Yaesu FT-747GX and Repair Archive

Yaesu FT-747GX - picture courtesy of RigPix

HF transceiver – Yaesu FT-747GX

The Yaesu FT-747GX was manufactured in the early 1990s and was the smallest HF transceiver of its time, with a nickname of ‘plastic fantastic’! The transmitter gets good results generally. Never had any complaints about SSB modulation and often get complements on the audio using the original MH-10B fist mic. My first outside-G contact was on 20m with 10 watts to France…on AM! FM has also received good reports.

The receiver is sensitive and most of the time I’m really happy with it. It could do with more effective IF filtering but this is why you buy a more expensive rig of course! During busy times on 40m, operating can be very limited by QRM. I’ve tried various tricks, such as using split transmitting (in SSB mode) and receiving (via the CW narrow filter). In theory I thought that might give me a chance but it only helps a tiny amount. Having said that, I’ve worked S.Africa, S.America, Asia and Australia on 40m so it can’t be such a limiting factor!

As standard, the PA-drive control is disabled in SSB (it isn’t on AM or FM). This can hinder accurate power control such as is needed when I need to run 1 watt into a 6m transverter. I found a mod on the web to re-enable the PA-drive control in SSB which allowed the rig to be used in a similar manner to most other rigs and with much better power precision. The mod involves removing the crystal filter and then snipping a diode on the main board just underneath the crystal filter, labelled D099. Thankfully it was simple to do and very successful. The power level on ssb can be run at 200mw now if I want!

When I acquired a new power/swr meter, I noticed the dial was a bit dim so a back light has thoughtfully been provided. I thought the best place to power this from would be via the ancillary power outlet socket on the back of the rig. This turned out to be disastrous – MY ADVICE NOW IS NEVER USE THIS SOCKET! In my haste I didn’t test the power lead, which unfortunately has a short-circuit in it. Yaesu rigs are notorious (I now know) for their lack of overload protection – the printed circuit board is the overload fuse and the result is a dead rig! Fortunately it wasn’t too difficult to find the problem and fix it – see below for more details.

After encountering a power drop to 50w, I did some digging and with help from HP1KL (Tony Conte), M0BCG (Ian WIlliams) & G3LLL (Harry Leeming) I have gathered together this info here.

If you accidentally fire RF in through the out door and find the receiver has gone deaf, don’t despair! There is a 8v 100mA bulb soldered to two pins on the board which acts as a fuse. Unable to get a direct replacement I’m now running with a 6v 80mA bulb with no noticeable difference in sensitivity..

FT747GX Manual

FT747GX Technical Supplement

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” is where the copper track burns through and splits and can be identified by having bare copper which was no longer covered in resin or whatever is used. The split in the track isn’t always visible but it is easy to test for continuity of course. I 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 rightUnderneath 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 right-hand 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