FT-747GX arcing or relay chatter?

Is my FT-747GX arcing or is it relay chatter?  Or…isn’t it embarrassing when you fail to spot the blinding obvious!

Wanting to do more portable operations due to my current apartment QTH, my Yaesu FT-747GX was awoken from a few years of slumber. Of course it makes sense to test the rig first doesn’t it and so I did. I plugged in the DC, connected the coax to my SWR meter and then to a dummy load.

FT-747GX PA brickI switched the rig into transmit with 10 watts and was very disappointed to hear what at first sounded like arcing between capacitor plates and the RF output going rapidly up and down. There was no difference with frequency which suggested the problem wasn’t within the Low Pass Filters. I thought I better open the rig up and see what was going on.

The noise was coming from the PA brick and almost certainly relay chatter but as it’s all enclosed I had to open it up. First I looked at the user manual’s circuit diagram and the most likely relay to have problems was RL13 the transmit-receive aerial relay which is on the LPF board.

To open the PA brick, first unscrew the four screws in the rig’s case which keep the PA brick in position. Then, unscrew the ground connection shown top right on the photo left of the PA brick.

FT-747GX-internal-wiringThe internal wiring within the FT-747GX is easily disturbed (and then you haven’t a clue what you’ve done!).

So I carefully eased the PA brick up on its side, trying not to have to uncouple any of the internal wiring. There is then just one more screw on the side to opening this side of the brick which revealled the Low Pass Filters board.

I then put the rig into transmit and it was clear that the noise was indeed chattering of the transmit-receive relay on the output of the LPF which connects to the SO-239 socket on the back of the rig.

FT-747GX-PA-Block-view-of-LPFIt was then that I realised I was running the rig on a Computer ATX power supply that I use for testing purposes. Not only that but I had 10 feet of DC cable between the PSU and the rig. The 12.0V output from the PSU was measuring 10.5V at the rig when in transmit. Although it would just about switch the relay when transmitting QRP, anything over 6 to 8 watts output was too much to sustain.

It is worth noting that the 10 feet of DC cable was clearly going to be inadequate if the voltage drop under only a few amps was over 1 volt. At 100W output the transceiver takes about 19A; 50W is about 10A. My cable was bought on eBay and advertised as 20A 12V cable but after consulting voltage drop tables I decided that I should probably look for at least 50A cable to minimise the voltage drop for my 10 foot run.

I tested the Ft-747GX with the same 12.0V PSU but with only 1 foot of DC cable and found that I could output 75W. Consequently, I thought it was likely that from a car battery with 10 foot of higher capacity cable that I would be able to run at least 80W which is fine for my purposes.

The moral of the story is of course to check your assumptions before taking anything apart! 😀