| I am restoring a Yaesu FT227 2 meter FM mobile transceiver. There is no transmit output. Tracked the problem back to low ~135Mhz signal out of a tuned RF amplifier handling the 135 Mhz PLL oscillator. The collector of the RF amplifier transistor has about 180 mv peak of the signal driving an LC tuned circuit, with the inductor the primary of a coupling transformer. Adjusting the tuning slug shows no variation in signal level. The circuit has a 5 pf capacitor across the coil so calculated the inductor to be about 0.28 uh. Pulled the cap and transformer. Cap is actually about 3.8 pf and the transformer about 0.09 uH. Calculated that the transformer should have about 0.36 uH to resonate with the 3.8 Pf capacitor. Rewound the RF transformer with ~ 7 turns for the primary giving about 0.35 uh. Looks like it adjusts from ~ 0,260 uh to about 0.400 uH. Put it together and it still does not seem to resonate and produce about 1.4 volts RMS of signal at resonance (according to service manual). Trying to figure a logical next step? I do not have a signal generator so thinking of using a NanoVNA to look at the cap and RF transformer and plot its impedance and figure out where it is resonant so I can correct it to the right frequency and get the circuit resonating. The drive transistor looks good with about 20 mv pk input at the base and the bypass caps around the tank circuit all look good. So the resonant frequency must be off somehow……..Does anyone have advice if this idea will work and if it makes any sense? |
