Hello Ed,
There are two ways to look at it. You can build or buy 3 power supplies with the hope that the board powers up and works properly and then those power supplies become the permanent replacement for the entire blown U39 module. If the counter is dead, you repurpose those supplies as lab supplies.
You could also build or buy three lab supplies that cover the voltages necessary. In that case, you end up with three nice lab supplies and then, if the board works, build or buy 3 dedicated supplies to repair the counter. You end up with three lab supplies. You can *NEVER* have too many power supplies in your lab! 
Yeah, agreed. What I would like best as end result is to have a working counter, with no visible modifications on the outside. Inside is a different story.
And I am willing to put some effort into achieving that result. That is why I would like to know up front whether or not it is worth while going down that path.
So my preference is the second approach, with as extra goal to build the 3 dedicated supplies into the counter.
Same thing applies to the Fluke 7261A. I'd like to know if it still works, before I decide to rebuild the primary side of the transformer.
One complication to keep in mind is that the input to the U39 module is high voltage DC rather than AC. I don't know if typical AC switching power supplies will tolerate high voltage DC input. You might have to rearrange the input power to feed them with AC.
The conversion from AC to DC is done with a straightforward rectifier. That rectifier should be able to deal with momentary mains peak voltages, so I would expect it to be ok, no?
And if the replacement power module has AC input, it can also be hooked up tot he AC line coming into the box.
I'm not seeing any analog switches. The oscillator itself is U1, 54LS320 which is a dedicated quartz oscillator chip, followed by a 54HCT393 which is a ripple counter, i.e. a divider. In any case, you had a solid 10 MHz signal coming out of the oscillator board and I don't see anything between the oscillator board and the power supply board.
I went back to the manual, and saw that indeed I misremembered: there are analog switches, but they are in the modulation circuitry, not in the 10 MHz path.
I don't think you stated what the amplitude of the bad signal was. Did it look like a solid signal or something that was basically noise being coupled in to the output line and then amplified by Q3? Bad solder joints or other circuit faults anywhere in the system are a possibility, but the clean 10 MHz coming out of the oscillator board but not on the output connector points toward the power supply board rather than the oscillator board.
Indeed I didn't, and indeed it may be that it was just a stray signal weakly coupled into the 10 MHz line. I wish I had made some scope screen shots...
In analyzing the fault location, I started at the connector, and worked my way back to the oscillator, making big steps. Once I found the signal,
I followed it forward towards the connector, making small steps. And to my surprise, I could then follow it all the way to the connector. (Simplified account, but basically correct.)
I expect that at some point in the future, the 10 MHz signal disappears again. Then I will do the same thing, but with improved recording of my findings!
Rubidium standards do drift over time. Considering the age of the unit, recalibrating it by comparing it to your GPSDO isn't a bad idea. It will be challenging to do it with a scope, you really need a time interval counter. The frequency will also shift slightly with temperature and input voltage. But since you've got that pot, that's a good starting point.
FYI, the PM6681 power supply is very close, if not identical to the PM6685. The service manual warns that if you touch the +5V trimmer, you have to adjust 'the complete instrument', whatever that means. The PM6685 service manual doesn't say that, but since it's so similar, keep that warning in mind.
Ed
Yeah, if I get the entire box working, then I can use that to measure my GPSDO, and adjust the pot for an exact 10,000,000.000 reading.
That would be the best I can do, as my other counters have less digits! I have a few more 74LV8154 lying around,
so I am tempted to use those to make a dual 12 (or even higher) digit counter, with the help of a micro dealing with overflows. Hmmm....
Cheers for now, variable supplies are ordered, I will report back once I have rigged up the whole shebang.
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