The crystal of 6.4 MHz is not critical. Each other frequency of 3 - 10.7 MHz is useful because the internal oscillator start easily and there no RF filters. The oscillator can be omitted if there is another HF oscillator. A good result was reached by coupling the carrier oscillator signal (PC-600) from pin 6 and 7 of PC-300 to pin 6 of NE612A.

The modification is simple; nothing has to be changed on any of the original print boards. Just disconnect a wire on the top of potentiometer R5 (MIC. GAIN). That point easily can be spotted because resistor R6 is connected to it. The wire must soldered on the top of the potentiometer (P) at the input of the processor. The audio from the output of the processor is then coupled to the top of R5. That is all! Preferably use screened wire for the assembly to or from the processor. The clipping level can be adjusted with potentiometer P. Do not run the gain to far, or you will decrease the intelligibility of your audio. So the gain must be carefully adjusted. If you always use the same microphone, the adjustment can be done once.

Adjust R5 as follows: whistle in the microphone for maximum output of the transmitter, set P so that the voice peaks are just below the maximum level. Eventually check with a scoop if the voice peaks will not be reaching the maximum level.

Probably the TBA120A or SN76660 are no longer manufactured, but it appears that they are offered still new (old stock).

Adjust the 10 m (= 17 m) VFO trimmer C415 for an accurate dial calibration of 18.000 – 18.500 MHz. The VFO range is then moved up to 12.750 – 13.250 MHz.

Check for a coupling capacitor C813 of 4.7 pF and solder parallel to C814 and C 815 an capacitor of 56 pF, eventually 47 pF. Adjust the iron cores for maximum signal on ± 18.110 MHz.

I did not remove the PCB PC-800 but used the soldering tags for the final modification of the 10 m receiver input tuning transformer.

Set the trimmers C921 and C922 for maximum output on ± 18.110 MHz.

The low pass filter provides full coverage of the 17 m band and require no further adjustment.

Note: the ATLAS RADIO INC. used CTC 2545, CTC A50-12 or CTC S70-12 transistors in the PC-500 amplifier!

Increase R516 (330 Ω) to ± 910 Ω for a correct indication of the collector current.

Eventually increase the output with an extra cap parallel to C505, but evenly decrease the output on 80 m and improves the overall stability of the amplifier with an one turn on T502 and feedback resistors to the collectors of Q504 and Q505. I was satisfied with C505 = 270 pF + 270 pF. The output of my PC-500D is not representative, may be you obtains more power on some bands.

For improved linearity and to prevent instability the output of my amplifier is limited with an extra feedback (fig») on transistor Q503. With a 1 kΩ pot and C270 I reduced the power to maximum 95 W.