I just found time to get into the scope power supply to do the cap and diode. Testing the old one shows forward voltage of 3.39v (I think it has been replace before) The new UF4006 is .56v. Is the UF4006 a good replacement?
I installed the UF4006 and it seems OK. The scope driver board is still not right with new fets and transistors. I have no 105volts on collector of Q1202.
Do you have a detector assembly from the donor receiver? If so try swapping it and see if the distortion changes. It could be that the detector is out of alignment. Not sure if you have the service bulitens so I attached the two that may help.
Warning..... Be very careful adjusting the coils on the MPX board. The little plastic adjuster that has a screw driver slot in it can shear right off of the coil core. I found out the hard way....
I have all the tuner parts. I will see what happens. The more I look at this unit the more I find that has been replace in past service. I have found that the transformer was even replaced.
I have seen this issue as well on one of my 18's. It was also missing a wire if I remember to the MPX board and maybe the muting switch? I'll see if I can take a picture of it.