Note: This is a fix to the overshoot modification by Bill, W3DUQ
previously posted here. 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 drawbacks:
- 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
- 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 components.
The Key Down Surge Problem
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.
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.
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 relationship.
Figure 1 - Scope display of a properly tuned and operating FT-102 on AM.
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 protection circuits.
Figure 2 - Scope display of the power surge: before modification at left, after modification on right.
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 out.
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
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 switched on.
nice thing with this modification is that this modified radio and the
unmodified boards are both forward and backward compatible.
Figure 3 - The regulator (7808) will be placed in the upper right hand corner of the AF Board.
Figure 4 - There is a little unused real estate just above the mounting hole.
5 - Using the 7808 pins as a template (or calipers, ruler of some other
measuring tool) mark the board and then drill the holes.
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 done.
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.
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 www.members.aol.com/NC4LMAL. And if you run into trouble, contact me for assistance at NC4LMal@aol.com.
High Resolution Images
Since first performing this modification, I have
perfected and 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 professionally.
To the Photos