Hoffy, the standard procedure of cleaning the contacts on the back of the lens is worth trying. They are gold plated, so use alcohol or similar on a swab or fine cloth, nothing abrasive. I doubt it's worth repair if repair is what it needs, but maybe. Contact Koh's Camera and see what they think.
EDIT: here's a nice tutorial on disassembly of a 150/3.5 S lens. A PS will be similar.
It shows which is the flash contact on the back of the lens. You could try to trace and look for a bad solder joint inside the lens; if the fault is in the shutter module itself that would be a delicate job if you were motivated to tackle it, but you might succeed. I would only do it for fun. If it came to that it makes more sense to buy another lens and sell for cheap the one you have, to someone who doesn't care about flash sync.
Actually selling the lens for a bargain basement price is probably not a bad idea. I had thought of keeping it and getting another focal length for flash work (keep uming and erring between a 110 macro or the 180).
I see KEH has 150/4 PS lenses in EX condition for $115-$139. If you got say 50 bucks for the one you have now, you wouldn't be spending much at all to get a fully functional one, almost for sure less than cost of repair.
I can probably fix them for you. PM if needed.
I have a lens I've been looking at, on ebay and used via amazon. but this scares me for some reason. the lens is in good/great shape but the dreaded "there is dust inside the lens but it wont affect image quality."
thoughts on that comment? experiences?
I have a zoom that was dropped and the front lens is chipped. There is light dust inside. There is no way I would ever be able to get someone to buy it. The images look like they came out of a pristine lens.
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You can sometimes blast the dust out of a zoom too. I have a '90s 100-300mm which arrived with "slight dust" and after strategic blasting with a can of compressed air it's nearly perfect. Just stick the nozzle into the same hole the dust got in through and give a few short blasts, the dust has presumably either been blown back out or just moved to a less obvious part of the lens.
Iin the 1980s, I used mostly Tamron lenses and can clearly recall my irritation and very visible dust right through the lens. Later, Nikon lenses also showed prolific amounts of dust, and only one Canon lens had it to memory. Personally I will not, and never have, now buy a lens with any trace of dust inside it. All equipment is bought in excellent to mint, and condition verified with seller/dealer with detail photographs and then again in person. A large amount of dust close to the rear element will have an effect on imaging quality (softening, flare).