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Nils, we did some checking of the voltage to the VCO on the PLL board as you suggest by looking at TP2002. On VHF band 108-139.99 the voltage varies from 2 to 6 volts measured (roughly) on my digital scope. When the radio is tuned passed 139.99 the voltage jumps to 7v and remains at that through to the end of the band and then switches back to 2v at 108MHz.

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Nils my technical knowledge is limited but I wonder what generates the variable voltage, is that done by the CPU or by the registers. I guess I am hoping that the failure is with Q2016 the switching transistor pair!

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My next test was going to be to check the voltage on pin 12 of Q2027 to see if that is varying, but before doing that I wanted to seek opinions. If Q2027 only switches the varying voltage between pins 4-7(HF) and pins 14-11(HF-VHF-VHF-UHF) then the fault could be back at the processor!

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At this point I’d really appreciate your thoughts as to the best next step.

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Sincere thanks again.

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Paul G0ODP and John G0KFM

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Hi, have you checked the voltage on TP2002 on the PLL board? Shall be between 0.5 an 6.5 from 108 to 139.995 and between 0.5 to 6.5 from 140.000 to 173.995.
The switching of the two vco's is done by Q2015 and Q2016, you should see the switching on TP2004 and TP2005 when passing 139,995.
The signal to Q2015 and Q2016 comes from Q2027 which is controlled by the main CPU on AF-CNTL?
I hope you can follow my thoughts.
Nils
SM7DVO