppm slow. It's likely most of this difference could be removed with some calibration (or its previous owner not dropping it ಠ_ಠ), but even just watching the stability makes for a marked demonstration of the difference the option 010 oven makes. The standard time base is rated for 2.5ppm drift between 0 and 50 degrees Celsius, where the option 010 time base has the much tighter 0.007ppm. Since these counters display eight (or nine) digits, 2.5ppm is very visible, where 0.007ppm will at worst only effect the least significant digit.
So, if you've been pining for test equipment that typically retails for a couple thousand dollars, and don't happen to have that type of cash laying around, eBay is an option to get equipment in the $100-$300 range that at the most need some trivial TLC before being a perfectly serviceable piece of equipment. Better still if you can either hack together or get access to a good standard to calibrate it against.