** Editors Note: This
is a fix to the overshoot modification by Bill, W3DUQ previously
posted on the
AM Window Website.
If you have a mod dated prior to 7 December 2004,
please read the following and take the appropriate actions -
either do the mods below or do not use your FT-102 on FM
(see number 2 below). TNX, WB3HUZ.
A Fix for the Previous Modification
The previous modification had two very serious
1. - The radio and board are 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
both forms of modulation mix and the output on FM is very
distorted by the simultaneous AM modulation
The Key Down Surge Problem
The FT-102 has a serious surge or overshoot of power when
first keyed in the AM mode. If you are driving a linear, the surge
may cause damage to the amp's tubes (or transistors), or cause
protection circuitry in the amp to shut it down. Further, even if
your amp survives, it is likely to be overloaded for a brief period
and the first few words of your transmission will be distorted and
cause splatter due to flattopping. Actually, this will occur even if
you use the 102 barefoot, although the splatter will be less
noticable due to the lower power. But good amatuer practice is to
avoid splatter no matter the power level.
Let's look at the severity of the problem. When I look at
the power surge on the scope I can see an enormous surge when the
102 is first keyed in the AM mode. I run my rig at 40 watts out on
AM. If my carrier is set to 40 watts and I key down the surge
extends to over 200 watts.
The reason that surge exceeds my usual steady state key down
power of 160 watts (when the mode switch is in the tune or CW
positions) is that the 102 is a tube amp and the caps charge higher
in any quiescent mode. That used to be called dynamic headroom in
the old Hi-Fi terminology. It is also the reason that the 102 can
easily exceed 100% modulation in AM on the positive peaks. That
cannot occur with solid state driven exciter amps, as the ALC in
these transceivers will stop any power increase beyond saturation,
stone cold. I usually get all the 102s to run at 110% modulation
after peaking the AM tank on the board and making sure that the
voltage inputs to the two circuits on the chip are in the right
Figure 1 - Scope display of a properly tuned and operating
FT-102 on AM.
The key down surge starts at over 200 watts and stays at
over 160 watts for 250 milliseconds (mS) and then gradually subsides
over another half second. After the modifications described below,
there is a minor peak (represents dynamic headroom of the amp) to
about 60 watts and then back to normal carrier within 30 mS of
keydown. This very small surge will not trip any linear amp
Figure 2 - Scope display of the power surge: before
modification at left, after modification on right.
Starting from an unmodified radio and FM-AM Unit (PB-2347),
cut the wire going to pin 1 of P60 (J01). 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
Then go to the AF Board/Unit (PB-2344) and pick up a lead
from pin 2 of J15/P38. This is
noted to be a 12 Volt AM line. It is energized only when in the AM
mode, and is on continuously in both receive and transmit. It is not
on in FM or any other mode. This will be your new power source for
the AM unit but it must first be converted to 8 volts. I recommend
using a 7808L regulator (cost is well under a dollar) to bring the
12 volts to 8V. The photos below show where and how to install the
7808L on the AF Board/Unit (PB-2344).
Connect this 8 volt line to the wire from pin 1 of P60 and
it is done. This will give you constant 8 volts at pin 1 of plug 60,
but only in the AM mode and will be activate in both receive and
transmit. This power line will not be active on FM or any other mode
so that AM cannot interfere with FM functions since it will not be
The nice thing with this modification is that this modified
radio and the unmodified boards are both forward and backward
Figure 3 - The regulator (7808) will be placed in the upper
right hand corner of the AF
Figure 4 - There is a little unused real estate just above
the mounting hole.
Figure 5 - Using the 7808 pins as a template (or calipers,
ruler of some other measuring tool) mark the board and then drill
Figure 6 - Scrape off the conformal coating around the holes
for about 1/16 inch. I used a Dremel battery operated drill and a
dental bit but many other things will work out.
Figure 7 - Scrape
away the copper between each hole to make a solder pad for each
leg/terminal of the 7808.
Figure 8 - Be sure to
insert the 7808 as shown. Because the radio was rotated 180 degrees
to get this photo, please make sure that the printing on the plastic
part of the regulator faces away from the board. Polarity
Figure 9 - Solder the
connections shown and described in the text above and you are almost
Figure 10 - View of
the back side of the completed AF Board (PB-2344).
Figure 11 - Reinstall
the AF and AM/FM boards and test. Again note the orientation of the
7808 at the time of final installation in the photo. The plastic
part with writing should face toward the PA board.
The FT-102 is a great rig for AM, FM, CW and SSB. The mods
above will make it a much better performer on AM, especially when
using it with a linear amplifier. If you are interested in more
information and other modifications on the FT-102, please visit my
Web page at
if you run into trouble, contact me for assistance at NC4LMal@aol.com.
High Resolution ImagesSince first
performing this modification, I have perfected and documented
the steps with a series of high resolution photos. These photos
are numbered and if followed in sequence show all the steps
that are required to do the modification