KC8AON's QRP PROJECTS

YAESU FT-747GX FREQUENCY JUMP FIX

GOT AN OLD YAESU FT-747GX THAT JUMPS FREQUENCY SLIGHTLY ?

READ ON AND I"LL TELL YOU HOW TO FIX IT !

I have 2 old Yaesu FT-747GX radios that are otherwise good radios, but both of them have the common 747 problem of occasionally jumping frequency slightly. The frequency jump isn't much - just enough to change your voice or CW pitch on the other end so that folks keep telling you that you are off frequency. The problems is caused by corrosion in the variable trimmer capacitors that sets the carrier frequency for CW, LSB, & USB. The corrosion actually causes the capacitors to change value slightly and it can happen unexpectedly at any time and often jumps back on frequency too. I tried cleaning the caps in both my 747's but this was only a temporary fix at best and the problem would return in a few weeks of operation. So I decided to replace the trimmer caps in 747s to permanently fix the problem once and for all. The original trimmers mounted 90 degrees to the circuit board, and there are 3 of them. Upper & Lower sideband trimmers are both 20pf maximum, and the CW trimmer is a 10pf maximum. I searched the Mouser Electronics catalog for trimmers that I thought might be exact replacements, but could not find any that really suited my needs because the 90 degree mount caps I found had much different pin spacings or locations than the originals, so I opted to get trimmers that mounted flat on the PC board with intentions of first mounting them on a piece of perf board and then mounting the perf board to the main board so that the trimmers was 90 degrees. The reason the originals are mounted 90 degrees is so that they can be accessed through holes in the back panel of the radio for adjustment as they are packed in too tight to get to for adjustment if mounted flat with the exception of the CW trimmer which could be accessed without being mounted at 90 degrees. So the replacement 10pf cap for CW was just mounted flat on the main board and the trimmers for LSB & USB was mounted on the perf board. In the photos and drawing, you can see how the trimmers was mounted on the perf board and mounted in the radio. Once I had them installed, I realigned the carrier oscillators to their proper frequency, and once set the radios have been rock solid steady ever since. To remove the old trimmers, I removed the circuit board (verticle board on the left looking down with the front of the radio pointing away from you). To remove the board requires removing one screw that also anchors the filter board in place, unplugging a few jumpers (color code them with markers so they get plugged back in right), then lift straight up on the board to unplug it. If it doesn't move at first, a slight rocking motion front to rear will help slide it offits strip plug near the center of the board. Once the oard is out, use a solder sucker or desoldering braid to unsolder the original trimmers making a note of which trimmer is which. Remember the LSB & USB trimmers are both 20pf caps and the CW trimmer is a 10pf and the ones I replaced them with was color coded - red for the 20pf and white for the 10pf trimmers. Once the old trimmers was removed and the solder holes cleaned up, I slid the new replacements into place making sure the pins was in the right holes and soldered them in place. The wire I used for mounting the perf board was stiff solid copper wire but still too flexible to allow adjustment without bending, so I placed an old piece of cable insulation beteen the perf board and the oscillator crystals to help support the board and you can see this marked in the after photo. Once I had them in place, I still needed to align them so the corresponded correctly to the frequency shown on the display. To do this, I simply tuned to a frequency that I was familiar with the folks that used them and then adjusted the trimmers so their voices sounded normal then asked them if I sounded normal. This got me plenty close enough and I can return to frequency now without getting reports that I'm off frequency. I could have used my freq counter to set the trimmers, but my quick method worked ok for now.

This is a photo of the original trimmers before removal

This is a photo with the replacement trimmers all mounted and ready to go and you can see the plastic insulation I used to help add support to the perf board.

Here's a simple diagram I made to show how the trimmers mount on the perf board. Notice the ground wires - this is a stiff copper wire that ties the common pins on the LSB & USB trimmers together and also mounts them to the main board. The 2 hot side wires are shown as blue wires that connects the hot side of the trimmers to their holes on the main board. Its really very simple to do, just make sure you have them mounted the correct space apart so that they align with the holes in the back panel of the radio for adjustment. The new CW trimmer is just mounted flat on the main board and can be accessed from inside the radio since it is closer to the top of the main board than the other 2.

Parts for the repair can be purchased from Mouser Electronics - http://www.mouser.com
The 20pf (RED) trimmers are part # 242-4221, and the 10pf (WHITE) trimmers are part # 242-3010 - cost about 48 cents each !

GOOD LUCK & 73 !