Best cartridge for very old worn vinyl

Howard, can you measure the P2S distance of the new position you found for the tonearm? According to the FR66 manual the mounting (or P2S) distance should be set at 295mm. Only then will the FR7 be correctly aligned to Stevenson geometry.

You mention that the FR7 and SPU came several mm's short on hitting the 'bull eye' at the previous dealer's position. This seems to indicate that the P2S distance has become slightly shorter in the new position you found. Correct?

If you prefer to keep the arm mounted at this new P2S distance you will need to reposition all your other cartridges, otherwise they will now be misaligned. Basically it means all cartridges should have a headshell collar to stylus tip distance of 51mm, same as FR7 and SPU.

FR7fc has a conical tip and this tip profile is less sensitive to the adjustment, this is the one and only benefit of a conical tip :) When all other profiles are a bit off the alignment, it will be irrelevant for a conical tip.

If one of your FR7 has longer cantilever than others then one of them is probably not the original (but a re-cantilevered version). Because they are all designed for FR tonearms first and you know PS of the FR arms. When FR cartridge is off with correct PS on FR arm then it is a bad sign (the cartridge has fake cantilever). You’d better check everything again slowly to make sure.

When you align any cartridge with one point protractor then you have to alight the cantilever, not only stylus tip position. You can reach the point with your stylus, but a cantilever and the hole cartridge will be certain degree off the lines. If you will change PS distance to reach the required point with the stylus then you will notice another problem soon, the problem is that a cartridge position (and cantilever position) is not parallel to the lines on protractor. You can fix it only with conventional cartridge on conventional headshell with slots.

@edgewear, hi again, I measured the s2p of the FR66 in the “new” position (ie the position of the tonearm whereby the stylus tip of an FR7fc sits in the “bullseye’ of the Brinkman alignment tool) - and its 295 mm (!).
Most definitely the FR7fc and SPU synergy both sound much (extremely) better in this position, in fact I venture to say better than anything I’ve heard in my system to date (including the cartridges that were each aligned in their headshells such as Koetsu Jade/Vermillion and London Reference (Decca).
I’m going to follow @Chaksters good advice about the Feicker protractor in any event but this is getting quite interesting as I wasn’t expecting such a jump in performance with the fixed headshell carts.
One issue I may face is that the other cartridges will all need to be moved back in their headshells and I’m a bit concerned as to whether there will be enough adjustment possible there as might be needed. I’ll cross that bridge when I come to it !

@chakster , thanks again.
i understand the point that fixed headshell cartridges can’t be adjusted for zenith (or azimuth). so how is this solved on the Feickert alignment tool ?
cheers

It is solved not on alignment tool for headshell integrated cartridges like FR/SPU, but with correct PS distance by tonearm designer if you're using FR cart on FR tonearm. On Feickert you can only verify Stevenson point for those FR/SPU headshell integrated if you're using them just like the manufacturer suggested to you them.  

For the rest of the cartridges you can adjust everything and altering the alignment by choosing the one you like (Baerwald, Stevenson or Lofgren). 

BTW some headshell integrated cartridges like old Dynavector and Technics EPC100c or 205c are adjustable, overhang and azimuth are adjustable, but you can't swist the cartridge anyway.