Trusting an old switching supply?

[jxm]

I've got a Peripheral Technology PT69-2 circa 1985 on the bench. Before I power it up with the original supply I'd like to check and see if anyone here has experience with this design.



The supply appears to be based on a MB3759 Switching Controller with 4 outputs, +5@6A, +12@3A, -5@1A and -12@1A. The -5 output is not actually brought out.



None of the electrolytics show signs of leakage or bulging, no heat discoloration on the PCB or components was noticed.



The scope capture was taken after 45 minutes of operation and shows.

• +5 loaded at 1A
• +12 loaded at 600mA
• -12 unloaded

Channel 1 is +5, channel 2 is +12 and channel 3 is -12. Ambient temp was about 78F/25C.



Using a hand held multimeter the DC voltages were 5.05, 11.64 and -11.16; these were spot checked several times during the 45 minute test and did not deviate by more than a few tens of millivolts.

The +5 and -12 rails appear to have less than 50mV of noise, +12 has maybe 70mV of noise. What appears to be the switching frequency seems high at 56kHz as the controller is spec'd at nom 40kHz, 36 min 44 max.

I would say that overall the supply is functional, but I've not done any switching power supply design. Maybe someone can tell that it's about to turn the switching transistors on one last time.

Thanks for any feedback.

Jesse

 

[Hugo Holden]

Your supply is just fine.

While you might wonder about the electrolytic caps in it, the fact is if it has not had much use, hours wise, they will be fine.

The best test they pass is if you power them up after a long time frame and they survive.. If they do, after that, due to re-forming under the applied operating voltages they will only improve.

If there is any doubt you could re-cap the supply. If you did, by far and away the most preferable electrolytic caps are Nichicon brand and the 125 deg C rate types.

But, if the supply you have shown were mine, I would simply leave it as it is and wait until a problem developed (if it did) before replacing parts.

 

[jxm]

Thanks for the feedback Hugo.

I think leaving well enough alone is a good choice.

In the time between taking the measurements and posting them I took the opportunity to power up the board with a bench supply, I'm happy to say that I got the HUMBUG monitor prompt as expected.

Now to clean the chassis up, install a suitable pair of floppy drives and fabricate cover.