I’ve been away on business a bit over the past few weeks and have only managed a couple of snatched hours on radio construction. However, I did manage a couple of hours last night to try and find out why the VCO is off frequency.
I started off by seeing how the VCO frequency changed as I adjusted the controls. The main control seems to have no effect at all. I connected my Rigol ‘scope to test point P1 and there was no change as I turned the tuning knob. However, changing bands did affect the frequency. That would seem to rule out the VCO itself as a source of error, for now anyway.
Looking at the circuitry, it seemed that the obvious next place to look was the MC14145170 PLL synthesiser, U4. This takes the VCXO output and compares it with the VCO output to generate a varying control voltage that changes the VCO frequency to keep the VCO locked to the VCXO. U4 changes the frequency in 5kHz steps and interediate frequencies are generated by varying the frequency of the VCXO using a pair of varicaps and a LTC1451 DAC.
Probing around, it immediately became obvious that something is amiss. There should be clock and data signals coming from the microprocessor on the Control Board and going to the DAC and the PLL chip. There isn’t.
The microprocessor is running OK, because the front panel is working and driving AUXBUS to change the VCO is bands.
Clock (SCK) and data (SDO) are created on the Control Board by the PIC U6 and they appear to be dead as well.
Next stage is to work out why the PIC is not generating SCK and SDO, or why the outputs are being held high; which is an alternative explanation.
