04-01-2016, 11:11 AM
Its much more difficult or impossible to set the broadcast band oscillator to the wrong peak. A mentioned previously, the radio will respond to two different frequencies. One 260 kHz below the osc freq and one 260 kHz above. On the broadcast band, the osc is always above the receive freq, so to receive 1000 kHz you need a 1260 oscillator signal. With a 1260 kHz osc, you can also receive the undesired image at 1520 kHz, but since it is so far away from the desired 1000 kHz, it is eliminated by the antenna stage tuned circuits.
So to tune to the wrong osc peak, you would have tune it 520 kHz lower than normal which would be out of the possible tuning range of the trimmer. But its a different story on the short wave band. Say for example you want to receive WWV at 10 MHz. You need an oscillator signal of either 9.740 MHz or 10.260 MHz. Either will work just as well, depending on which was chosen by the designers, as it has to track with the ant and RF tuned circuits. But it is easy to tune the osc to the wrong peak in this case, as the freqs are only a few percent different and well within the range of the osc trimmer.
With the 118, I believe the SW band was designed with low side injection, that is the oscillator runs 260 kHz below the desired receive freq. By changing the padding cap from 3000 pF to 1800 pF, you may have effectively converted the circuit to high side where the osc operates 260 kHz above the receive freq. This may improve the dial calibration, but may disturb the tracking with the ant and RF tuned circuits. Check the SW sensitivity with the 1800 pF padder, as the Ant and RF stages may no longer be synchronized. See how the shadow meter responds and see if you have the same deflection with a known SW signal as you did previously before the padder change.
As I recall, I played with the padder value to get the dial cal right on, but then lost sensitivity due to mistracking with the ant and RF circuits, so I went back to the original padder.
So to tune to the wrong osc peak, you would have tune it 520 kHz lower than normal which would be out of the possible tuning range of the trimmer. But its a different story on the short wave band. Say for example you want to receive WWV at 10 MHz. You need an oscillator signal of either 9.740 MHz or 10.260 MHz. Either will work just as well, depending on which was chosen by the designers, as it has to track with the ant and RF tuned circuits. But it is easy to tune the osc to the wrong peak in this case, as the freqs are only a few percent different and well within the range of the osc trimmer.
With the 118, I believe the SW band was designed with low side injection, that is the oscillator runs 260 kHz below the desired receive freq. By changing the padding cap from 3000 pF to 1800 pF, you may have effectively converted the circuit to high side where the osc operates 260 kHz above the receive freq. This may improve the dial calibration, but may disturb the tracking with the ant and RF tuned circuits. Check the SW sensitivity with the 1800 pF padder, as the Ant and RF stages may no longer be synchronized. See how the shadow meter responds and see if you have the same deflection with a known SW signal as you did previously before the padder change.
As I recall, I played with the padder value to get the dial cal right on, but then lost sensitivity due to mistracking with the ant and RF circuits, so I went back to the original padder.
