28 Mhz is unusually open for the last one week. I realized that our ubitx output is woefully low. Here is a fix that I made today, if some of you can try it out, let me know if it works as well.
Step #1 Increase the predriver gain towards the higher frequencies
The predriver Q90 has a emitter degeneration capacitor C81 (0.1uf) and and R83 (10 ohms). I replaced the C81 with a 470 pf and the R83 with 5 ohms. With this, the emitter reactance decreases with increasing frequency, yielding higher gain beyond 14 MHz.
Step #2 Take off the feedback from the IRF510s.
Just remove the R261, R262 (both are 220 ohms) to open up the gain.
Let me know if this mod is replicable?