tlw
The amps are most definitely fine.

It'll be at least the end of the week before I can dig into the thing properly.

The bridge earth as I remember it is just a wire trapped between the Tunomatic post bush and the laminated maple top. I had a try at getting a soldering iron to it a few years ago but there isn't enough spare length in it to get the wire clear enough if the lacquer to want to get a hot iron anywhere near it. Just soldering an extension length on to the existing wire is asking for finish damage.

Some aluminum foil should insulate and protect the finish pretty well. And if your soldering iron tip is as it should be (clean, well tinned, right temp), it should be on the metal for no more than 2 or 3 seconds.

Well, having finally had a chance (and built up the patience) to haul the entire wiring loom out of the guitar the mystery is solved. Or if not solved has gone away. Which I'll call solved until it comes back.
 
Not the string earth.
Not a bad pickup.
But two things.
 
One being a dodgy solder joint. The wiring goes from the pickups->pots->switch->jack and that pots->switch run is across the diagonal of the guitar body, like a Les Paul only longer. And no access plates on the back to let you get at it.
 
Part way down that long run of uninsulated Gibson braided wire the braids are soldered together - the 335 etc are similar but at least the switch is near the pots not over a foot of stiff cable away. That joint being just out of reach if I pull the switch out through the neck pickup route. But with the entire loom out that joint looked like it had seen better days and my meter was showing some inconsistent readings, the connection making and breaking and having a high resistance. So I removed the old solder, cleaned everything up and re-made the joint. Why it lasted 20-odd years then decided to misbehave is one of life's mysteries.
 
As is why anyone thinks 2.5x1" holes in the sides of the pickup routs are sensible as the only access to the loom and pots. Could have done the whole job in under 10 minutes on an SG.
 
The second problem being the tone capacitors were quite a bit off-spec. So in went a couple of Orange Drops and everything seems to have settled down again.

tlw,
I'm happy for you that you got your guitar fixed
 
Kenny

Great that it's fixed. My guitar tech has tricks for dealing with hollow body guitars. He attaches dental floss to things like the switch and jack so that when they are removed he can pull the threads to get those impossible to reach parts back to where they belong. Works great and the floss is strong enough that you don't worry about it breaking.

Yep, thin strong string tied to everything is the way to go. No chance of getting anything back where it should be otherwise. It doesn't work reliably for the jack socket, especially an edge-mounted one like the 135. For that the answer is stiffish wire soldered to a jack plug that's missing its cover. Insert into socket and remember when refitting not to pull too hard on that wire.

Not tried dental floss, I usually use twine, but floss is a good idea.

There's lots of Youtube videos where people extract the wiring and pots from semis through the f-holes - really hard work that. Fortunately Gibsons have a cut-out in the treble-bout side of the centre block where the bridge pickup is and another the other way round at the neck pickup if the switch is mounted like on a Les Paul. The wiring went in through the pickup holes and can be extracted the same way. Fiddly and can be frustrating, but with care not too difficult.

That's unless for some unknown reason the cut-out isn't there or the centre-block was glued in the wrong way round, in which case it's protect the top and try the f-hole method. I once came across a 335 like that and still bear the mental scars.