I have an original Jem 77 BFP & PMC which I am considering putting up on ebay ( no longer play them ). They come with the original cases. The PMC was modified a bit, new pickups and an extra switch to operate them. The BFP model has a new ( well maybe 7 / 8 year old ) low pro edge tremolo system ( same type as the original )
Would these two guitars, with the modifications described above, still get a good price by collectors ?
The market for classic (I'm reluctant to use the word "vintage" just yet
) JEMs isn't quite yet as picky as the market for vintage Fender or Gibson guitars. As such, restorative modifications, things like in this instance, the replacement of the BFP's trem with an identical but less worn bridge, doesn't tend to hurt the value at all. Another common restorative modification is new pickguards, as the clear ones for guitars with direct mounted pickups are very susceptible to cracking above the neck pickup.
What does hurt the value of these types of guitar are things like new, different, pickups (unless the originals are still available) especially if they're not the appropriate colour and also, usually even worse, are permanent modifications like extra switches.
however, the good news is that the PMC is a very rare guitar, only 271 were made, and they are all very unique. They are also very prone to fading, especially on the pink and peach coloured parts of teh swirl. As such if you have a great swirl, you'd still get a healthy price for it, if it's been protected from sunlight, it'll be worth even more.
My own experience is that if the swirl isn't so hot, then the modifications would have a greater adverse effect on the price than for a similarly modified example with a better overall "look". That said, the swirls are very much a case of "beauty is in the eye if the beholder" so you never know.
I'm sure everyone here, myself included, would love to see pics!