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

Glenn Little WB4UIV glennmaillist at bellsouth.net
Sun Apr 20 00:04:07 EDT 2008

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.


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 ?
>Roy UP
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