Kenwood TS870 has two antenna jacks ANT1 and ANT2 and external receiver antenna jack (RCA type) that can accommodate external receiver when activated through the program menu. This adds some attenuation to TS870 receiver signals. There is no provision for external receive antenna switching (Beverages, preamp., etc.) When I inquired Kenwood about this option, they sent me modification instructions that required cutting the traces and rewiring the ext. RX connector. When I received my Service manual and had a look at the circuit, I came up with this simple modification:
Parts needed: 1 diode (any type) I used rectifier diode, generic NTE125
Step by step instructions:
- (In the following references the front panel of the rig is facing you.)
Remove the top cover.
- Remove 7 screws holding the cover over the FILTER and RELAY unit (in the middle) and remove the cover. (Where the ANT1 and ANT2 connectors go.) This exposes two PC boards.
- Locate the connector CN301 on the RELAY unit (adjacent to ANT1 and ANT2).
It is about 1 inch back and 1 inch left from the ANT1 connector and has three wires going to it - two gray, left and one black, right.
- Take the diode, clip the leads to about 0.5 inch length, bend them down and form them so the are spaced about 1/8" and parallel (kinda forming letter Y).
- Plug the diode into CN301 connector from the top, pushing the diode wires between the plastic housing and the metal contact of the connector. Cathode (the end with the band marking) of the diode should go to the middle contact (gray wire), the other end to the left contact (gray wire). Make sure that diode makes good contact with terminals of the connector. Black wire should be free.
- That completes the modification. Replace the covers, do not pinch the wires going to the antenna tuner!
Connect your transmit or main antenna to the ANT2 connector. Connect your receive antenna or preamp output to ANT1 connector. Now you can transmit only through ANT2 connector.
Front panel button ANT now allows you to select ANT2 (receive/transmit on ANT2) or ANT1 (receive on ANT1, transmit on ANT2). Antenna tuner would always see the impedance on ANT2. The way the switching works, whenever you transmit, you will switch ANT2 on transmit, regardless of ANT1 or ANT2 selection. Much better use of connectors than originally devised by "knowitalls" at K...d.
Notes on TS870 usage: Normally there is enough gain on the receiver for beverages. In case that you need to use the preamp or preselector, than turn the AIP on TS870 on, and if needed add the attenuators. Try to keep the noise level peaks close to low end of the Smeter scale. Helps with intermods. If using DSP, play with different settings and different levels of AGC setting, it has effect on overall behavior.
TS870 looks like great contestpedition radio, it's light, fits in carry on luggage. I got speech module for phone CQing and small switching PS for it ($160). I still miss the tunable audio peak filter and ability to go lower than 400 Hz on CW note.