Hi-Fi AM with a Kenwood TS-940S

by Don - KE1IZ

 Modify your Kenwood TS-940S for Hi-Fi AM? You bet! Just follow the step-by-step instructions below. Let me first describe the modifications and the rationale behind them.


The key to giving the TS-940S Hi-Fi AM is removing the low level AM modulation process from the IF strip where it is limited by the characteristics of the filters and balanced modulator. We then apply the modulation to the power control gate of the first RF stage through a transformer, after the audio driver stages have been modified to reproduce Hi-Fi.

Audio Stage Modifications

Replacing the first caps C188 through C199 simply allows more bass response to arrive at the input of IC 3, the main speech amp. IC 3 as used in the stock TS-940S introduces both high frequency and low frequency limitations do to the original caps used. C200 and C203 form a high frequency negative feedback loop that kills everything above 3 kHz. C202 and C207 are bypassing caps for internal transistors in IC 3. Replacing them fills out the low end. At this point both the mic pre amp (Q36) and IC 3 are modified for flat response from below 20 cycles to well over 20 kilocycles. However, there are two problems. IC 3 needs further decoupling without its negative feedback loop otherwise it will oscillate and its high frequency output is far too much. Installing a .01uF cap across it's input pin 1 to ground solves both of these issues with a gradual roll off starting above 7 kHz. Installing the 22uF cap directly off the output of pin 5 gives us good low end to drive both AM and SSB independently through the common relay terminal and eliminates the need for C206.

Balanced Modulator and RF Stage Switching

You might have noticed the modification also removed the old C204 coupling cap from the circuit. It has but you can leave it in place on the board. Installing the 1K resistor inline with the place of C206 slightly reduces the audio gain arriving at the balanced modulator. The balanced modulator is now used only on SSB and the 1K resistor makes it possible to go from AM to SSB without having to reduce the mic gain.

The relay is used to switch the modulating point from the balanced modulator to the RF stage in AM only. In SSB the relay still feeds audio into the balanced modulator through the 1K resistor. The 2SC945 transistor accomplishes the relay switching automatically when changing modes by triggering off a control voltage used to select AM filtering. I used the 2SC945 (C945) transistor because it seems to draw lower base current then other common NPN transistors. This can be an issue as the 100K base resistor used cannot be of lower value otherwise it will excessively load the radios control voltage.

RF Stage Modulation

Audio is fed into the RF stage trough a 1:1 Radio Shack audio isolation transformer rather then a coupling capacitor because of the characteristics of the stage we are modulating. If a large enough coupling cap were used to pass low end, the cap begins to hold a charge after heavy modulation and holds the carrier high. The transformer eliminates this problem.

The place were we decide to inject the AM audio into the RF stage is important. I picked the power control gate of Q3 on the RF board for several reasons. Simply adjusting the power output of the stock rig showed this circuit has the capability of effecting carrier power from 0% to 100%. Further inspection with the audio generator revealed this location was unique in that it had the same effect on the carrier at audio frequencies without the need to change any capacitors or bias levels in this stage to make that happen. The stage could easily handle extreme positive peaks without overloading or flat topping.

The Radio Shack 1:1 transformer has windings on it very close to the 56 ohm value of R66 on the RF board, seamlessly allowing us an audio injection point into this RF stage without affecting any of it's normal operating characteristics. I used the Radio Shack transformer in reverse because it's normal primary winding, the black and white wires were closer to 56 ohms and more suitable for use as the secondary to replace R66 with.

The ONLY features lost on this rig as a result of the modification are the use of the speech processor and TX monitor in the AM mode. The processor would have been useless for Hi-Fi AM due to it's required filtering stage that cleans up the processors dirty output. The processor still functions fine in the SSB mode.

As mentioned in the modification, you will need to increase the level at which the ALC begins to clamp your audio peaks to unlock the full potential of this rig. Ideally there should be nearly no ALC action under normal AM modulating conditions.

You might ask what are the safe limitations of the power output on the TS-940S. My answer is more than you may expect. For over 8 years I've run my rig at 50 watts carrier with over 100% positive modulation peaks as measured on the Bird 43 with peak kit and oscilloscope. This is all possible WITHOUT disabling the ALC, VSWR protection, or the 14 amp current limiting of the FPA. Make sure the ALC and Maximum power alignment are set so that nothing higher then 225 watts on modulation peaks are possible otherwise the 28 volt power supply will begin to fall out of regulation.

Feel free to set Max power below this point if you make lots of old buzzard transmissions. Additionally you can replace the 6 amp fuse on the back of the rig with a 5 amp fuse and the fuse may actually blow before you do damage to the rig rather then just prevent a house fire after the fact.

The Modifications

Take a look at the over all layout of the boards involved in these mods to get your bearings. The red circled areas are shown in more detail later.

