SD1405 are NOT FT-920 replacement finals
The MRF255 is an FET. The SD1405 is a bipolar transistor. The SD1405 will not work in the circuit without significant rework, nor is it designed for operation above 30MHz. FT-920 has 6 meter band and uses a common chain for stage gain throughout the intended operating range of the apparatus. Furthermore, the FET device operates at much higher input impedance. Without modifying the PA, RF gain will begin fade at around 8MHz and fall at a logarithmic rate as operating frequency increases. By the time you reach 50MHz, there should be significant insertion loss through the devices. To suggest the MRF255 is interchangeable with the SD1405 is liking a golf club to a hockey puck. If you are using this radio with bipolar transistors without performing the requisite modifications, you're transmitter is likely unstable and causing undesired emissions.(which could attract kilocycle cops. To date, the Polyfet SM724 is the closest counterpart to the MRF255. A properly operating FT-920 is a fine radio set and deserves quality technical support.
Jim Warner WA9Z
Exline Sig, LLC
"Expensive radio service, on time, one time"
