YAESU FT 102 AM SURGE MODIFICATION
Please note - this surge modification in its complete and detailed phototutorial form is Available Here.
And the phototutorial mod can also be seen at this address: http://www.amwindow.org/tech/htm/ft102overshoot/ft102overshoot1.htm
Many folks have asked me to come up with a good AM surge modification for the 102 and so I listened and have come up with an excellent modification that can be done in an afternoon or less.
This modification was prompted by an email from Bill W3DUQ who has also published a mod for the 102 overshoot modification. Bill correctly found that the cause of the overshoot was the 8 volt AM transmit supply line which was unstable at initial onset of transmit while it was going from a resting voltage of 0 to the working 8 volts. He correctly undertstood that a stable 8 volt supply was needed and had to be on in receive as well as transmit so that there would be no variation of the voltage at the onset of AM transmit.
Below is my email to Bill with the new and simplified modification and its explanation.
OK, I have it figured out and it overcomes all the short falls of the initial modification.
Your initial modification had two very serious drawbacks:
1. - The radio and board were neither forward or back compatible. By that I mean that the modified board could not be used in another unmodified 102 and that a modified radio could not accept a normal unmodified AM-FM board.
2. - Because AM is also powered up when FM is selected in your modification both forms of modulation mix and the output on FM is very distorted by the simultaneous AM modulation components.
Starting with an unmodified radio and board cut the wire going to pin 1 of P60 (J01) of the AM-FM unit. Leave enough room (about 2 inches) to solder another lead to it and insulate the other cut end which carried the old 8 volt supply to pin 1, so it doesn't short out.
Then go to the AF board and pick up a lead from pin 2 of J15/P38. This is noted to be a 12 Volt, AM line. It a yellow wire and is energized only when in the AM mode. It is on continuously in both receive and transmit. It is not on in the FM or any other mode as was the case with your mod.
This line (yellow wire) will be your new power source for the AM unit but it must first be converted to 8 volts. I recommend using a three terminal 7808L (L is for low power) regulator (cost is well under a dollar) to bring the 12 volts to 8V. Then connect the 8 volt output line from the regulator to the wire from pin 1 of P60 and it is done.
This will give you a constant 8 volts at pin 1 of plug 60 - but only in the AM mode and will be activate in both receive and transmit so there will be no variation when going from receive to transmit. This power line will not be active on FM or any other mode so that AM modulation components cannot interfere with FM functions since AM will not be switched on.
The nice thing with this simplified modification is that a modified radio and unmodified boards are both forward and backward compatible. Give it a try. The Modification and phototutorial are available
73 de NC4L Mal
This surge modification in its complete and detailed phototutorial form is Available Here
The modification can be done in an hour - just click on the blue link at the top of this page.
73 de NC4L Mal