Hello Ed,
After giving the situation some thought, I think I will construct a replacement three-output supply first, delivering +5, +15, -7.
The +5 V can be fixed, that is needed more often. The other two voltages variable, so I can use it in other situations too.
(Side note: I happen to have a Fluke 7261A that needs the same treatment. Same transformer problem: short in the primary windings.
Completely different transformer, though. Also three output voltages to replace. With a bit of work, I should be able to rewind the primary.
Transformer disassembly started, E-I core removed. But before going any further, I want to know if it is worth while!)
So, now I am thinking about constructing a fixed+variable power supply. That would come in handy more often, I'm sure.
I have some surplus computer supplies, that should take care of the +5, and gives a decent +12 for free.
It is the variable outputs that require some thought.
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!
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.
Coming back to the Rb oscillator.
Magnetics:
I have the same situation: the bolts for the outer shell are non-magnetic, the bolts for mounting on the base plate are magnetic.
And additionally, and this is where I had not taken care enough to keep things separate: the same is true for the washers!
I have four magnetic ones, and four non-magnetic. I have sorted them using a weakly magnetized screwdriver.
The mix of magnetic and non-magnetic screws didn't make much sense to me. I dug through my stash of non-magnetic screws and found four more non-magnetic ones so now I've got a full set.
Root cause of the failure:
The signal that I saw on the output before the unit repaired itself, was a completely different and well-defined signal.
That tells me that the connector itself is good. I remember reading that there are some analog switches that the 10 MHz signal must go through
before it reaches the output buffer on the supply board. That means that other signals can be routed to there as well.
If that is correct, the failure was in that switching part! Would you agree? Was it truly a failure to begin with?
What is the purpose of the analog switches? (I do not recall reading anything about that.)
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.
The signal observed earlier is some kind of modulation signal. From the frequencies observed, most likely it was the 127 Hz signal.
If I am not mistaken, the 127 Hz signal is used to modulate the Ghz resonator frequency. But is that 127 Hz signal itself modulated as well?
I think I need to go back and read more carefully about how the modulation and locking mechanism works.
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.
Calibration:
In order to get the oscillator out for inspection, I also removed the 20(?)-turn Bourns pot from the front panel.
(So now I know how to circumvent the calibration sticker, which is still very much in tact!) After removal from the front panel,
I carefully measured the position of the pot, so that I could always put it back in position in case it would be accidentally turned.
The resistance measurements have in 5 digits, thanks to the 8842. The unit was originally calibrated to 0.2 mHz.
I have not touched the Bourns pot, nor have I touched the setting on the aux power supply of the unit (the one that delivers 24 V).
Would that suffice to get me somewhere close to the originally calibrated value, or is that just wishful thinking?
Regards, EJ
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
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