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.

PROBLEM
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!

VHF FM Amp

Hacking the Baofeng Radio

I just read a great little article about hacking the Baofeng type Chinese built amateur radios.  You know those $30 throw-away 2M/70CM radios… we’ll someone took one apart and spent way to much time learning how it works.  If you have the time it’s worth the read.

Go here: http://hackaday.com/2013/02/28/hacking-a-ham-radio/ and while you are at it make sure to add www.hackaday.com to your favorites list.  Great website!

Baofend UVR-5

 

7MHZ VFO with Arduino and AD9850

A little while ago I picked up an AD9850 DDS board.  Basically, these are little boards you can pickup on eBay for about $10 shipped that can be programmed to output a sinusoidal wave from 1hz to about 40mhz.  The output is about 1Vp-p and is fairly clean.

To drive these little boards you have to program them using either a serial interface or a parallel interface.  You send over a couple of bits according to a code specified in the spec sheet and the AD9850 outputs the appropriate frequency.

AD7C DDS VFOI used an Arduino UNO board, LCD 16×2 display, and a rotary encoder to create a little all-in-one VFO.  It can output the full range available to the AD9850 but I simply limited it to only 7.0-7.3Mhz in the Arduino code.  I will be using the VFO as part of a DC receiver I am putting together for fun.

You can see a video of it in action here: http://youtu.be/r6nRMikOOmI

The code to drive the AD9850 is fairly simple, however, the hardest part I found was getting the large integer (frequency) split up into the appropriate numbers to display on the LCD screen.  Some creative code fixed that problem. Full documents and discussion on this item is on the permanent page located here.

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.