Astron switching power supply repair

Astron SS-18/SS-30 repair

I bought a broken Astron SS-18 switching power supply on ebay for $20. Perhaps to add to the misery of the broken R2200 power supply, maybe to learn a bit more about repairing these damn things. Having found the schematic on-line, I decided to bid.

What I've learned is that switching power supplies all seem to be about the same. In general there's a high-voltage (i.e. can kill you) AC and DC side, a switching transistor or two to chop up the HVDC, some rectification, a low-voltage side with some filtering, and switching control.

Let's start with this: repairing a PC power supply is just a waste of time. Go buy another for $35 and be done with it. Maybe spend a few more bucks this time for a better supply that won't fail so easily. But most commercial 12V smps are a lot more expensive and it's somewhat more specialized for communications equipment -- so maybe worth spending some time.

I started in the LV section. I pulled the caps and tested the LV rectifiers and filter caps. The filter caps had somewhat high ESR so I replaced them with some new low ESR caps that I had on-hand from a different project. I don't think this was strictly necessary but whatever. Caps lead a hard life and good new caps tend to be inexpensive.

The HV side I didn't bother testing because the supply didn't lend itself to safely testing anything without a lot of work and preparation. And it wasn't blowing input fuses. Skip it.

So, what about the the PLL switch control? Turns out they start with flea-power windings off the switcher transformer at startup and then switch to running from the output voltage. So, you can back-feed 12VDC into the supply, completely disconnected from AC mains circuitry, and more safely investigate the switch functionality.

There are two outputs from the PLL switch -- on this unit a TL494 -- which drive the HV switch transistors. I started here. I saw no output on my oscilloscope. Hmmm.

As luck would have it, I had a cast-off PC power supply that was good but didn't have the proper connections for newer motherboards. It had a TL494! I replaced the TL494 in the Astron with the one from the PC power supply and that fixed it! Amazing!

So now I have a lightweight, nicely-built mid-current 12V PS for ham radio stuff, repaired for a cost of $0 and I learned something along the way. Good all the way around.

Astron SS-30 repair

Another switcher. Purchased this one cheap off ebay, not working. Physical condition looked good, 10A 5x20 fuse completely vaporized inside.

Initial suspicion: bridge rectifier. Why? There's not much circuitry between the fuse and the input switching transformer. Something caused a dead short to vaporize the fuse. That leaves a shorted capacitor, shorted rectifier or shorted input transistors. Bridge rectifiers are easy to remove, easy to test. I've seen this failure before. Start here.

My initial suspicion was correct, the input bridge (PBU605) had one shorted diode. Upon replacement of the rectifier, the power supply once again works fine.

So to summarize, if you're blowing input fuses, check the input rectifier for proper operation followed by the input switching transistors. If no output, check any bad caps in the output circuitry and verify proper operation of the switch control chip as appropriate.

Sat Oct 21 20:10:15 CDT 2017

Another broken ebay Astron SS-30. This one was a little different than the last -- good fuse, but it goes 'tick-tick-tick-tick' and has no power output. Without any diagnosis I order a couple of TL494 PWM control ICs from DigiKey and a set of output caps. The four output caps are cheap as are the TL494. After replacing those two items, the power supply is back in operation. I opted for the TL494IN variety as it has extended an extended temperature range on the specs and is the same price as the other flavor. Why not?!

This page last modified Sat Oct 21 20:14:51 CDT 2017 by timc!