M3EZP Yaesu FT-747GX Burnt trace caused by 13.5V output socket overload Go to home page
Yaesu FT-747GX Burnt trace caused by 13.5V output socket overload

What happened
I plugged in a device into the rig's 13.5v ancillary output socket. This socket is intended to carry up to 200mA loads but there is no internal protection for overload! In my case I plugged in a cable with a short circuit - the result is that the copper traces on the printed circuit board act like fuse wire, the thinnest one burns up and leaves an open circuit. The symptoms were surprising to me but reading news groups they seem typical of this problem...

On receive - no audio; s-meter at FSD;

On transmit - "on air" lights up; s-meter still at FSD; external power meter shows about 0.5w output on cw (and oddly on ssb) no matter how the drive control is set - I'm guessing this might be a good indicator on the transmit side as far as PA stage?

Notes - the front-panel controls and LCD give the impression of functioning normally. I checked the RF-overload bulb on the main board - it hadn't blown.

Both Yaesu Technical Support (via Yaesu.com) and Harry Leeming (G3LLL) advised me that a "burnt trace" was the likely fault and that these are usually easy to spot and fix; component damage was unlikely. So rather nervously I pulled the rig apart (see info below). A burnt trace doesn't always look burnt, mine looked like bare copper which was no longer covered in resin or whatever is used. It is easy to test for continuity and I then just soldered a jumper wire between the most convenient points. On the 'bad' photo below, my fix is the right-hand yellow jumper wire (note that it isn't close to the 13.5v socket!).

Underneath rear of main board, 13.5v output sockets is far right Underneath rear view of main board, 13.5v output sockets is far right. This did fix the problem. However silly me plugged the meter in again (now with fixed lead) - the radio worked at first but I then unplugged and replugged it back in and the radio was dead again!

MY ADVICE IS NEVER USE THE 13.5V OUTPUT SOCKET FOR ANYTHING!

The left-hand yellow jumper is my fix for the second burnt trace. This being done the radio now seems ok.

Advice on how to take the rig apart
Follow the instructions in the manual for taking the lid off. The front panel comes off in a similar manner - you ease out the front from the side lugs and then the lugs beneath the panel. The middle underneath lug is the same approach as it was taking off the lid.

1. Take off the Crystal filter board
Unscrew the earth tag arrangement. Squeeze the plastic lug to the right hand back of the board and lift it up vertically. There are pin connectors on the underside of the board to the front and back which you need to be careful off.

2. Take out the FM board - No problems

3. Unplug internal board connectors You don't need to remove everything. As a first I would suggest you unplug everything coming out of the RF 'brick' on the righthand side of the main board. Make a careful note of what goes where! My scribbles are below...

My bad sketch of the connectors, you need to draw your own

4. Take out the 5 main board screws
There's one in each corner of the board and one in the middle of it. To the front right-hand side there's a bracket arrangement - you'll do well to get this back as you found it!

5. Slowly ease out the board from the back
It slots into the back panel and the main and side boards are one piece. I found I didn't have to get the board out fully, after it was free from the socket holes in the back panel it should tilt up 90 degrees, now sitting on top of the front panel.

6. Do the fix - As the board is vertical, be very careful not to drop solder where it shouldn't be! Screen off 'surgeon' style the rest of the board if possible.

7. Refit the board - Be aware of the sprung earth chassis connectors on each side

8. Refit the screws - The one to the front right hand side is a real $%^$^&!!!!! You could do with 3 hands, 2 pairs of radio pliers and a screwdriver. I ended up fitting the front panel ring connector underneath the bracket arrangement.

9. Replace the FM and Crystal filter boards
The FM One is straight-forward. The Crystal one is more difficult, pay attention to the underside pins and try to just push straight down - if you don't get this right, you find the receiver has lost virtually all of its gain but otherwise behaves normally.

10. Refit the internal board connectors

11. Refit the case

12. Switch on and Good Luck!

Please note that I am not a qualified electronic technician just an average radio amateur with very limited construction or soldering skills at this point in time. However I took a deep breath and managed to fix my rig fairly easily. If this worries you, then please don't attempt it and let a qualified technician look at it. I wouldn't want you to dig yourself into a deeper hole!

David Brewerton - Last updated: 19th June 2004