modulation is great stuff. But there are other ways to get on the ham
bands using amplitude modulation.
The Kenwood TS440 is a very popular H.F. rig. It sold well and had
quite a few useful features. The component and assembly quality was
reasonable and there is some spare room inside the cabinet. Circuit
layout makes for easy modification. I believe that rigs similar to
these will be the "boatanchors" of the future. The term "boatanchor"
here means radio equipment sought after by experimenters and no longer
appreciated by folks who don't. One important feature of the 440 is the
final I.F. stage frequency choice. The use of a final I.F. at 455 Khz
allows substitution of a wide range of filters available as surplus at
The TS440 had several drawbacks which can be overcome by modification.
The changes highlighted here are those which will have small or no
drawbacks. Kenwood introduced a great radio for it's time but they did
have to keep an eye on cost in order to be competitive. Fortunately,
the basic rig is excellent. Certain components such as the SSB and AM
final I.F. filters could use an upgrade. There are also a few issues
with the VCO which have been thoroughly addressed. If you know which
end of a solder iron to hold this can be an advantage as the rigs show
up half dead for sale frequently.
The modifications presented here are a combination of things found and
used previously as well as some new mods. It is a good idea to check
out this article published by Rich Measures.
The modifications here focus on AM. SSB modifications are also shown
but are not necessary for AM use. It is possible to keep your TS-440
working as delivered from the factory on SSB with most of the changes
The factory service manual is required to do the modifications. The
component layout foldouts are indispensable.
It is worth mentioning that any older radio subject to modifications
like these should be thoroughly tested over some period of time on the
air. You want to be sure that the rig is in good repair first!
Importance of Filters
The mods here will allow the radio to transmit and receive AM with a
comfortable bandwidth and excellent attenuation of the transmit signal
off to the side. One advantage of low level AM is the shape of the
transmitted signal can be very close to that of a well designed I.F.
After many hours of experimentation it has been determined that a
Collins 8 kc mechanical filter pulled from an R390A also makes a very
good transmitting filter. These filters are available from several
sources. They show up on EBAY from time to time. Fair Radio still has
some also. Prices are not too bad. Many sell for $25-$45.00.
The filters are physically large and will not install in the footprint
of the stock unit. The filter can be remoted using mini coax removed
from a junked VCR and mounted where the antenna tuner would normally be
if that option is not installed. Otherwise the R390A filter will just
sandwich on top of the I.F board where optional filters would go. The
filter's ears will have to be removed for this and the housing should
be insulated with tape or heat shrink.
If you have the optional auto-tuner AND optional 8.83 MHz filters then
good luck finding a place to put the new filter. Removal of the factory
speaker makes a little room but this has not been tested. Filters will
also work outside the rig. Avoid mechanical shock when trimming filter
mounting ears. Do not use a bench grinder. A good pair of heavy duty
diagonal cutters can be used to nibble the ears off. If in doubt, call
There are several ways to go about changing transmit audio. If one is
comfortable with the use of audio processing gear then this can be
effectively married to the 440 while allowing the stock speech circuit
to be left alone. The external audio chain can be hooked directly into
the balanced modulator with excellent frequency response. This way you
can still revert to the original sound. The operator will have the
ability to hear how far they have progressed. This gives a quick
reference. One thing about using external processing gear is that the
mic gain on the TS-440 front panel will not control levels. You will
need to have some kind of gain pot on the processing chain output for
The other way is to do the usual (usual meaning boatanchor typical
speech amp modifications such as increasing coupling capacitors)
changes in coupling in the built-in (existing) speech amplifier. This
works exactly the same as it does with old tube transmitters. Very
respectable transmit audio can be had with the use of a D104 driving an
MPF102 impedance convertor built into the stand. This can be powered by
the 8 Volts available at the mic connector. As the D104 supply seems to
be drying up one can also experiment with numerous other inexpensive
microphones. The Turner C.B. desk mics so popular in the 70's are a
good runner up in sound quality to a D104.
One other word about D104's: Many mic elements out there are shot. It
seems no (2) D104's sound the same. I've personally tested about 25
elements and find about 10% to be of superior sound quality. If you
only have one mic head it might be a good idea to borrow a known good
spare otherwise your newly modified rig could sound like a tin can.
