Yaesu FT-840 Repair
Although the FT-840 is a VERY reliable radio there is one fault which has caught a number of Hams out over the years.
The fault is easy to trigger and when done , it looks like a very extensive repair.
I believe that many rigs have been trashed over this which is a shame , because the repair is actually quite straight forward.
The cause of the problem is a short of the auxiliary power output connector on the back of the radio. This connector is designed for powering small devices , a common example would be an external digital mode interface.
If the connector is accidentally shorted , which is VERY EASY to do , then instantly -- you have big problems. The resulting symptoms are:
No RX audio , yet system beeps are heard correctly.
S meter is at FSD
PLL , synthesiser and LCD all work correctly.
If you have these symptoms then this is your fix..... read on.
When we investigate the fault we find that the 13.5V rail is down to most sections of the radio. We can discount the possibility of a still existing short due to the fact that the audio amplifier is still getting its power. We know this because systems beeps are still heard when we press keyboard buttons.
So it must be a break in the circuit.
Probing voltages around the place we find that on the board that has the audio chip , the same board that has the backup battery , we can see that the regulators are still up and running , yet on the connector that takes 13.5V off this board is down.
Removing this board you should find a burnt section of track., top view shown here in drawn in red.
The section of track that burns is very short. It is a very narrow section that obviously has a low current handling capability.
Yaesu should not have left this line unprotected and have an external power jack hanging off it. For the external power jack a small fused and regulated board would have added mere cents onto the cost of the radio and would have been much more useful to the user ,,, and of course , then this fault would never occur.
To fix the radio simply clean up the area and make a wire jumper across the affected area.
Nothing else needs to be repaired .... reassemble and your done. :-) OH2FFY