Or...
Build Your Own...
I'm sure others will come in with their own ideas and methods...
But to help you with the above, Many just remove R123 - which is your prerogative, but to me it also plays an important role in how much Gate Drive and Bias - if not kept separate - will eventually destroy the IRF520.
Now, you may want to adjust values to meet the power levels you want;
You cannot do a JP36 MOD and also this MOD and expect both to work using "specific" values shown - it may work, or not - you will need to make the EN-1230 part provide the Gate the Bias as needed, change the resistors of the EN1230 to help with this - else you may need to apply other methods shown further below.
- the above shows a 1N4148, and 470 ohm - you keep the diode, but MODIFY the resistor from 390 to a high as 680 ohms to lower, or raise - BIAS power.
The EN-1230's 3.3K can go as low as 1.5K to as high as 3.3K - but affects the OVERALL level of signal DRIVE present in the line that goes to the final.
Note the "R123" - it's important - this area many techs just remove and jumper, which again, you choose your poison, but really you need to keep it in to prevent another problem that tends to creep into this.
If you want to use Variable power to adjust for amp drive, then the values to replace the EN1230 used for the above will be different. Use the higher (greater ohmic resistance) values to allow more RF signal to apply and keep the MOSFET working.
IT's when you drop the dead key too low, does the MOSFET have trouble working linearly let alone properly.
So R123, if jumpered, exposes the Gate to potentially higher drive currents and voltages that can exceed the parts' Gate Drive Rated limits - R123 installed, reduces / impedes the RF signal - so to offset the Drive to Signal ratios, especially on low power levels - install a small value Disc Cap across R123 as needed to allow more RF signal to arrive to keep the MOSFET on.
To adjust the carrier- you have to modify the EN1230 part - it's better to change the 3.3K Resistor (shown in position piggy-backing the 1N4148 and 470 ohm) to higher or lower values to offset the VOLTAGE you'll need to keep the part working on low, and R123 as needed to keep the MOSFET from blowing up if you need / send the Gate full RF power - as it (RF power) and the Gate Rectified Bias from the EN1230 can raise the level of power applied to the Gate and destroy the MOSFET.
The Range of Adjustment is the problem, and R123 is needed to help with this. It' is better to lessen the GATEs' DC voltage (from the EN1230) by increasing the R123 resistor value - and let RF from DRIVER go into MOSFET across the Cap - in enough level to offset.
There is another reason for R123 - is;
IF your variable power mod can't go as low as you want - you may need to install a 10K "Tickler" resistor to apply a trickle voltage from the TX transistor located close by - Locate JP29 and positive side of C116, solder one leg of the 10K there, with the other leg to the R123 side of R56 to help keep the MOSFET on when driven in low power mode.
If you want a lot of variable RF power (like Low 1.5 watt to high 20W) find another method, this is only for 4 watt radios being "refurbed" with the IRF520 (or Equivalent) MOSFET