The D104 has a frequency response which lends itself to medium fidelity
communications without the use of an equalizer. The same does not hold
true for many other types of mics. This is only true if the mic is in
good shape and is run into a very high impedance. The need of a high
impedance load to the D104 will dictate the preamplifier's physical
location. Any run of microphone cable will effect the high end
frequency response due to the shunt capacitance of the cable. If a
pre-amp is constructed (as shown below) within the mic stand the output
impedance can be conveniently low and cable will no longer roll off the
The D-104 preamp is
shown in Figure
Notice that provision has been made in this circuit to invert the
polarity of the audio. This is a very important feature. Human speech
is not a mirror reflection about the zero amplitude line as expected.
The peak amplitude will be much larger in one direction than the other.
This varies with voice types. For you engineers who think this is a
hoax just remember this: total energy under the curve is the same
either side of zero. What this means for anyone using an AM transmitter
of any type is that they will obtain more P.E.P. with the proper audio
polarity while avoiding cutoff overmodulation. Properly "phased" audio
is as important as all the other things we are covering here. Check out
the amfone.net website for a few links describing use of an
oscilloscope for envelope waveform measurement if you need to brush up.
Add 1K Ohm resistor in series with R27 on FINAL unit (near Q8 and 9).
R27 has a loop on 1 lead which can be cut. Scratch any paint off first.
This will allow the cooling fan to come on sooner. See dk.mods for
While at mods.dk check out the mod to lose the attenuator pad on the AM
broadcast band. Clip R13 and R14 (on the RF unit board) but the jumper
mentioned does not make much difference.
Remove C33 on the
I.F. unit and replace with circuit shown in Figure 4.
Remove and replace CF2 on I.F. board with circuit shown in Figure 2.
All of the following are done on the I.F. board unless otherwise noted:
C53 change from 0.47 to 4.7 uF
C51 removed or reduce by factor of 5. Removal works OK
C67 change from 0.47 to 4.7 uF
C66 same as above.
C69 (which is next to Q12) change from 0.1 to 4.7 uF
C60 change from 0.047 to 1.0 uF
C175 change from 220 to 1000 uF (Factory part very prone to failure)
Use extended leads and relocate capacitor.
C170 change from 10 to 100 uF
If you wish to use the built-in speech amp driven by a mic plugged into
the front panel:
Remove C124 Remove C 207
C132, C133, C135 change to 10 uF.
To use an external
audio chain, apply the schematic shown in Figure 3.
The existing spare RCA jack on the rear panel can be removed and
replaced with a mini stereo 1/8" jack. The RCA can be left in place and
used in a single ended configuration but this will not allow for
balanced audio input and may make audio polarity reversal more
difficult. Balanced audio input is a good idea in an R.F. environment.
Sometimes the ground connection can be left unconnected at the audio
input jack. The shield will remain but it is simply not connected to
the terminal on the jack.
The 13 pin ACC2 jack is also a good place to make a few external
connections as there are at least 6 unused pins. The plugs are
available for a few bux on EBAY. The balanced audio can be brought in
either through the 13 pin connector or by replacing the original unused
RCA jack. You will have to remove the internal antenna tuner to get the
little board that has the jacks mounted on it. The tuner can be put
back into place after the board has been modified for the external
Again, the IF filtering is one very important part of this transceiver.
The stock AM filter, CF2, located on the I.F. board is so bad that the
substitution of one from a scrapped CB rig is an improvement. There are
many choices of replacement filters. Some have poor shape factor. I've
seen many filters with bad shape factors talked up on the air and on
the Web. Some homework will be necessary if you choose to forgo my
recommendation here. I've tried dozens of filters on the TS 440 on AM.
I even had a 440 clip-leaded to 2 R390A's to try out their mechanical
filters as well as some other old style mechanical filters.
My conclusion on the filters is that 8 kHz makes for good quality AM
audio on the ham bands. Six kHz is not enough room and more than 8 kHz
is bad form on a crowded band. Provisions can be made to eliminate the
filter on transmit altogether and transmit audio will then be shaped by
the 8.83 and 45.05 MHz I.F. filters. This might be nice on 10 Meters or
on an early winter afternoon on 75 Meters, but I'd not want to make
friends this way during prime time hours in a crowd.
