This is probably the best symptom to check. The 2N2219A is the driver for the final. The current flow through the emitter resistor determines not only the amount of drive,
but the amplifier class. If that 10 ohm resistor is being fried, either it is not a heavy-duty enough part, or it is being over-driven. Note that R136 provides the drive to the amplifier.
A proper procedure to check through would be:
Tools: a good DVM. a RF probe feeding a DC voltage to the DVM.
If you are replacing the IRF510, drop R136 and RV1 down to near zero. If not, adjust RV1 FROM ZERO for a drain current draw of an additional 100 ma. of current draw
for the whole rig. Note that RV1 adjusts the gate voltage; the increased drain current is an indirect effect. Note that at a gate voltage of 0-3v the drain current will be minimal,
and then increase quickly around 3v as the gate voltage is increased. At some point about 5v on the gate, there will be a sudden and very heavy current though the drain.
That is the avalanche region. If you hit that, you have gone too far. It is probably a good practice to put a 1/2A fuse temporarily in the drain supply line to limit
the current draw if you should hit the avalanche region; often, even that is not enough as the effect is sudden and catastrophic to the device if the current
is continuous. And every IRF510 is different. This can happen at ANY point! If it does around 3-4v on the gate, replace the unit with a better one.
Then, when you have decided that you are in the linear region of the IRF510, look to the driver. The 2N2219A should be only warm to the touch. The 10 ohm emitter resistor
should be cool and remain pretty much so, or heat to a bit warm. It should never be hot. The amplifier is operating Class A with the warmest feel (most current draw) to the resistor Measure the RF voltage at the top of RV1 when transmitting a more or less continuous tone. It should be in the region of 1-2v pk to pk. If not trace back and find out why. Usually that is somewhere in the mic drive. Gradually increase R136 while observing the power output into a 50 ohm dummy load. When you have about 5W output, stop. That is about right. Check your audio with a QSO and tweak the drive to make it right. Don't adjust too much. You may have to iterate some between R136 and RV1 to get an adequate result. But once you have hit an output region, further adjustments are small.
If you decide to use 24v at the drain of the IRF510, the procedure is about the same. Current draw will be different, of course, because you will be transmitting at a 10-20W level. But not that different! Never use anything over 14v in the main board. For one thing, power is applied continuously to the VFO and BFO, and these units have power limits. Even if you are using a DDS, Q8 and Q7 are still active. In fact, Q7 is usually the hottest area of the board, no matter what the BITX version.
john
AD5YE
