Electronics > Repair
Help needed for repair of Efratom FRS-C rubidium oscillator
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edpalmer42:

--- Quote from: ejd.pol on April 11, 2024, 08:20:40 am ---Hi Ed,

Quick reply (before rushing off to work):

--- End quote ---

What is this 'work' thing you speak of?  I don't remember ever hearing that term.  ;)


--- Quote ----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.)

--- End quote ---

True, but missing a blown fuse is high on the list in the Face Palm Hall of Infamy!


--- Quote ----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!

--- End quote ---

Agreed.  However, we're starting to run out of obvious things to check so I'm starting to think about less obvious possibilities.


--- Quote ----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!  :o

-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?

--- End quote ---

One other point that you can check for signal is 'pad G' which is shown on the parts drawing and is silkscreened on the power supply board in my unit.  It's between the main connector and the 10x1 connector labelled as J1.  If the signal is present there it proves that L1 and C9 are good.

It seems unlikely that the failure of the main power supply would affect the output of the FRS-C.  Perhaps a pre-existing fault that either wasn't noticed or wasn't important.  Just use an external reference and carry on.

Have you confirmed that the counter's main board is dead?  Many (most?) failures in SMPSs just make the output disappear rather than go high.

Ed
ejd.pol:
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.  :D

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!  8)

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.
edpalmer42:

--- Quote from: ejd.pol on April 12, 2024, 09:08:54 pm ---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.  :D

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)

--- End quote ---

Yes, another thing we can check off the list.


--- Quote ---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!

--- End quote ---

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.


--- Quote ---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?

--- End quote ---

Google is your friend.  Also, fixing the original power supply is a reasonable thing to consider.  But don't go probing it with your scope unless you're REALLY sure what you're doing.  FYI


--- Quote ---But before putting significant effort and time into that part, I would like to know if the rubidium oscillator is still ok!  8)

--- End quote ---

Why?  I mean, yes, it's fun to fix a Rb standard, but it's basically a seperate project from fixing the counter.  AFAIK, your PM6685R is also known as a PM6685 (without the R) which has a built-in oscillator.  If you want better accuracy, you can use an external reference or purchase a plug-in OCXO internal reference from the web site we love to hate!  Actually, there's only a couple available right now.  Search for pm6685 and pm66xx.  There are add-in oscillators and RF prescalers that just drop into place.

By the way, I sorted out the confusion in my unit regarding TTL vs. Sine output.  I think this unit left the factory with the TTL option.  Somewhere along the line someone converted it to Sine output - all the component values match the requirements for a 10 MHz Sine output.  But they forgot to remove the factory-installed jumper between pads A-B and didn't bother to remove the TTL sticker that's on the flex board.  When I clipped the jumper the waveform purity and amplitude improved dramatically.  Nice!  I didn't notice before because since the unit worked, I had no reason to pull it apart.


--- Quote ---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.

--- End quote ---

Thanks, but I have too many GPSDOs .... and too many Rb standards .... and too many fancy OCXOs ..... I think I'm seeing a pattern here ......

Ed
ejd.pol:
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!  8)

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!



 
edpalmer42:

--- Quote from: ejd.pol on April 13, 2024, 04:51:50 pm ---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.

--- End quote ---

Okay, I was getting confused between J1 (the single line connector) and P1 (the main I/O connector for the unit).


--- Quote ---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...

--- End quote ---

The fact that you touched it and it started working is very suspicious.  Check the solder joints on J1 and P1 *VERY* carefully.  I use a 20X magnifying glass.  Connectors, switches, even some component leads tend to unsolder themselves over time.  This get particularly bad if there's power and/or heat involved.  Even at 20X magnification it's almost a guess whether you're looking at a good solder joint or one where the component lead has completely broken free of the solder and is just sitting in the hole.  Is that a crack that I'm seeing or just a trick of the light?  It's often easier to just add some liquid flux ( that's usually necessary ) and resolder all the pins. 


--- Quote ---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!  8)

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?

--- End quote ---

I didn't find any washers in mine, but it was messed with before I got it.  Mine has a plastic sheet that's folded into the inside of the cover.  Does yours use different screws to attach the base plate and the cover?  Mine has 5 screws for the baseplate and 4 for the cover.  They look identical but I noticed that 5 of the screws are non-magnetic and 4 are magnetic.  Coincidence?  I think not!


--- Quote ---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...

--- End quote ---

No, he did the disassembly off-camera.  He referred to an earlier video where he repaired a similar unit so he didn't want to waste time doing the same thing again.  Be sure to check out that earlier video.


--- Quote ---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!

--- End quote ---

Working on a mains-connected power supply, particularly a switching one, can go really bad, really fast!  You should do as much as you can with the supply unplugged and only after you made damn sure that the capacitors are discharged!!  Then check the rectifiers and all the electrolytic capacitors on both the primary and secondary sides of the transformer.  They are easy to check and are often the culprits.  The fact that the fuse blows instantly suggests a shorted capacitor.  It's possible that such a short could then take out the rectifier associated with that capacitor.

Ed
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