Always follow the current

[TheCBDoctor]

My electronics teacher gave me the best advise when it comes to repairing radios, "Always follow the current." There are two reasons you may not have a proper voltage. The first is there is an open, which can give you the supply voltage or zero volts. The other is a direct short which can give you an incorrect voltage or zero volts. The only way to tell if you have an open or a short is to monitor the current.

In an open circuit your current draw will be low. In a shorted circuit the current draw will be high. The problem with solely using a digital multi-meter is the input impedance of the new meters is 12 Mega-ohms. It does not interfere with you voltage reading, but it also does not tell you if you have an open or a short.

I always use a current meter when testing radios. One of the most common problems I have come across is when the antenna connector at the SO-239 is broken. One would think that the radio was not transmitting or receiving, but watching the current meter draw 2 amps tells me the radios is transmitting. You can use the same concept in a circuit on the board.

If you are not using a CB radio with a 50 ohm dummy as a test radio on your bench, then you may be spinning your wheels. Many times a radio will not appear to be transmitting. The current draw may be low, but the test radio on your bench shows a signal. That tells me that the radio is transmitting, but not at enough power to show up on a power meter. If there is no modulation on the test radio it may be the Final. If there is a signal, but no modulation, then the Final and Driver may not have a supply voltage. (What is the current reading?) If the Final is open, then the Driver can modulate. If the Final is shorted, then the Driver is being loaded down; you will see a high current draw. The signal you will see is from the pre-driver which is not modulated in most radios. (The Uniden PC-244 and PC-122 is the exception along with the Cobra 146 GTL and sideband radios in the SSB mode.)

If you don't have a current meter or are not using one you may be wasting precious time going around in circles. The other advantage of using a test radio with a dummy load on your bench is that you can "see" if the radio you are working on is working, and you may also use it to hear if your radio is receiving. It may not tell you how sensitive your receive is, but it will tell you that you have receive.

One last thing, your radio seems to be transmitting because of the current draw, but your test radio does not show a signal. You probably are off frequency. A good frequency counter is a good thing to have. Many times it does not even have to be hooked up to show you a frequency.

So to my former electronics teacher, Mr Shays, I say thank you for the advise so many years ago. It has saved me thousands of hours of bench time. He is long gone, but I still remember his advise ......"Always follow the current."

[Slyguy]

Thanks for the advice CBDoctor. Its always good to remember things like that.

[FarmeroBrown]

Thanks for the advice CBDoctor. Its always good to remember things like that.

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