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Mine is a TH II.
Near the front, middle of the main board is the PLL crystal. There are two through holes already in the board and I believe they are marked for a capacitor if I remember correctly though there was none installed.
I used a PC board mount air variable trimmer cap. The value was 10 to 160 pF.
My frequency was high and the trimmer brought it down. Once in a while the frequency increases slightly and I just bump the chassi and it generally comes right back where I want it. Obviously something must move slightly when I do that.
This mod only corrected the off frequency situation which got rid of the warble. If your frequency is already low this probably won’t work.