Starting on the IF Unit:

Remove C188 a 1uF and replace it with a 4.7uF.

Remove C191 a 10uF and replace it with a 100uF.

Remove C192 a 1uF and replace it with a 4.7uF.

Remove C195 a 1uF and replace it with a 4. uF.

Remove C199 a 1uF and replace it with a 4. uF.

For now, simply remove C200 (.01uF) AND C203 (.0018uF)

Remove C202 a 10uF and replace it with a 33uF.

Remove C207 a 33uF and replace it with a 100uF.

Install a .01uF decoupling / high-end limit cap on the bottom of the IF unit, across the input of IC 3. It should be located under IC 3 and connects from pin 1 to ground (short leads).

Install the positive side of a 22uF cap into the old place of C203 (leave the negative side of the cap bent up for now). The positive side should solder into the hole just above the zero in "C203". This is the output pin 5 of IC 3. See photo for the and the next three steps.

Remove C206 and install a 1K resistor in the negative hole (nothing in the positive hole). Connect the other end of this 1K resistor to the normally closed contact of a 12-volt DC SPDT relay.

The negative side you previously bent up on the 22 uF cap connects to the common terminal of the SPDT relay.

Connect the normally open contact of the SPDT relay to a wire that will later feed the primary (red wire) of a 1:1 audio transformer that you will mount on the RF board.
Pickup coil voltage for the SPDT relay by connecting one side of the relay coil to pin 2 of connector 10 (+15 red wire) on IF board.

Pickup AM trigger voltage for the coil from pin 4 of connector 12 (AMG) on IF board. Feed this trough a 100k resistor and into the base of a 2SC945 (C945) transistor.
Solder the emitter of the C945 transistor to ground.

Connect the collector of the transistor to the relay coil terminal with nothing connected to it yet.

You are done with the IF board unless you wish to enhance the high frequency response capabilities of the SSB transmitter further. In this case replace C186 (.047uF) with a .01uF.

Moving on to the RF board:

Mount the 1:1 audio transformer as shown. Connect the primary (red wire) of the 1:1 audio transformer to the wire you soldered onto the normally open contact of the SPDT relay.

Ground the other end of the primary (yellow wire) on the 1:1 audio transformer.
Remove R66 (56 ohm) on the RF board and replace it with the secondary wires of the 1:1 audio transformer. Black and white wires in either hole.

You are done with the soldering iron and ready to re mount the boards. Some further alignment of the ALC is required and alignment of the maximum power output is at your discretion. Without increasing the ALC limit, the radio will clamp your audio peaks and hide much of the transmitters potential to reproduce positive audio peaks.

Final Thoughts

There are some variations you can make to this modification to suit your needs. For example the coil wire for the relay could be connected with an inline switch (the dim / norm switch could be rewired) to see the before an after effects of the different modulating points within the radio. If the relay is not allowed to key in AM the radio will modulate through the balanced modulator. It will still sound better then stock do to the capacitor modifications however, it won't even be close to the fidelity of the RF modulation technique.

Should you happen to already have an existing rack of Hi-Fi audio equipment in your shack, you could skip 95% of this modification by driving the audio transformer externally. Simply remove R66 on the RF board and insert the black and white wires of the 1:1 isolation transformer. Drive the red and yellow transformer wires from your external audio source on AM. Line level audio may not be enough to fully modulate the AM transmitter. You may have to add amplification or try different audio transformers with a step up ratio to replace the 1:1 isolation transformer. This shortcut modification will stop the mic gain and VOX functions from working. It will also require removing the audio from the transformer on SSB and using the mic jack or phone patch RCA to inject SSB audio.

I wanted my radio to be Hi-Fi just by plugging a quality mic into the front panel. If you take the time to modify the caps in the mic pre amp and speech amp, the usable frequency response is just as good as external audio. The use of a passive notch filter in your mic line can go a long way in eliminating the need for external audio equipment and the possible RFI associated with it. The notch should occur between 350 and 750 cycles depending on your preferences. The filter is nothing more then a series resonant circuit at the notch audio frequency comprised of one inductor and one capacitor. 24 to 48 volt DC relay coils with a resistance around 750 ohms often have the right inductance when used with a series cap value around .1uF. The filter must be installed following a stage with an output impedance of 10K ohms or more in order to give the filtered AF signal a place to drop its voltage.

The filter works great in Astatic amplified microphones using an electret condenser element feed with a 33K bias resistor off the 9 volt battery and automatically turns the element off since these mics switch the negative line on the battery. The notch filter installs directly across the condenser element. Install an appropriate coupling cap to block DC and limit low end response and windscreen foam is a must on these elements otherwise they sound muffled. The coupling cap should be around .01uF.





29 October 2009