From the Time-Nuts mailing list:
Having just gone through the process of finding, acquiring, and fixing an HP 5061A, here are a few pointers for qualifying one for purchase. The biggest problem you're likely to face is a beam tube that's at end-of-life. If there is an electronics failure (like mine, read on), these clocks are really fairly simple to troubleshoot and fix; almost everything is discrete components; the main circuitry is mostly analog.
If the seller says the unit locks and goes into continuous operation mode, the quickest check you can ask them for is to report the beam current. It should be above 15. However, note that this is a relative measurement. There is a meter adjustment control that sets the meter sensitivity, and it's possible it's just not properly set. So, a low beam current isn't an absolute failure indication. With low beam current, if it locks, then you can still have a working clock. The primary impact of low current is more noise in the signal, which leads to greater short-term frequency variation. Even with that, it's still going to be in the 10e-10 or 10e-11 range. If you average, over time the accuracy will be about as good as a newer tube.
If it goes into continuous operation for a while but then loses lock, you're taking your chances. The tube could be so depleted that it can't maintain lock, or the clock could just not be adjusted properly, or you could have an electronics failure. A quick check is to ask the seller for the ion pump current reading. If it's not less than 10, then the problem could just be that the unit has been sitting around for too long without the ion pump having been run. This is curable just by having it run for a few days, or in extreme cases, using an external 3500v 5 ma supply to run the pump more energetically than the clock itself can. In any case, if you're feeling ambitious and can get a good price ($500 or less?), give it a shot.
Finally, if it won't lock at all, then either the tube is gone, there is an electronics problem, the clock is way out of alignment, or the ion pump hasn't run in a long time. Have the seller report the ion pump current. If it's over 10, then you might want to take your chances if you get a good price. If it's less than 10, buy it if you like a challenge and can get a good deal.
I got mine for $300. It wouldn't lock. The ion pump current was high, so I decided to give it a shot. I almost got lucky. After running for a few days, the pump current went to zero, which is good. But, the clock would only lock for a second or so, then lose lock. After a bit of testing, I found that the crystal oven had fried itself and some wiring inside the can. (The design is really stupid; can't imagine why it was packaged the way it was) Anyway, I rebuilt the oven, fired it up, and now have a nicely-working clock that locks, stays locked, gives a nice 20 reading on beam current, and has a high-output tube. BTW, the tube is 25 years old! ('82).
Finally, if you do need to troubleshoot and align the clock, you can easily get by with a good ac/dc DVM (10 meg or higher impedance) a 100 Mhz scope, and a reasonably good counter, one that can reliably read 12 Mhz to 1ppm).
If anyone wants any more tips or info, feel free to ask, and good luck! Bill
Bill Ezell