Glenn Thanks for the very incisive "inside" perspective on SMPS - I would certainly agree that the average SMPS unit is very much a 'budget' design, but this one is rather well made, and provides a DC 'back up' facility, so it would be nice to restore it. There is also the very real danger of contact with the input stages , with the UK AC supply power at 240 volts, this could be quite lethal. This particular item has both AC (90-240V) and DC (20-60V) input switching stages, making it even more difficult to unravel. I'm sure most engineers would rather bin a faulty unit than repair it. Long live the 'good old' linear supply, perhaps I should build one, as there is certainly space within the equipment. Roy ----- From: "Glenn Little WB4UIV" <[EMAIL PROTECTED]> To: <time-nuts@febo.com> Sent: Sunday, April 20, 2008 5:04 AM Subject: Re: [time-nuts] Robert G8RPI -- Odetics 325 PSU
> Roy and the group > > SMPS have a very classic failure mode. As the design is finalized by > a bean counter, and/or the person that designed the SMPS was not > qualified to do so, we have capacitors pushed beyond what they were > designed for. If you are lucky, the electrolytic capacitors are 105 > degrees C. If not they are 85 degrees C. In all probability, the > capacitors are not rated for the service that they are being used . > > To fix the supply replace all of the electrolytics with pulse rated, > 105 degree C parts. The SMPS uses high frequency for the energy > transformation. The designer had to use fast diodes, but, because the > cheaper capacitors will continue to, somewhat, operate for the period > of the warranty, the cheaper capacitors are used. > > Use the largest capacitor that will fit into the available space. A > higher voltage capacitor will work fine. If you can measure the > capacitor ESR, you will find that the ESR has climbed to a higher > than acceptable level on the capacitors that are causing the problem. > > If you check the new capacitors before you install them, you will > probably find that they are just above the lower tolerance value. If > the next higher value will fit, use it. > > I replace failed electrolytic capacitors almost daily. I am the chief > engineer at a TV station and see more problems with capacitors than > anything else. Our equipment is highly abused by being left on 24/7. > > Almost all SMPS problem that I have seen are capacitors. Many > "technicians" cannot fix a SMPS as they cannot find a bad > semiconductor and the capacitors will check close to the correct > value with acceptable leakage. They cannot check the parameter, ESR, > that is causing the problem. > > Hope this helps. > > 73 > Glenn > WB4UIV > > At 02:04 PM 4/19/2008, you wrote: >>Hi Roy, >> I've looked at two of these PSU's that had failed and was unable >> to repair either of them. (I'm a professional electronics engineer >> and can fix most things) There were no obvious faults like leaking >> capacitors or burnt up components. The topology of the >> auto-switching between 28V DC and 115/230V AC is not obvious and >> the PCB is very densely packed making it very hard to trace the >> circuit or make measurements. The high voltage on the PCB makes it >> hazardous to work on. I gave up and replaced them with a standard >> mains input multi-output unit. I don't recall the voltages and >> ratings (they were marked on the unit), but if you let me know what >> they are I'll see if I have a suitable unit. email robert8rpi (at) >> yahoo.co.uk >> >> Regards, >> >> Robert G8RPI. >> >>Roy Phillips <[EMAIL PROTECTED]> wrote: >> Hi Robert >> >>You may remember that we exchange info on the 325 SatSyc Receivers. >>I remember that you said that the PSU was a weak point with the >>system - you were right on , after finding a suitable antenna and >>replacing the lithium battery in the Rx., it was running sweetly for >>about 18 days, but yesterday it failed. The power fuse (AC input) >>had blown, and following a quick inspection within the PSU - nothing >>appears to be burnt out or visibly damaged. What was your experiece >>of fault finding with this item. Before I proceed further, do you >>have the schematic for the PSU - this is not in the normal manual - >>its an AC to DC, and a DC to DC unit. Its a quality item, Model >>VF100 -494-10/CX ,made by "Converter Concepts", and should be worth >>restoration. Any ideas ? >> >>cheers >> >>Roy UP >> >>_______________________________________________ >>time-nuts mailing list -- time-nuts@febo.com >>To unsubscribe, go to >>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>and follow the instructions there. >> >> >> >>--------------------------------- >> Yahoo! for Good helps you make a difference >>_______________________________________________ >>time-nuts mailing list -- time-nuts@febo.com >>To unsubscribe, go to >>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.