[Kenwood] TS-120S PTT problem

[Steve]

At 09:54 AM 5/2/2010, you wrote:

>I've got an old TS-120S which seems to have a problem with the PTT
>circuitry.
>The rig seems to transmit if I switch to "Send", or in VOX mode, but when I
>key up the mic, nothing happens. The mic is a Kyokuto. I checked the mic
>itself by running a DMM across the PTT terminals in the plug and it does
>switch from open to shorted when I key the mic, so I suspect something in
>the radio.
>
>Is there anyone that can offer any suggestions?

To confirm it's the radio, short the PTT pin in the mike socket on 
the radio to the metal frame of the socket. If it doesn't go into 
transmit, the wire may have broken off the pin on the back of the 
socket. I've found cold solder joints on the mike socket before. You 
can resolder most of the pins without removing the front panel frame 
but it may be difficult and you may burn other nearby wires. Removing 
the front panel frame requires removing the top and bottom covers (be 
careful when lifting the top cover off, and unplug the speaker from 
the PC board), then remove the VFO which is easy to do... simply 
unbolt the four Allen-head screws on the radio's front panel which 
hold the VFO in place, then slide it out and carefully unplug it. The 
front panel can then be unbolted and tilted so you can get better 
access to the back of the mike socket.

If the bulbs on the VFO and S-meter are burned out, you can get to 
them easily now. You may not be able to find exact replacement bulbs; 
I've had to use larger colored bulbs from Radio Shack. The dye on the 
bulbs comes off easily with some alcohol and hey presto: a clear 
bulb. You may have to get a lower-voltage bulb such as 6 volts. In 
that case, you'll have to install a resistor in series to drop the 
voltage. You should install a resistor even if you have a 12 volt 
bulb to drop the voltage to, say, 10 or 11 volts which will make the 
bulb last much longer. For a 12 volt bulb, start with something 
around 33 ohms; a quarter watt resistor should handle the power up to 
a bulb current of about 120 mils. For a 6 volt, 60 mil bulb, try 150 
ohms; if it's too dim, parallel it with a 1K to drop the resistance 
some and increase the voltage. However, the 120 ohm resistor will 
have to dissipate 1/2 watt; so use a pair of 330 ohm, quarter watters 
in parallel. I cut one lead of the bulb in the middle then solder the 
resistors in, and cover them with heatshrink, then place them in an 
open space far away from the S-meter's soft plastic or the dial plate.

The resistance and power for other bulb voltages will have to be 
calculated using Ohm's Law.

Steve, K0XP