I am using the Expressimo heavy counterweight on my Rega RB-900, and have the VTF disabled (by turning the VTF gauge all the way up). And unlike Dopogue, I was really quite amazed at the improvement wrought by the Expressimo counterweight.
Fact - When you install the new c/w and disable the VTF spring you will have to adjust VTF by measuring it with a scale. (This is good.)
Fact - For the scale to work accurately you must zero out AS.
Conclusion - You will have to reset AS afterwards.
Recommendation - The AS device on Rega arms is notoriously inaccurate. Most people find the optimum value to be lower than the scale indicates. Nsgarch's world-famous visual AS-setting method is easy, effective and free. (I'll let him type it - again!)
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