Front Panel Sidetone Adjustment for the TS850S
Front Panel Sidetone Adjustment for the TS850S
by Tree N6TR/7 - Boring, OR
(originally published in the March/April 1994 NCJ)
One of the few complaints I had with my TS-850S is its lack of provision for controlling the sidetone volume from the front panel. Many times, when using two radios, I do not want to hear the sidetone. It seemed that making the MONI button control the sidetone, or using the MONI control to adjust the sidetone volume, would have been an easy thing for Kenwood to do.
Fortunately, it is also an easy thing for you to fix. The modification I have done allows you to adjust the CW sidetone volume with the MONI control. To see how this is done - locate the STON (sidetone level) pot on the IF unit schematic. The wiper of the pot goes to a 330 k-Ohm chip resistor and is summed with the output of the SSB monitor circuit after the MONI pot. This means the MONI pot will have no effect on the CW sidetone level. If you remove the 330 k-Ohm resistor and couple the sidetone audio to pin 2 of IC7, the MONI pot will now control the CW sidetone level. This was done with a 330 k-Ohm resistor in series with a 0.1 uF capacitor.
To perform this modication - perform the following steps:
1. Disconnect the power cord.
2. Remove the bottom cover.
3. Locate R239 on the IF unit. This is near the back of the radio, about one inch away from the sidetone level adjust pot (VR5, STON). There is a visible circuit board trace going between this resistor and the pot.
4. Remove R239 with a soldering iron and discard it. Be careful not to drop it into the radio.
5. Locate IC7. This is about one inch away from the STON pot towards the front of the radio. Locate pin 2 of the IC (pin 1 is marked) and follow the trace until you find a feedthrough between two components. You will be putting a thin wire into this feedthrough.
6. Cut a 2 inch piece of thin wire from a small signal diode - or other component with leads thin enough to go through the feedthroug. Tin one end of the wire.
7. Being careful not to melt any other components or cables - insert the wire into the feedthrough just enough to make a good physical connection. You will probably have to be applying heat while inserting it. If you can, put a little solder on the joint (requires a very thin soldering tip and solder).
8. Assemble a 330 k-Ohm resistor and 0.1 uF capacitor in series (values are not critical). Solder the connection between them. Bend the leads so that one end of the assembly can be soldered onto the side of the STON pot, and the other end can connect to the thin wire installed in step 7. Install the assembly and solder the connections. Make sure it isn't shorted to any other conductors.
9. Turn the STON pot fully clockwise (max volume).
10. Put the bottom cover back on. You should now be able to adjust the CW monitor level with the MONI jack.
73 Tree N6TR