Repair of two 13.8V 10A power supplies (ICT12012-10AX)

Just last week I took a look at a couple of bad power supplies that I received. Along with a pair of channelized UHF radios. The two power supplies were of the same make and model. Make: "Innovative Circuit Technology Ltd." Model: "ICT12012-10AX" They had different symptoms. One would power up but after a minute or two the voltage would drop off and the radio would shutdown. It also would not supply necessary transmit current. The second had zero output on the 13.8V line, apparently dead.

I took a look at both and decided to start with the first one that had some output. I measured the AC ripple on the 13.8V line and it was showing on my DMM around 300mv rms. The spec for the power supply claimed no more then 25mv of ripple. With that I suspected the output electrolytic capacitors. This is a typical failure with switching power supplies. They require very low ESR (equivalent series resistance) at fairly high capacitance out of the output capacitors. I replaced these capacitors and the UHF radio remained on indefinitely. However on attempting to transmit the extra current draw caused the radio to shut off as the voltage dropped too low. In this case I decided to swap out the remaining capacitors on the power supply including the electrolytics on the primary side. This made the first power supply fully functional.

The second power supply having no output I at first suspected a failed semiconductor. The fuse was fine but I still checked diodes and the main switching transistor with a DMM for shorts across semiconductor junctions . There were none that I could find. With an O'scope, 100x probe and an isolation transformer I checked the main switching transistor for any switching action. There was none. I then looked up the datasheet for the IC that should be controlling this transistor and verified power to the supply pins and checked for switching on the transistor control pin. The voltages seemed reasonable per the data sheet however there was no switching/pulsing occurring at the transistor control pin. After some poking around the power supply came to life briefly with some audible whining. I found that the switching action was intermittent, not consistent. I decided to swap out the capacitors on this power supply as well. Just to be on the safe side I touched up any solder joints on the board that may have been questionable.

The main switching IC has it's own voltage supply on the mains input side. It is made up of simple discreet components providing only enough power for the control circuitry. If the main switching IC is not getting decent power itself as in there is considerable ripple on it's supply line then the power supply may not function properly. A bad capacitor could cause this. After replacing all of the same capacitors that I did on the first unit the second one came to life. The switching was consistent at 100khz without the sharp edges typical of cheap switching power supplies that can cause RFI. This is likely why it is used as a communications power supply.

As a side note, removing the power supply PCB from the chassis required cutting or desoldering the wires going to the output terminal.

Below is an image of the power supply with some descriptors (if you can see them,,,). Circled in green are the capacitors that were replaced in both units. Near the transformer I had to series two capacitors to get the voltage rating that was on the original.

Repaired 13.8V 10A power supply. -AC9LF

The copyright printed on the PCB shows 1997 so these supplies are around 20 years old now. Maybe they will last another 20 or more.

With these power supplies in working order my home PC of 12 years stopped booting up last Friday. That will be another post.