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Old 01-19-2011, 11:23 AM
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On the bench at this moment is a Uniden PC 66XL. This is the same radio as the Cobra 25. I still use the basic 29 schematic theme when checking these radios.

This radio has the no TX or RX problem. I am not just talking about audio, but there is no detectable 27 mhz signal on TX or RX. This is when your test radio comes in handy. Key the radio under test with the test rig set to the same channel. Do you hear a slight signal? Now key the test radio and listen to the rig under test, do you hear anything? does the meter move?


First test was at pin 2 of the PLL chip. I used a frequency counter to check the 10.240. And there is was. Good
Next test was at pin 4 of the audio chip with a metal probe. Yep, plenty of noise.
Next I used my frequency counter to check TP 3. Set channel selector to 19. There should be a 16.270 mhz signal there. Nope, counter reads zero. Now lets go to pin 10 of the PLL chip. This should give a frequency of 5.140. That is half of the 10.240...The PLL chip has a diver circuit that produces this signal so it is internal to the chip. As I test this pin I find no 5.140 mhz signal present. With this condition there will be no 16.270 mhz at TP 3 to deliver 1st mixer energy to the RX mixer. And since the signal is not present at TP 3 or pin 10 of the PLL, then we determined that the PLL chip is bad.

This took about 4 minutes to check after the radio covers were removed and power was applied.
This is why I say it is easier to check what is working in a radio instead of spending hours looking for the faulty component first.
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