Ashhar Farhan VU2ESE notes that 28 Mhz has, unusually, been open for the last week. As a result, he realised that the ubitx output was woefully low on 10m. Hence the experiment below:
Ashhar suggests the following final fix, and asks for others to try it out and see if it is replicable:
Replace C81 from 0.1uf to 470pf
Replace R83 from 10 ohms to 2.2 ohm
(you can short R83 as well)
Replace 97, R98 from 47 ohms to 220 ohms
Remove C261, C262.
So, what happens is that removing the C261 and C262 increases the gain of the finals. They are run open. Hence greater gain at 28 mhz.
However, the gain is very high at lower frequencies. So, in order to reduce the gain at the lower frequencies, the 0.1 uf cap is replaced by the 470 pf. As the frequency of the signal drops, less and less RF flows through the 470 pf, decreasing the gain of the predriver. 470 pf is not a magical value, 220 pf works almost 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). Replace C81 with a 470 pf and R83 with 2.2 ohms. Altenatively, short R83.
With this, the emitter reactance decreases with increasing frequency, yielding higher gain beyond 14 MHz.
Step #2 Take off the feedback from the IRF510s.
a) Replace the existing R97 and R98 with a value of 47 ohms with 220 ohm resistors.
b) Remove C261, C262.