Modifications for the TS-440 for use on AM

Or how to get on AM without spending an arm and a leg.

by Dave - W2VW

 Plate 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 minimal cost.

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 here.

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!

The 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. filter.

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 Mike Tyson.

Audio Options

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 this.

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 high end.

The D-104 preamp is shown in Figure 1.

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 website for a few links describing use of an oscilloscope for envelope waveform measurement if you need to brush up.

Component Changes

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 complete blurb.

While at 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 audio connection.

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.

SSB Notes

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 and D2.

Importante! The 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 watts.

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 email
won't work.





19 January 2008