Many people may not imagine it, but this SLT-12U is simply the best Marantz turntable and a real gem. You just have to understand how it works. It's a balancing act with top notch sonic performance if the cartridge is quality (Shure V15 III, IV or Empire for example). As good as Thorens TD124 with a good arm and even better.
First, remove the belt by disassembling the small chainring "Marantz slt-12U", then remove the chainring.
Second, adjust the base with a real spirit level (not the little spirit level on the turntable). The level must be perfect both lengthwise and widthwise.
Now secure the arm in the black box with tape all the way around the box and reinsert the tray with the belt.
Now you need to adjust the height (about 0.15 -0.19 inch) between the base and the tray with the two wing nuts under the turntable and adjust the perfect level of the tray.
And finally, you can adjust the counterweight on the arm and try.