I got this information from a friend and wanted to make it available to everyone that has an FT-817 - without their having to spend hours searching the stupid YAHOO forum.
I have discovered that under some conditions, the FT-817 may be susceptible to an INTERNAL OSCILLATION of the final and driver stages, which will result in excess current being drawn, and POSSIBLE damage to the affected stages. I ALSO have determined that this problem can be solved by simply REVERSING the PHASE of the driver signal to the input of the final output board, and this is done by un-soldering and reversing the shielded cable from the driver stage output balun, at the input to the final board.
It DOES take a good eye, steady hand, proper tools, and a little skill to SAFELY and properly accomplish. (You simply swap the shield and the center conductor connections of the input coax to the final board)—Do this at ONLY the final board end, the coax COLOR on the correct balun is gray/white-NOT the blue one!—Once the modification is complete, as further insurance, I advise checking the final stage IDLING BIAS current.
Idling Bias current may be checked by un-soldering the 13.8 volt bare wire power jumper from the P. A. (final) board, and connecting a milliampere meter in series, to read the idling current. The radio should be connected to an EXTERNAL power source of 13.8 volts D. C., and operated in the SSB mode with NO AUDIO input (Turn SSB mike gain menu to ZERO)-When you energize the PTT circuit, in SSB mode, with no audio, you SHOULD see a current of 70 to 90 milliamperes, and if you adjust the bias pots, one at a time, COUNTERCLOCKWISE on the final board, each pot should take it DOWN to approximately ˝ the value previously noted. (35 to 45 milliamperes each), for the TOTAL of 70 to 90 ma. (If all is normal, RETURN the pots to their original setting. Do NOT exceed 100 milliamperes when adjusting!)- After this simple, no-parts fix, you will ALSO likely note that the “Ferrite Bead” on the external power cable is NO LONGER NEEDED!
Be certain that the top cover screws are properly secured before using the front BNC antenna jack, or you could induce feedback via this route. Hopefully this will put an END to the mysterious “Finals Blowing.
Lastly -a word of WARNING- if you are not sure, or do not fully UNDERSTAND this procedure, or do not have proper tools and skill, seek qualified assistance! Failure to do so MAY result in DAMAGE to you transceiver!
73, and good DX!
KC3VO, Bob Curry
I have opened up two of the FT-817s and made this change to them and made some notes that might help you do the job. First, you get to the final amp board by removing the bottom cover of the FT-817 (the one with the battery cover on it.) Have the rig upside down with the mic connector socket pointing toward you and the front of the rig to your right. When you look into the rig, you will see a balun coil in the lower LEFT corner of the box. The driver transistors are along the edge of the circuit board closest to you. The balun has 2 holes in it with a gray colored coax cable looped through it with one end connected to the main circuit board. The other end is connected to the final amp circuit board.
In both my units, the shield was connected to the left input side and the center conductor to the right. The cable terminates into some large solder lands on the top of the board and is easy to exchange the way its wired. There is another balun towards the center of the main circuit board-this is the output balun and you don’t want to touch it. The 13.8 volts powers the final amp board by a bare wire from the main circuit board (where there is a large + sign on the board) up to the final amp board on the RIGHT side in the upper right corner of the amp board. If you hook up a meter, the plus lead goes to the main board and the minus side to the amp board.
Another way to save you finals is do NOT run a full 13.8 volts from your external power supply, there is almost NO increase in output power if you run over 11 volts but do try it with your rig.
73's and GOOD Luck!