Here's the problem, as soon as I turn on the transmitter, it seems to be ok. The longer I leave it on, the hotter it gets and the more the power output decreases. This is despite the rear fan working and low SWR with a dummy load or jpole antenna. I can set the SWR meter to forward and watch the needle drop.
Also, it doesn't sound as good anymore. Very weak, distorted and thin. Right before i noticed the problem I was testing the signal in my car and noticed a sharp decrease in volume.
Here are some pics of the circuit board. I'm an idiot with electronics but was wondering if someone could point me in the right direction to fixing it.
For starters, that final appears to have very little heat sink compound on it. The thing uses the cabinet for its heat sink and has no fins for cool airflow to go over and remove the heat.
I suggest installing a proper heat sink inside the unit by lifting up the final so it stands vertical, then mount a good heat sink to it with holes drilled through the heat sink so that the airflow properly cools that final.
The reason for poor sound is that when the final in these heat up, the specifications shoot off to la la land and cause internal VSWR right back into the 1415 chip and other circuitry.
Peace!!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
Here is a good heat sink to use for that transmitter. Take two of them and place them back to back so that the fins face opposite directions. Then bend every other fin outward a little so that the fins are in a zig zag pattern and catch more cool air. This should help considerably with the heating problem.
Thanks I'll definately install the heat sink as you suggest, however do you know why this just started happening? The transmitter didn't do this when I first got it.
Plus I was able to keep it cool by taking off the top part of the case and putting a cpu fan on it. This prevented any part of it from heating up. But the sound was still very bad whereas before it was fantastic. Could something else be going on here?
Yeah even if the RF output transistor has worn out it still shouldn't effect the audio greatly, only unless it is causing excessive current drain from your power supply which in turn is causing less power than normal to get to the on-board audio circuitry.
If it were me I would remove the RF output transistor temporarily and then test the circuit just using the buffer output transistor. If the audio is fixed then it is the RF output transistor, if not then the problem is before the output stages. It's very possible that it is the buffer transistor that is bad. That would prevent it putting out full power, and also would drag down the oscillator stages which could make the audio sound bad.
With circuitry that small it's not easy to fix unless you know exactly what you are doing.
If you have the money and know-how I would suggest replacing both the RF output transistor and the buffer transistor.
Plus I was able to keep it cool by taking off the top part of the case and putting a cpu fan on it. This prevented any part of it from heating up.
You may have created more of a problem by doing that. The fan certainly was a great idea, but just kind of blowing air over everything doesn't really help. It needs to be directed to where the heat is and pull that away.
I found this out the hard way with one of those HLLY units from China. It overheated from the beginning even with the incorporated fan it came with. To "fix" it I decided to do the same as you did. With the top cover off and a fan right on the heat sink, it overheated and shut down several times until it fried the MOSFET. To make a long story short, I drilled the heat sink (just as RF Burns told you to do) then I put the cover back on (with a new output transistor of course!) and its never had an issue again.
I agree that attaching a heat sink and replacing the output resistor is the best way to go.
As a side note, I was playing around tonight and found that I was able to significantly increase the power of the transmitter by stretching the coils of wire (that go in front of the BNC output) outward.