Why does my PLL go out of lock?
Emily Sparks
Published Mar 13, 2026
Why does my PLL go out of lock?
As the reported problem (unlock) indicated one of the PLLs went out of lock, we started measuring the VCO voltages and compared them to the expected values as given in the service manual (pages 103,104). The results were mostly in line with the expected values, but for PA1DYK not all:
What causes PLL unlock on a ts-450?
PLL UNLOCK. If your are lucky, the cause of PLL unlock could be loss of HET Oscillator VCO2. Although I haven’t seen this problem myself, John, WB4LNM, discovered this problem in the TS-450 at K4RY. In this case, TC1 in the VCO2 sub-unit of the PLL UNIT had broken causing the 64.22MHz VCO to lose lock.
Can a tm-721a be used in cross band mode?
To operate the TM-721A in the repeater cross-band mode, chip resistor R121 on the control unit (X53-3130-XX) (B/3), must be removed. The following modification will detail how to access the resistor. Disconnect the power supply and antenna from the transceiver.
How to fix Kenwood tm-721a cross band Repeater?
Kenwood is to be congratulated for recognizing the problem with a large installed base of TM- 721A’s and making this field fix available. While you have your radio open, you may want to activate the cross-band repeater function. Lay the CPU board down so the two ribbon cables come off to the north.
As the reported problem (unlock) indicated one of the PLLs went out of lock, we started measuring the VCO voltages and compared them to the expected values as given in the service manual (pages 103,104). The results were mostly in line with the expected values, but for PA1DYK not all:
PLL UNLOCK. If your are lucky, the cause of PLL unlock could be loss of HET Oscillator VCO2. Although I haven’t seen this problem myself, John, WB4LNM, discovered this problem in the TS-450 at K4RY. In this case, TC1 in the VCO2 sub-unit of the PLL UNIT had broken causing the 64.22MHz VCO to lose lock.
Kenwood is to be congratulated for recognizing the problem with a large installed base of TM- 721A’s and making this field fix available. While you have your radio open, you may want to activate the cross-band repeater function. Lay the CPU board down so the two ribbon cables come off to the north.
To operate the TM-721A in the repeater cross-band mode, chip resistor R121 on the control unit (X53-3130-XX) (B/3), must be removed. The following modification will detail how to access the resistor. Disconnect the power supply and antenna from the transceiver.
