Having torn apart and managed to fix over a dozen film era lenses, I think I'd opt to give it a go. I typically record the entire tear down so I can play it back, which makes it easier. Given the description of general softness, I would be inclined to expect one of the lens groups has come loose. I had that happen on an old Canon F5.6 400mm L. I was able to see where the adjustment had moved and move it back. I also tore down and managed to fix a Canon 100-400 mk1, thanks to a complete teardown video on youtube and a 2nd dead lens I acquired an AF motor from. Point being, it sounds like you have an alignment problem. Screwdrive lenses are wonderfully mechanical. It may be worth it to take some time and try to disassemble it. Those big hunks of glass in that bazooka may just need adjusted, and once inside you may see exactly where they used to be aligned. The front is almost always the easier disassembly point too, and conveniently that's exactly where the big elements likely need adjusted. If you want to watch a few teardown videos to get an idea, check out CamerasLensesEtc channel.

I have the Pentax 600mm f5.6 A lens, my first true ultra long telephoto. It is a lovely manual focus lens, super-sharp but has now undergone a couple of rounds of 3rd party repairs. The lens works beautifully, but its time has passed. Even the repair fellow sadly passed away and I know if anything happens to it in the future it will be unrepairable. My eyes are not as good as they once were so nailing focus isn't as easy as it once was either. While I still regularly use a 150-450, I finally have to admit it is time to move onto a different manufacture for my long lens options. I imagined that someday I would get a Pentax 600mm f4 F or FA, but for the price, and the fact these lenses are so long in the tooth, it is better to move on. So with a big birthday that came up I reluctantly moved over to a maker that has eye detection for wildlife. It's hard, because I have been a very loyal Pentax user, stuck to it through thick and thin (and ridicule) because I loved everything about Pentax. Ultimately it is a tool though and I have to respect that. I do hope your lens come out of it in flying colors.

As someone pointed out it is most likely an alignment issue. This is usually causes if you bump the lens. You can usually fix it by taking out the screws with a plastic rollers around them and rotate them. These rollers are responsible for travelling on the elements in the lens and migh deform by use or by bumping the lens and by rotating them you bring them back. Sounds confusing but it is rather easy.

Toronto Camera Service might be an option too, they advertise service for film era gear?

try remove rear filter and/or replace with multi coat digital era filter, this may reduce reflection between rear element and sensor

The zoom lock button on my very new, but previously used 150-450 broke of some 2 years ago. I bought it on the internet in the USA and had it sent to me here in happy South Africa. The local agent sent it to a Pentax repair shop in Germany and it came back after 3 months 100% mint. Ricoh charged me ZERO, not even postage! Can you believe it, I still cannot. So maybe that is an option for you. Send it to the repair shop in Germany for an opinion.

That is one monster lens! I do hope you find someone to fix it 🙂👍

Hi Kobie, I understand why you would like to get it fixed. I am still using my 40-year-old Pentax film equipment because I like it. I happen to use exactly same digital equipment for birds & wildlife as you do and with K3ii+150-450+1.4TC I no longer need ("want" is another matter) a big prime. Much as I like Pentax though, if I ever really needed e.g. a 500f4 then an older generation Canon would be lighter than your 600f4, probably sharper, and have better AF, and a 7Dii body is very cheap now. Lens+camera combo easy to find used, and cheaper than the over-priced P 560/f5.6. Sad but true?

also have this goal and similar problems. I have been able to prove that by removing the rear filter my images are sharper and without problems in the image. With the filter they always come out soft. Have you tried it?

Уважаемый Коби! Если бы я хотел отремонтировать такой объектив, я бы связался с Pentax (Japan) и выяснил могут ли сделать? И если да, то отправил бы объектив к ним. С уважением, Анатолий

Hi Kobie, Many years ago I tried to get my FA*80-200 F2.8 Lens cleaned due to Fungus! My local couldn't do the clean. They mentioned that it would most likely need to be sent to Japan as many of the elements were glued in place and some kind to Lazer would be needed to get it back in perfect alinement. Just what they said, no idea if this is true? They did try but said that they just could not get it apart with the tools they had. Your 600mm F4 is from the same era, so it may relate? Cheers Justin

Thats a big boy. Wow.

Too Be Continued !!!

I was wondering have you checked on the Pentax forum to see if anyone might be near you that knows of a place ?

Kobie, could there be a filter in the lens internal filter holder ? Or could the holder have moves out of alignment and be disturbing the light / image path ?

AF Adjustment doesn’t fly? Can you get a sharp with manual focus?

A pretty heavy paperweight. 😢

You show try to call them in advance...😅

Supposethe only way to get this monster fixed is to send to Japanese service. Yeah, logistic must be not pleasing...