Hi Ed, Many thanks for your response! I'll try to answer your questions accurately.
1) I have taken the unit out of the broken frequency counter. That counter also contained a 24 V supply private for the oscillator,
so (after verifying by DMM and scope that it functions correctly), I have also removed the supply, and use it to power the oscillator.
It is amazing: one third of the counter box is reserved for the oscillator and its power supply!
Actually, I also took out the base plate on which the oscillator was mounted, and use it as the heat sink.
2) So there are two completely separate supplies, the only have the 220 V mains in common. They are both switching supplies.
The supply for the counter electronics has one fuse on the primary side, which is blown. A replacement fuse blew immediately.
The supply for the oscillator works, and is completely functional, as far as I can ascertain.
3) I have a PM3375 (100 MHz, 2 channels). The output is indeed a suspicious 60 Hz, but not precisely so.
Moreover, It changes frequency in unlocked state in a sweeping manner, reaching DC on the low side (It seems to "cross" DC),
and reaches about 160 Hz maximum before it goes down again. Indeed in NL we have 50 Hz, so it is certainly not some kind of stray mains voltage.
4) Indeed, after digesting some Efratom manuals (older ones with decision charts, newer ones with schematics), the xtal oscillator was my first stop as well.
I measured a nice 20 MHz signal on pin 9, and 10 MHz on pin 11 of the oscillator board. That was reassuring.
I followed the signal into the power supply board, where the output buffer is located; no problems on the flex board.
Q3 is the output transistor (2N2222A), but it has no heat sink mounted on it, as shown in the assembly diagram in the manual.
I felt it, and it was only a bit warm to the touch.
I probed the base, emitter, and collector, and all three have reasonable DC and 10 MHz AC signals.
Q3 is the last transistor in the 10 MHz chain, so it seems that the active electronics are fine! :-)
This is where I stopped for today, as it seems impossible to trace the output signal from Q3 to the connector, at least, not without severe disassembly.
It might be as simple a problem with the coax cable, or some short on the power supply board (although I find that unlikely).
For now, the only way I can see to probe a point between the connector and Q3 would be to supply the box through crocodile clamps or such,
so that I do not need to plug in the external connector. And I could actually start by doing a continuity/short check on the coax...
In case you have some more suggestions on how to continue, please let me know!
Hi Ed,
Quick reply (before rushing off to work):
-Have not checked yet the fuses on the power board. Will do so!
(given the high functionality of the unit, I would expect they are intact, but it is good to be certain.)
-The 160 Hz occurs just at the top of the frequency sweep, and only lasts a short time before it sweeps to lower frequencies.
-Even if the crystal has drifted so much that it can not be pulled into the proper range, the coax output should have the crystal oscillator signal present.
My unit does not. That is the first thing to repair. After that has been fixed, the locking can be examined. Mind though that my unit does indicate lock after the warm-up!
-I did test Q3 with power off, and saw the diodes between B-E and B-C. Did not check for a C-E short though, will do that too!
-Your last remark about the flex board is an important one, much appreciated! Together with the complexity, the inexperience in dismantling these units (if unavoidable)
were the two reasons for me to ask for help. On second sight, the complexity is not as overwhelming as I first thought ,
but the handling of the unit (flex board, and especially the physics package with the rubidium lamp and heater and such) is certainly beyond my experience.
I will increase my paranoia level!
-My unit has indeed only the analog sine output, not the TTL output. At the factory, L1 and C18 are always installed, as the default is the sine wave output.
I have read in the manual somewhere that conversion to TTL is done after factory sale. That may explain the presence of L1 and C18.
-Final thought: this morning it dawned on me that the fail may have to do with the main power supply fail. Reasoning is as follows.
It seems that the active components in the 10 MHz chain are fine, up to and including Q3. The reason there is no 10 MHz to be seen on the coax
might be something mechanical (a break in the coax at or near the connector, for example), but there is _some_ signal, so that's unlikely.
Alternatively then, it could be, say, a failing capacitor. And for the AC-coupling capacitor, that might have been triggered by the fail of the main power supply.
If so, then I have lost all hope for the main board, but it looks like the failing capacitor has protected the oscillator very well. Makes sense?
Hey Ed,
The work thing is something I inherited from my dad, he was inflicted too! No need to worry though, it is not contagious or anything like that.
After some puzzling, I checked both fuses. It seems my power supply board is a different revision from what is depicted in the manual I am using.
(Yet another thing to be paranoia about: incorrect or outdated information in the official documentation!)
Both fuses seem to be intact: they both measure almost as a short, but not quite: they measure 0.05 ohms, while shorted leads measure 0.02 ohms.
So that seems to be OK. No face palm award yet! (DMM = Fluke 8842)
Good advice about testing pad G, I had missed that! I found a nice steady 10 MHz sine wave on that point. That means that Q3 is also in the clear,
no need to test for shorted C-E anymore. So tomorrow, I will have a good look at the connector and the coax cable!
