50w PA failure, Where to start?


Hello, Recently, I was utilising my 50w PA with my QCX+, when spontaneously, a fault occured. The PA kept turning on and off repeatedly, First i checked to see if it was my power supply, and, it was not. I unplugged the PA from power,rf,etc as to not cause any more damage, and have only gotten around to investigating now.

Do note, The behaviour of it turning off an on repeatedly WAS a powersupply thing (Laptop PSU's do this, apparently), And on other, dumber powersupplies, it would flash on, then turn off indefinitely.

After investigation, I noticed that the leads off of q2 were breaking off, however, after soldering them again, the problem did not go away. I noticed specifically, that q2 got very, very hot fairly quickly, but q1 did not.

Where should I begin to find the fault? Should I preemptively replace both q1 and 2?

Ben Bangerter, K0IKR

Sounds like Q2 is shorted.  It is easy to replace Q1 and Q2.  Cut the leads close to the package, or bend the transistor back and forth until the leads break, then carefully remove them individually.  I found that heating one lead/pad with the soldering iron until the solder liquifies, then rapping the board sharply on the bench results in the lead remnant and solder being rejected from the through hole.  Be sure to use IRF510s from a reputable supplier, the same make (Vishay) and lot preferably. Good luck!

73, Ben K0IKR