TS-850S Amplifier Interface Page

The first interface is one that I am using to key my Drake L-4B. This would apply to any of the older amplifiers. Positive keying only, not for the SB-200 or FL-2100B. The problem that I was having was the relay in the TS-850S would key the relay in the L-4B.  This was too slow, the RF from the TS-850S was already outputting.  This caused a high swr on the rig.  In amplifier circles this is known as Hot Switching and can cause damage to the rig finals.  So what I did was to by-pass the relay in the TS-850S.  Using the 12 volts on transmit output( pin 7 on the remote connector on the back of the TS-850S) to key a High voltage/High current transistor.  (See figure 1.)  I mounted the transistor and resistors in a small aluminum box.  I put RCA sockets on each end.  Transistors that can be used is MPS-A42 or 2N6515, Resistors are 1K 1/4 Watt.

figure 1.
The following is how N0AJ, Jim constructed this circuit.  Pictures are from Jim.  I built two of these and Jim also built two. 

The box is a standard BUD box and measures 1.5 X 2.0 X 2.5. A smaller box could also be used. The connectors are RCA phono jacks.

 As can be seen in the photo above, very few components are required. The pin connections shown on the schematic will work for both the Kenwood TS-850S and the TS-570D. The amp connection will depend on your make/model. The transistor can be either a 2N6515, or an MPS-A42. I have built two of these now, and have used both types of transistors with success. The resistors are 1K and can be as low as watt. The resistors I used were from my junkbox, so the resistors shown are watt.


Assembly is very simple. Solder the transistor collector to the center conductor of one phono jack, and the emitter to the ground lug. Solder one resistor to the center conductor, and the other resistor to the ground lug of the phono jack on the opposite end of the box. Bend the base lead of the transistor, toward the resistors and solder the resistor leads and the base lead together.

The interface with your rig and amp will depend on type. The box described here was to interface with the Kenwood TS-850S/Drake L-4B, as described by KBX. I used it to interface my Kenwood TS-570D with Eric, WTTs Alpha. We used it during the CQ CW contest last November, with good results.

The completed unit, with rig interface cable is shown below. All components were purchased at Gateway Electronics. The cost is very minimal.


Jim, NAJ

Joe, KBX

One little note on this circuit from Wayne VE3EFJ

I really enjoy your TS850 web page. It is just terrific.

Regarding your amp keying circuit I have one comment for
"improvement". There really should be a diode from collector to
ground to stop the transistor from being spiked by the relay.
Cathode (bar end) on the collector). A 1N4007 should work fine.

Regards -

Wayne VE3EFJ