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Using the Kenwood TS-940 with Modern Linear Amplifiers

Tom Wheeler, NØGSG

Johnson County Radio Amateurs Club April 2013

The Kenwood TS-940 is a classic radio. Introduced in 1985, it is the successor to the TS-930 and is highly valued for its nearly bulletproof reliability, excellent receiver, and high-quality transmit audio. One of the TS-940's strengths is the use of PIN diode transmit-receive switching, which enables the radio to be used for full-break-in CW keying (QSK). In QSK mode the radio quickly switches back to receive in between transmitted CW elements.


This strength is a weakness when interfacing the TS-940 to modern linear amplifiers, which are very sensitive to transmit signal timing. Most new amplifiers will go into a fault-protective mode if RF energy is present on their input connector during the off-to-on transition of their send relay; because of the QSK nature of the TS-940, and a relatively slow transmit-receive (T-R) relay internal to the radio, this issue is very likely to occur when it is interfaced to modern amplifiers. There are many possible solutions; two of them are as follows:


  1. Use an external MOSFET on the TS-940's accessory connector to switch the linear amplifier into  transmit mode. This solution is good if you want to preserve full QSK nature of the TS-940, but unfortunately may still be dangerous to the health of your amplifier, as the TS-940 only presents about a 3 ms delay between activation of its internal transmit signal and emission of RF. That's not enough margin for safety in my book; hot switching an amplifier is a sure method of causing its eventual demise.


  1. Slow down the TS-940 RF output timing so that there is no possibility of ever hot switching the linear. This will eliminate full QSK on CW, but the amplifier will be protected much more fully. This is the method I have chosen. The modification is quite simple and easy to perform, and is shown in Figure 1 below. Essentially the modification increases the R-C time constant of the enable signal (TB) driving the second transmit mixer in the TS-940. A non-polarized capacitor is necessary because this signal swings both positive and negative. The values shown give about a 30 ms delay between keydown and emission of RF from the transmitter.

Figure 1: Modification for Delayed RF Output from TS-940

This modification is easy to perform - - and easy to remove, should you change your  mind at a later date. The capacitors are added at the junction of R51, R57, and R49,   which is next to connector J2 on the RF Unit. This can be completed with the RF Unit PC board in place. The resulting transmit timing is shown in Figure 2.

Figure 2: Transmit Timing after the Modification.


The top trace in Figure 2 is the TB (Transmit B+) signal inside the TS-940, and the bottom trace is actual RF at the antenna connector. The unit is in RTTY mode in this photo, but similar timing is enforced for all modes, as the delay circuit is active in all modes. Once this modification has been performed, QSK capability in CW is still present, but the first transmitted CW element may be truncated. With some additional thought, this could be prevented by adding an electronic switch to take the capacitors out of the circuit in CW mode.



Transmit timing is critical to the health of linear amplifiers, especially modern solid-state models. The TS-940 will provide excellent service in these applications with the addition of this modification.