How to adjust the dead key in a uniden pc78ltw
You change the dead key with the addition of 2 components to the board. There is no dial or pot inside to turn to adjust the dead key.
What you want to do to the radio is called a swing mod. If you are trying to adjust your dead key, it sounds like you are trying to run a linear amplifier. If that is the case, you should probably set your dead key to around 2 watts. Most amplifiers are good with 2 watts. If this is so, then do this:
Go to radio shack and get a 1/2 watt 100 ohm resistor, and a 100uf 35 volt electrolytic capacitor.
Open the radio and look for JV117. It is a jumper. It is located in the back left of the radio in front of the final resistor in front of the heat sink.
Use a de-soldering wick to remove the solder from the trace side of the board. Remove the jumper.
In the jumpers place put the capacitor. Make sure the black tab is facing the back of the radio, towards the final. This is very important because there is a positive and negative to the capacitor. If you do it backwards you can blow your radio.
Solder in the capacitor, then take the resistor and on the side you soldered, solder the resistor bridging the 2 pins of the cap on the trace side of the board.
How this works:
The resistor determines the amount of dead key. the bigger the resistor, the smaller the dead key, the smaller the resistor, the higher the dead key. (dead key will only go as high as it was before you put in the mod.) A 100 ohm resistor gives you about a 2 watt dead key.
ALSO! you are going to want to find VR4. Its a little pot towards the back right center of the board. Then turn it all the way to the left. This turns up your modulation, and will make the radio swing to full PEP wattage, around 20 watts.
If you have a watt meter that reads avg wattage then you can gain more wattage by adjusting L14. It is just right of VJ117. it is a white or brown plastic tank with a copper coil around it with a slug in the center covered by a little way.
*do this only if you have a meter, also, do not transmit on your radio without an antenna connected or a dummy load or you will blow the final.*
Remove the wax and then while saying a steady ahhhhh sound, slowly back out the slug until you have peak wattage,(when the needle no longer increases.) You do not want to back the slug out any more then is needed for full wattage, other wise you over bias the final and lessen the life of it. So once the needle stops going up while giving a steady ahhhh into the mic, back it in a hair just to make sure its not over biased.
And as always, if you are not comfortable doing this your self, take it to a radio tech at a radio shop. They are usually found at truck stops.
Oct 15, 2009 |
Uniden CB Radios