Switch Mode Power Supply Unstable Output Cure

I got this switch mode power supply from a friend and my plan was to use this on my 80W linear amp. Things went the other way when I tested this supply with my 80W amp. The output voltage won't stay at 12V and every time i keyed my transceiver, the output voltage swings to 15V and sometimes the worst hitting a peak at 17V which by any moment will surely blow the PA transistor of my amp. It took me a quite a while to figure out what was causing the issue but then it was the stray RF reaching the PWM switching regulator IC was the main culprit. My verification includes bringing a 6W portable hand held transceiver close to the switch mode power supply and pressing the PTT while monitoring the output voltage of the power supply. Every time i pressed the PTT the output voltage jumps to 17V  and the proximity of the rubber ducky antenna is directly proportional to the increased in output voltage of the power supply.

I got the courage to disassemble the switch mode power supply to access the circuit board and found the only chip mounted on the board. It was a AZ7500E pulse width modulator regulator chip that runs this power supply and found some important pins that are responsible for its regulation. I tried to bypass to ground pin1 and pin16 (both were (+) inputs of its built-in error amplifier) using a 0.01uF ceramic capacitor. After the modification I quickly reassembled the power supply and test again for stability and surprisingly works now normally. --- 73 de du1vss

Home Brew 60W RF Dummy Load

Another great way of making your own 60 watts dummy load is by the use of Caddock thick film power resistor.  Purchased some of these at http://philippines.rs-online.com/web/ which cost  at 270 Pesos each. Two 100 ohm / 30W resistors were paralleled to make a 50 ohm / 60W dummy load. The good thing about this product is that it comes in TO-220 package and mounting this on a suitable aluminum heat sink is a big help in heat dissipation during actual use. Been able to test my prototype on VHF and provides a very good matched at 144MHz exhibiting an SWR value of 1.1:1 across the amateur band due to the fact that reactive components can be easily minimized because of the unique package of this resistor.

Thermal compound was added between the aluminum surface in contact with the power resistors. A pig tail coax to PL259 connection was also included but a slight increase in SWR was noted. 73 de du1vss