When you say the photo chopper is aligned, do you mean the infrared diodes and the photo transistors? Taking that at face value, Turning the knob clockwise should produce a high on pin 4 (IC 10) and turning the knob counter clockwise should produce low on pin 4 (IC 10). This output goes to the LSI and is the up/down control.
If you do not have that change of logic state, then you need to look at the input to IC 11, pins 8 and 6. This is the input to this circuit and comes from the photo transistors in the photo chopper. You will need to look at that signal with a dual channel oscilloscope. You should see to signals shifted by 90 degrees and the lead and lag should reverse when the direction of the knob is changed. If you have good waveforms here then you need to follow the waveforms through the rest of this circuit to IC 9 and then to IC 10.
The output of IC 10 on pin 3 is the CK (clock) pulse train that occurs then the tuning knob is rotated and since you can tune up but not down, this must be okay. You need to look at pin 4 of IC 10, this is the up/down control and it may be stuck in a logic high state. Also note that this is also connected to the ACC socket, make sure there is not an issue there.
You say the RF power output is low, (okay you know what is coming).
How low is it, just a few watts or very low and is it the low in all modes and are you able to adjust the output power in the various modes, even if it is low?
Your issue with low output power can be from a number of sources.
Of course it is never a bad idea to start with the power supply (to the RF PA). Then check the drive to the PA. If the drive is good and the power supply is good, then you may want to look at the output filtering and associated circuitry.
If the drive is low, then things like the APC (automatic power control) circuit and the circuits the feed it. Alignment should be close unless someone has been inside this unit.
Your DMM and your oscilloscope will be your friend.
This is about the best I can do right now. My poor health and medications poke holes in my thought process and make make it fuzzy, so if I have some of the designators incorrect, please bear with me.