| Electronics > Repair |
| Help needed for repair of Efratom FRS-C rubidium oscillator |
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| edpalmer42:
--- Quote from: ejd.pol on April 17, 2024, 08:16:18 am ---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. --- End quote --- 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. --- Quote ---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. --- End quote --- 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. --- Quote ---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.) --- End quote --- 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. --- Quote ---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. --- End quote --- 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. --- Quote ---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 --- End quote --- 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 |
| ejd.pol:
Hello Ed, --- Quote from: edpalmer42 on April 19, 2024, 08:27:37 pm ---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! :) --- End quote --- 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. --- Quote ---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. --- End quote --- 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. --- Quote ---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. --- End quote --- 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. --- Quote ---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. --- End quote --- 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... |O 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! --- Quote ---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 --- End quote --- 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.... 8) Cheers for now, variable supplies are ordered, I will report back once I have rigged up the whole shebang. |
| edpalmer42:
--- Quote from: ejd.pol on April 23, 2024, 06:28:07 am --- --- Quote ---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. --- End quote --- 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. --- End quote --- It isn't clear to me why a switching supply would have trouble with high-voltage DC input. The only thing I can think of is that with DC, the input voltage won't ramp up at mains frequency, it will just hit hard at maximum voltage. Maybe the internal surge limiter wouldn't be able to cope with that. :-// --- Quote ---Cheers for now, variable supplies are ordered, I will report back once I have rigged up the whole shebang. --- End quote --- Good luck. Let us know how it goes. Ed |
| ejd.pol:
Hello Ed, the adjustable supplies arrived some days ago. After a bit of thinking on how to attach them to the PM6685R, I soldered six wires to the underside of the main board, onto which I connected female connectors. These allow to plug in the barrel jacks of the various supplies. See the attached pictures. Once I had rigged up everything, I triple checked all connections, making sure the right supply was connected to the right pad on the board. After turning on the supplies with the switch on the power strip, nothing happened. However, pressing the power button gave some response: the back light of the LCD turned on, and the fan turned on. The display contents remained empty, though. Nevertheless, as the power button is read by software, this means that a lot of circuitry on the main board is operational, and that software is being executed properly. I was puzzled by the empty display. By then, given that the controller appeared to be up and running, I had expected at least an error message of some kind. I had not connected the oscillator yet, so I decided to hook it up, to see if that would make a difference. And it did! The counter sprang to life, and displayed nicely that it was picking up some signal, with all associated bells and whistles. So, this meant the main board is still functional. This is way more than I had expected, so I was (and still am) delighted. In the provisionary setup with the adjustable power supplies, I had also included some voltage and current meters, to give me some sense of the power consumption from each of the three supply rails. All three rails were drawing comfortably less than 1 A of current each. So, I ordered three miniature power supplies of 5, 15 and 9 volts, capable of supplying 1.5 A for the 15V one, and 2 A for the other two. If all is well, the three supplies together will fit into the space of the original supply module. On the primary side, I will hook them up to the AC voltage, not the DC. I will let you know how the story ends, and post some more pics of the final solution. After this adventure, I do have a few questions, I would appreciate your opinion on the following: * The fan only turns on when the counter is taken out of standby. The remarkable thing is that the fan is cooling the supply of the Rubidium oscillator, which is on all the time. That power supply needs to work just as hard when the counter is in stand-by as when it is on. So I don't get it... Is it perhaps the case that when the counter is on, the fan is supposed to create a more stable temperature environment for the oscillator? (By the way, that would also explain why the fan is allowed to be operated from a different power supply, which may break...) * When the 10 MHz clock is not there, apparently the display remains empty. But the software is very much alive. So why is there not some error message displayed? Or at least some kind of sign of life? This boggles my mind. Anyway, after installing the replacement supplies, it looks like I will be able to see how accurate the counter is, after I hook up my GPSDO. I will keep you posted on that part as well. Again, many thanks for the advice and moral support! EJ P1: after all these years, the flex board is like new! P2: after taking off the shell, you can see the bent capacitor P3: after removal from the counter, the original supply really looks battered more pics to follow in a next post, due to size restrictions |
| ejd.pol:
P4: backlighted supply connections. You can even see the thin sense wire P5: soldered wires to hook up the provisional supplies P6: wires and connectors, with a tie-wrap to a post for strain relief P7: voltages and currents drawn P8: This made me happy! P9: error messages are not unheard of |
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