[time-nuts] Robert G8RPI -- Odetics 325 PSU

[Roy Phillips]

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" <glennmaillist at bellsouth.net>
To: <time-nuts at 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 <phill.r1 at btinternet.com> 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
>>
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