No, I don't know the status of the main board yet. I do have another 10 MHz generator that would come in handy there, a home-brew 10 Mhz GPSDO.
(Designing and building that one was a nice adventure. Based on an Arduino Uno, but the micro does not do any counting. I use a 74LV8154 for that.)
I would have to find a replacement power supply first, though. Do you know if there is a PM6685 service manual floating around on the web?
But before putting significant effort and time into that part, I would like to know if the rubidium oscillator is still ok!
Cheers, and may thanks for all your help and guidance!
P.S. in case you're interested, I can post the design-and-build doc of the GPSDO. The costs run only in the $50 to $60 range,
about half of which is the OCXO. I definitely learned a lot from it.
Hi Ed,
>> Do you actually have a coax cable on the connector? Mine, and every picture I can find online, shows the power supply board as a single board
>> with the connector soldered directly into the board.
Yes, there is a connector that is indeed soldered onto the power board in my box as well. There are two other connectors though. At least, I would call them connectors too.
There is some wiring between the power supply and the oscillator, and these wires are soldered to a second connector, which mates with the connector on the power board.
And the coax I was referring to runs from that second connector to a third one (single line with regular 0.1" pitch) that plugs into the main board.
So, this morning I sat down to see if I could find some short or discontinuity in that part of the signal path. To my surprise, I found that there is a 50 ohm resistor
somewhere in the coax/connector assembly. I can not locate it exactly, but I would expect it to be at the end of the third connector.
After measuring, and checking again, I could find nothing wrong with the cable. So, in desperation, I hooked everything up, to see if I had perhaps missed something
in my earlier measurements. And to my astonishment, I now have an accurate 10 MHz signal at the third connector!!
Of course I am very happy about it, but I really have no clue what has changed to make that happen...
And now that it seems to work, it is impossible to find the fault...
I have hooked up my lowly Tek CDC250, which I have repaired some years ago by replacing the dead reference by a TCXO.
I have calibrated that reference with my GPSDO. It has been a while since I last used it (more than a year...), but when I hooked it up
to the Rubidium reference, and allowed for some warming up, it is giving a very steady 10,000,000 reading, with every now and then 10,000,001 .
Not bad at all!
A few remarks/questions in hindsight:
-Given your warning, I have taken a good look at the flex board, and it seems to be in excellent condition. No sign whatsoever in the direction of it being brittle.
-I found that the screws to mount the box on the base plate are just a bit too long, and they nudged a capacitor and bent it out of ts original orientation. i did not bend it back.
-On lifting the outer shell (after removing the four bolts on top), I found only two plastic washers. Are there supposed to be two, or four? Or perhaps none?
So, it seems that we can now shift attention to the "rest" of the PM6685. And I agree completely, that is a entirely independent project in and of itself.
It is clear that the supply needs to be repaired or replaced, so I want to disassemble it from the counter, at least to protect whatever is still alive on the main board.
Heh, I was laughing when I looked at the video you posted: at 7:45, the guy simply lifts the supply board from the main board, effortlessly.
That will not be possible at all in my case! In fact, due to the construction, I think it is impossible to remove it without actually destroying one thing or the other.
During assembly, the board was mounted on the main board with nylon pylons, and then soldered in place on both ends at multiple pins...
So for disassembly, I see no other option but to destructively cut the pylons, and to make a jig from copper wire that allows me to heat all connector pins at the same time,
allowing me to desolder the board one end at a time. The heat sink close by does not make it any easier, though.
I truly wonder how they serviced the power supply in the factory.... And please do let me know if you can think of any alternatives!
If and when I can assemble a repaired or a replacement power supply back onto the main board, I will use nylon standoffs with ditto nuts and bolts!
>>Google is your friend.
Indeed it is, and I found the service manual without much difficulty. I am reading the relevant parts on the power supply.
And actually, the first thing I would like to do, is to power up the main board with a stand-in supply to see if there is still some life in it..
That replacement power supply would need to generate +5, +15, and -7 V, and the 5V is used as-is on the main board. Hmmmm.....
These supplies are not offered on FlukeBay. Fortunately, my unit already has the 2.7 GHz option and the GPIB option.
Thanks for the warning about doing measurements on the supply. I will be careful with that. But first it need to get out!
I already did some measurements on the transformer, and what worries me is that on the primary side, I do see the coil on the 1 - 6 pins (0.5 ohm),
but the coil on the 3 - 4 pins measures in the Mohms. I guess that means conclusively that my transformer is broken, doesn't it?
If so, is it worth-while to try to fix the transformer, or can they be found somewhere? Should I start looking out for a replacement supply?
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.
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.
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.)
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.
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
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.
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 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.
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
QuoteOne 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.
Cheers for now, variable supplies are ordered, I will report back once I have rigged up the whole shebang.