With the TS440 properly adjusted using 8 kHz transmit bandwidth through
a good (shape factor) filter you will find off channel energy is far
less than that of many other AM transmit schemes. Two drawbacks of the
R390A filter are it's physical size and it's additional 8 dB insertion
loss over the stock ceramic filter.
Remember that the filter used here will affect both transmit and
receive audio. It would be a good idea to look around for a Kenwood 6
kHz 8.83 MHz I.F. filter to use in addition on crowded bands. The
optional 8.83 MHz filters do not operate in transmit on these radios.
The signal path
through the final I.F. filter is bilateral. That is to
say it travels in one direction during transmit and another on receive.
The use of an I.F. filter with insertion loss significantly higher than
the original will require the installation of a small additional I.F.
amplifier. The device used is a simple MPF102. See Figure 4.
The amplifier is in use both on SSB and AM and only affects the
receiver. The stage gain can be switched automatically by mode by
hooking to the internal diode mode switching bus.
The factory CFJ455K14 filter is not suitable for anything other than
communications quality SSB audio. Measure's recommendation for
replacement with a CFJ455K12 is still the easiest way to go after all
these years. The problem with these filters is finding them. They were
used in a number of Kenwood SSB rigs including the TS-940.
There are a few people who junk TS-940's and sell the boards on EBAY.
The TS-940 I.F. board has the 455K12. A CFJ455K5 is also one way to go
but the K12 is much better.
The SSB filter can also be replaced with a good old made in America
Collins 3.1 kHz mechanical filter. Several case styles of these filters
are not collectable and can be had at reasonable cost. Again check out
EBAY. The filter gets put in the same way as is shown for the
mechanical R390A filter on AM. With the old style Collins mechanical,
the receive gain can be brought back where it was before additional
filter insertion loss by referring to Figure 1. Unhook the cathode of
D1 from the juncture of R1 and R2 and connect it to the juncture of R2
carrier oscillator alignment procedure must be done
after replacing the SSB filter. No shortcuts here. It will NOT work
properly without alignment.
One important step is to reduce the TS440 ALC action. The ALC is bad
news on AM. ANY indication of ALC means nasty distortion. It is no
bargain on SSB either. You are much better off providing most of your
gain reduction in the audio processing where it can be done with a look
ahead system like the Behringer DSP9024. The ALC is not removed. It
will be adjusted enough so as not to provide any gain reduction during
normal use. The ALC pot VR1 on the FILTER unit should be adjusted for
available carrier output of 140 Watts. Put the rig on the bench in FM
mode and crank the carrier pot wide open on transmit to make this
adjustment. Do not leave it at this level too long. Twenty to thirty
seconds should be plenty of time. The decreased ALC level will allow
you to keep the AM carrier around 10-35 Watts with enough dynamic
headroom to keep the output clean and out of ALC gain reduction.
This modification will also change the rig's behavior on SSB. Even
though you will now be seeing more power output, it is not such a great
idea to pound the final into the ALC range previously used. Instead use
the rig at 100 Watts out on peaks and enjoy your cleaner signal. The
reduction in distortion on SSB without the ALC controlling loop gain is
quite dramatic. The TS-440's ALC is really nasty.
The ALC screwdriver change is important. You will not be able to
produce much of a clean AM signal without increasing the maximum power
out with the 440. The target maximum carrier output power is 30-35
After the mods are completed the rig can be run anywhere up to about 35
Watts carrier and a little over 100% positive peak modulation. You can
get 200% positive modulation if desired, with reduced carrier levels.
Since the TS-440 is using an AN612 balanced modulator (which is similar
to an NE602) over modulation will not cut the carrier off as in some
other rigs without a balanced AM modulator.
Another important note: The TS-440 must be transmitting into a 1:1 SWR
as indicated on it's internal meter. Anything other than this will
result in increased ALC threshold which causes all manner of
distortion. If you plan to use this with a linear amplifier then be
sure that the amplifier's tuned input is in good shape. Alternately, a
small antenna tuner can be placed in-line between the 440 and amp if
the 440 doesn't already have the built-in tuner.
Any comments or questions, send an email. Email:
Leave out the ssb sucks not because it does not suck but because the