This one is for folks that know the Kenwood TKR-820 repeater :-) Or how to modify a VCO frequency range :-)
I have a known working on 451.125/456.125 TKR-820 UHF repeater that I'm attempting to move down to 441.350/446.350. I've got the chip reprogrammed and the transmit VCO had no problems retuning to the target frequency (441.350). The receive VCO is another story. The manual says that PLL/VCO lock happens when the voltage from the PLL's integrator is between 4 and 5 volts, with a target of 4.5 volts. After adjusting the VCO's variable capacitor (the only adjustment available), the best I can get is 2.5 volts, and I'm getting a slow sawtooth on the integrator output from the PLL. This is surprising to me since it seems the RX and TX use identical PLL and VCO modules, and the RX VCO only has to tune down to 446.350. 5mhz higher than the TX. In any case.. the 2.5v control voltage is obviously incorrect since I cannot key the repeater with my HT at that adjustment. I have pulled the CTCSS configuration chip as recommended in one set of retuning instructions so I should be able to key the repeater without a PL (the repeater controller will be handling that in any case, so that chip will likely never be reinstalled). Any ideas what I might be doing wrong, or any tricks on how to get everything tuned up ??
