The Wet Finger Method

[TheCBDoctor]

The Wet Finger Method


Twenty years ago a technician that I apprenticed under taught me a trick to fixing radios without having to resort to a volt meter or oscilloscope.

This method should not be used on tube radios or base stations that are plugged into the wall using 110 volts AC. When testing base stations an external regulated power supply should be used. This is especially important when testing Cobra 2000 GTLs. Unlike most base stations the 2000 does not cut the voltage supply by opening the positive lead, the radio cuts open the ground. This leaves a dangerous voltage at the power ON/OFF switch.

The method is affective and will cut the repair time in half. Simply wet your finger and run it over the solder side of the CB. Many times if you are missing a signal, wetting your finger acts like both a capacitor and a resistor.

By “jumping “out a defective circuit you may get the radio to work somewhat; this will allow you to localize the area of the board that has the defect. This method is especially affective when the radio has a shorted decoupling capacitor.

A decoupling capacitor filters out any RF on a DC supply line. There is always a resistor in series with the cap to prevent a direct short. The resistor dissipates heat by dropping the voltage across it and limits current going to ground in the event of a decoupling cap shorting out. Most of the caps that short out are rated at 10 volts; many of these caps are on the 8 volt supply. I have fixed a number of President Jackson radios using this method as well as many other radios.

Another instance of using this method involves the audio chip. Let’s assume that the radio does not have any audio receive or transmit. If you touch the input of the audio chip you should hear noise coming out of the speaker or see modulation on transmit.

Most of the time the problem is either the decoupling cap on the receiver or transmitter audio circuit. On the TA-7222AP the input is pin four; the output is pin nine. If pin nine is not half the DC supply voltage of 13.8 volts then the audio chip may be bad. Using the wet finger method will help determine that the audio chip is defective without having to use a multi-meter right off the bat.

Audio chips rarely go bad, unless the radio was hooked up backward. If the polarity protector is open, then the audio chip is shot.

A few other methods I use are pressing on the board to check for cold solder connections, freezing the board, or applying heat. These methods also help in determining the problem. Whatever the problem may be, going directly to any test equipment can have you going around in circles. A good instructor at my Electronics School had some good advice for me that have help me over the years; always follow the current.

A volt meter can only tell you if you are missing a voltage or indicate a short in the circuit. My measuring the current you can tell right away, which problem the radio has.

Just some observations that I hope will help those learning about repairing their own equipment.

Respectfully,

[hammers]

cool tip remind me of a trick i learnd while working as a auto mech if ya think ya got a bad plug wire or someother secondary leak hoese it down it will tell ya right away it will start skippin or if bad enough just die out