I will add to Allison and others advice:
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No wires must run parallel or near your heat sink or finals. RF will get in and play havoc.
At 28/04/2020, you wrote:
*** The heatsink must be grounded.
*** If you added wires to get from the board to the MOSFETS (IRF510) that is
an absolute bad thing to do. You just can't, it will be unstable.
Note besides TO220 insulators you also need the shoulder washers to
keep the fets from shorting to the heatsink.
There is a capacitor in series with the 220 ohm Drian to gate. Thats C262 and C261
hard to see in the manual schematic unless enlarged. No need to alter.