Converting a PC Switching Power Supply to 13.5V

This information was published by VK2EVB, Peter McAdam, in the December 2001 edition of Lo-Key #72 (CW Operators' QRP Club Inc.). As I have done nothing but reproduce his results, all the credits and kudos belong to VK2EVB.
Power supplies from old personal computers are "a dime a dozen," so to speak. Go to any hamfest/computer show and you will find tables of them. I can get all I want from a local computer vendor who has stacks of old "useless" computers. Typically, these power supplies produce outputs of +12V/9A, -12V, +5V/20A, -5V. VK2EVB's circuit converts the +12V line to +13.5V. By keeping the original rectifier in the +12V line one ends up with a +13.5V/9A power supply. (VK2EVB swapped the 20A rectifier from the +5V line with the 8A version in the +12V line to end up with a supply rated at +13.5V/15A.) I choose to keep the 9A rectifier in line and not do the swap as I do not need a high current capability while running QRP.

Doing the Conversion
Use Care When Working on the Power Supply as the AC Mains Lines are Exposed When Opening the Enclosure!!! Unplug the power supply when working on it and put it back together before testing it!!!
The process is fairly simple and straight forward (refer to the schematic below).
Basically VK2EVB removed three resistors from the power sampling circuitry, and added one.
1. Start by removing all but one output lead from the +12V line and the GND line. Keep the leads with the largest wires. These will serve as the output lines to whatever jack you supply.
2. Trace Pin 1 from the TL494 through the 27k resistor (R33) to the +12V line, and insert a 2k2 resistor in series with the +12V output and R33.
3. Trace Pin 1 from the TL494 through the 4k7 resistor (R32) to the +5V line and remove the resistor.
4. There is also on that same line a 470R (R21) and a 270R (R20), going from the +5V trace on the board to -5V and -12V respectively. Remove both of these resistors.
5. Secure the circuit board back into the enclosure and plug in the AC line. You should see +13.5V where formerly there was +12V. (I didn't, at first! I had removed the wrong resistor and ended up with about +8.8V!)
Improving the Filtering
1. Typically there is a 2200uF/16V electrolytic from the +12V output pad to GND. VK2EVB replaced that capacitor with two 2200uF/25V electrolytics. I didn't have those values in my junk box so I replaced the original capacitor with a 6800/35V electrolytic instead. Remember to observe the polarity of the capacitors!
2. VK2EVB also installed a filter in the output line of the now +13.5V supply to remove any stray radiation from the AC mains. He used a PI filter consisting of two 0.047uF capacitors and a 100uH inductor using a ferrite core from another junk supply. I didn't install the filter, but I did install a similar RF choke in series with the +13.5V output line. I used a ferrite core from an old computer power cord, winding as much insulated 18AWG stranded wire through it as I could. I ended up with about 550uH.
3. A 220 ohm resistor was also added across the output +12V and GND terminals to serve as a load for the supply, attempting to keep the noise to a minimum. VK2EVB used a 1 watt resistor; I used a 2 watt carbon composition.
4. VK2EVB also soldered desoldering braid on top of the +12V traces to improve current handling capability. I chose not to do this since I didn't swap the 9A and 20A rectifiers (see the note above).

Almost Finished!
1. In my supply there were two holes where the original output wiring and computer on/off switch wires exited the enclosure. These holes were on the end of the enclosure opposite the fan and AC jack. The end of the enclosure was slotted for ventilation.
2. I bolted a rectangular piece of circuit board on this back side of the enclosure, using the slots for screw holes, and mounting two banana plug terminals in the space provided by the larger holes.
3. I also enlarged one end of one ventilation slot to accept an SPST toggle switch. (This switch should be rated for the AC line current and it should be installed in one of the AC lines coming into the power supply. Use fairly heavy wire.)
4. VK2EVB also installed a power on indicator LED. He picked up +12V for the LED from the original +5V circuit board pad which after the conversion is at about +12V.
5. Stick four rubber feet on the bottom of the enclosure and test the supply at various frequencies while listening on your radio. Hopefully, you'll hear only signals and background noise, and no spurious signals from the switching supply.

My Results
My converted supply output is +13.49V/9A and, thus far, I haven't heard any unwanted signals from the power supply itself. The conversion took me about 2 hours to complete. Have fun and credit VK3EVB for a fun homespun project!