Tuesday, March 25, 2008

Problems with my 40 Meter Radio

So this year my dad decided I should try and get my ham amateur radio license. This would allow me to transmit radio communications on different frequencies and at higher power than the family radios you can buy at your local drug store. For Christmas, he bought me the study guide for the rules exam, and a kit to build your own 40 meter radio receiver.

This kit was the HR40 from Ramsey. Putting it together wasn't that bad at all. Maybe 30 parts total, soldered onto the board, and had to go out and buy a new 9V battery to plug into it.

Then the horrors begin. I plug it in, turn it on, and... nothing. The greatly feared hiss was all I got. I double checked every solder joint, nothing, tried adding a random length of wire as an antenna, nothing, went in my back yard and built a 20 meter long dipole antenna, nothing.

I was stumped. At this point I had run out of time for winter break and left this kit at home as I sludged through winter quarter. Now I have returned for spring break. Great be my luck, my dad managed to pull a working Tektronix oscilloscope out of the salvage bin at his job, so I am now better armed.

I sat down and read the first three pages of the 6 inches of instruction manual. Whoever came up with RTFM forgot to add a clause for how much of the manual you really have to read. I knew enough to be dangerous and started trying to get pretty waves on the scope which I has success with. I found where the signal came out of the oscillator IC and measured it with the scope. Figured out how the digital time measurer worked on this 25 year old oscilloscope, and finally got the money shot.

The sine wave being fed into the audio amp was on the order of 0.260 microseconds. This works out to be... 1/t = f... grumble grumble... 3.8MHz! Now comes the horrible realization: Do the math for 40 meters. Divide the speed of light per second by 40 meters (m/s * 1/m = 1/s), and you get the frequency 7.5MHz. Something is horribly wrong.

I started playing with the different frequency controls. The tuning knob and an adjustable inductor inside the radio, and managed to get the frequency to drift around 3.6MHz - 3.8MHz, but that isn't good enough... I need something at least 7MHz.

So I find the Ramsey website, spend some time reading through other users having problems with the HR40 (useless), and finally break down and email Ramsey tech support yesterday afternoon asking what to try next. This morning I got an email back asking if so and so part was an ISK6883 instead of a K6886, and if so, send them my mailing address and they'll send me the correct capacitors for my radio. Hooray! I love it when technical support correctly guesses the part number. I just emailed them back and we'll see how long it takes them to get me ~1 gram of parts to solder onto my radio to supposedly make it work.

Life just wouldn't be as much fun if everything worked. That's why I'm an engineer.

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