['LIVEWIRE?']

I'm in the process of repairing a Bush SRP30C fitted with the above deck. Someone has fitted one of the Chinese Ceramic Cartridges in the detachable headshell by gluing it in a BSR Turover bracket, fixing the t/o knob & 'flag' in place. So far so good, but (1) they were only using the LH channel of the cart. with the -ve terminal taken to one pin of the 3-pin plug on the headshell, and the +ve terminal to the other pin. No common negative lead was fitted. The cartridge actually worked that way, how I don't know, since one of it's pins was only connected to the 'stereo' socket in the player. I've now fitted a negative lead, commoning the 2 -ve terminals on the cart.
Problem #2 is that I cannot set a tracking weight no matter which way or how much I adjust the screw at the back of the PU arm which is meant to set the stylus pressure. My trusty Garrard Pressure gauge, which I checked for accuracy using the 5g. test weight, will not show a balance at any point on it's scale from <1 to about 12g., though the arm did track several records OK before, out of curiosity, I decided to check the VTF (Stylus Pressure). AFAIK the arm is as Garrard designed, and I can't beleive the VTF is more than 12g, so what can I do to check and set the tracking weight?