Here the driver transistor (2SC3133) was burnt. The transistor is no longer produced and it is hard to find in the online store. Need to be careful when ordering one as we found that the transistor that I receive was a fake.
So, we need to find a replacement for 2SC3133 and the similar pin arrangement may be2SC1945, 2SC1972 they are with lower power output and also hard to find.
The next option is to replace with new RF MOSFET transistor RD16HHF1. It will need bias circuit modifications. It may not perfect, but still doable.
Since the original bias circuit only use one trimpot for bias adjustment, I keep it simple bias for the MOSFET. It will not balance the operating point of each MOSFET, but a small imperfection we can tolerate.
The bias circuit is made by a simple voltage divider. R5018 is replaced by 1K5/0.25W in series with 1K Trimpot. While Gate resistor R5008 and R5009 is replaced by 3K3/0.25W.
Seen above is the gate bias transistor (3K3) in place of the old 2x470 ohm SMD resistor.
The MOSFET require less feedback, so replace R5010 and R5011 with 560/2W resistors.
The final modification. Signal output from the driver observed with oscilloscope shows that it has 20Vpp and seems promising.
Transmit power test gave a very good result. The PA now can deliver up to 180W of RF Power. But for safety, the power is adjusted to 100W.
I am glad that the MOSFET modification can save the S600.
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