the mods that I performed on mine were:
1.replace entire switching board with this:
2. Replace both pots (B100K dual gang for speed and B50K for intensity)
3. Install SPST switch across 100K resistor for vibrato mod:
4. added fet preamp booster to output with B50K volume control:
on mine, where it says 'volume' 2, I used that as the input and wired the B50K volume control's lug three to the 'output', and lug two to output.