VHF FM Amp Repair

About three weeks ago while rummaging a junk box  at a local hamfest I saw a TPL PA3-1AE-2 amplifier in pieces in a box. I’m familiar with TPL products as I have a similar version of this amp so I knew exactly what it was.  I like them as they tend to be more commercial quality amps and are fairly easy to repair.

While it was in pieces it had a bag of screws and everything somewhat neatly stuffed together in the parts box. The box owner told me to make him an offer and while I stared the parts for a bit he blurted out, “How about $10?” The price sounded good to me so I bought it.  Worse case scenario… I can use the heat sink and box for something.

This amp if a VHF (130-174mhz), commercial quality amp.  FM service only as it’s a class C.  It takes 5-10 watts of drive to produce 80-120w of output.  The schematics are available online but you can email me (ad7c @ arrl . net) if you need them.

While I didn’t expect the amp to be perfect as it was in pieces and you don’t take a working amp apart for fun… I was a little sad to find out it had a blown power transistor.  It took a while to diagnose.  I lifted (un-soldered) the emitter and base of all the transistors, there are four of them, and checked them with a multimeter.  This one below was bad.  Failed open.

MRF224 TransistorI ordered a new MRF224 from eBay and I was pretty worried.  RFparts.com, a very reputable transistor supplier, wanted almost $50 for the MRF224.  The one I found on eBay was $29 shipped.  I was nervous about buying from a China supplier on eBay because of the rampant fake parts problem they have but I took a chance.   About 8 days later my transistor arrived, I have installed it, and I now have a great VHF FM amp that I have $39 total investment.  I have an old Radio Shack HTX202 that outputs 7 watts and that yielded 110w from the amp.  Nice!


Analog Devices AD9850

I’ve been searching the cheap electronics on eBay lately and I ran across a couple of items that look interesting. One of those items is an Analog Devices 9850 Square/Sine wave 1-40mhz signal generator.  What makes these things amazing is that they can be purchased for about $10 and that includes shipping.  So… I bought one!  It arrived and I was a little shocked at how small this little device actually is.

AD9850 Module

I have an Arduino that I just starting tinkering around on so my hope is to get the Arduino to drive the AD9850 and see what kind of oscillator I can make.  There are some other people who have already done this so I should be able to borrow some code rather than start from scratch.  Unfortunately my AD9850 arrived without any pinout schematic so I have to track that down first.