| Electronics > Repair |
| Help needed for repair of Efratom FRS-C rubidium oscillator |
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| ejd.pol:
Hi Ed, Yeah, a bad solder joint, or a crack in the flex board could be the culprit here. The solder joints are not easy to check though, as they are all hidden inside the box. So on one hand, it would be nice to check everything, on the other hand, that check could easily cause a new problem! So at the moment I am considering to err on the safe side: not mess with it now that it seems to work. Of course, I would like to have proof that the system achieves a good lock. What is a sure sign of proper lock for you? Yes, my unit has the plastic sleeve also on the inside. There are holes in the sleeve that are much larger than needed to pass the bolts through. So, given the flexibility of the outer shell, when no washers are used, the bolt will bend the shell locally. I can imagine that washers are used to prent this bending, but that couls also have been achieved by making the holes in the inner sleeve small. So I do not understand the function of the plastic washers. (It is not isolation, as the bolt will provide electrical contact. Thanks for the tip about magnetic and non-magnetic bolts, I will check mine for that! After some searching, I found the video that the repair guy was referring to in the video that you posted. It is about a repair of a PM6681, and indeed it shows the repair of the supply, which seems pretty much identical to the PM6685 supply. In his case, the TL431 needed to be replaced. But... the video does not show what technique he uses to take the supply out! :-/O That second video does show how he examines his supply, rigs it such that he can probe and such, that is definitely useful. So, I did what I have described earlier: destroy the pylons, and then desolder the connecting wires on both ends of the PCB. Removing the pylons was easy: the nylon has become brittle, so only using a needle-nose pair of pliers, and a bit of twisting did the job. For the desoldering, I first went in with some solder-wick, and that worked just fine on the primary side. The secondary side has some more pins, and it took a bit more effort to get it loose, but in the end I got that too. Phew! :phew: What was clear immediately was that the two 470uF/35V secondary end caps definitely need to be replaced. The green plastic sleeve is discolored to black at the base, and there is some signs of leakage in the from of gunk between the caps and pcb. And while I am at it, the 5V end cap of 10,000uF/6V3 will be replaced as well. I plan to use a 10V type for that, if I can find one that fits in the available space. But before actually doing anything with the caps, I have two more checks to do: the rectifiers (as you rightfully mentioned), and the transformer. 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? Cheers, and many thanks for your help in this adventure! |
| edpalmer42:
--- Quote from: ejd.pol on April 14, 2024, 07:23:35 pm ---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? --- End quote --- It seems unlikely that the transformer is blown. Note that the service manual says that the only time the fuse should blow is if there's a catastrophic failure on the primary side of the transformer. The winding from 3-4 is a low voltage, low power winding that only provides power for one small IC. I don't see anything there that would be capable of frying that winding. R57 is shown on the schematic as 100R while the parts list says 47R, but only 1/8 W. That would blow open long before the winding would fail. R33 is 10R 1/8W. It would blow as well. Also, V08 is set up as a current limiter that won't allow more than about 130ma to flow. Recheck your measurement. If the transformer is blown, it's not practical to fix or replace it. It's a custom part that only Fluke has and I doubt they would sell it to you. Your only option at that point would be to do a complete replacement. That would be a significant, but perhaps straight forward job. Ed |
| ejd.pol:
Hello Ed, I did do some more measurements to get a better view on what may need repair, and doing so, I can follow a path of destruction. All measurements are done in circuit. What follows is a list of broken primary components. Primary components not mentioned are ok. Secondary components have not been measured. -The recheck of the winding on pins 3 and 4 of the transformer gave the same result: 17 Mohm, and rising during the measurement. -Both diodes in D09 measure open. -R57 measures 4 Mohm -R29 measures 2 kohm -Transistor V08 shows a short between C and E, and that brings us to ground again. I can not tell easily if IC U03 (UC3842A) is still intact. I was thinking about this a bit, and the only way that I can think of for these failures to happen, is if the two primary windings shorted. In turn, that may have been caused by the secondary output caps failing? Large secondary currents heating up the transformer perhaps, combined with a weak spot somewhere in the insulation of the primary windings? In any case, I think it is very likely that I do have a broken transformer. So, before sinking anymore time or money into the supply, I would want to see the main board functioning! If the main board is toast, then the PM6685 can be scrapped, except for the reference, of course! (Unless the physical parts may be of use to someone.) So, some thinking is needed on how to power up the main board with some kind of emergency supply. I'll keep you posted on progress, and as always, your feedback is highly appreciated. |
| edpalmer42:
Wow! Some serious carnage there. Yes, something obviously went horribly wrong! I'm not hopeful, but since the blown circuits are nothing more than power for U03, you could try, with the unit unplugged, injecting power from an external supply to see if U03 wakes up. I'd recommend a variable lab supply. Set it for maybe 200ma current limit and slowly increase the voltage to a maximum of 10 - 12 volts. Be sure to check the resistor chain (R24 - R26, R27, R30 - R32) and D12, D13 to make sure there are no blown components there. If U03 draws some current, but doesn't cause the supply to current limit, you can consider whether to try a power test with the unit plugged in. I'd STRONGLY suggest that if you do a power test, you use a dim-bulb tester. If you're not familiar with that, search the forum. There are a few good threads that describe the tester and its use. It's trivial to throw one together if you don't have one. I've had a few cases where I needed to simultaneously power up multiple supplies. I just used a switched power bar with multiple supplies. That would be appropriate for the above power test and for powering up the main board. If the main board survived, you could replace the three supplies. +5 and +15 modules are easy to find. -7 would be a variable unit. You might be able to find a Fluke parts unit for sale that uses the same power module. There seems to be lots of commonality between the various PM66XX models, so that might be a practical option. Determine which units use the same power module and then start looking around. Ed |
| ejd.pol:
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 |
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