Fingers crossed..

 I dismantled my vivitar 24/2.8 (s/#37xxx =Tokina) and poked around. I unscrewed the rear element, cleaned and examined it - and that was about all it seemed I could do without major disassembly. 

Testing showed the same problem as before: the view approached infinity focus only when it was focused nearly to the minimum value! Almost like an extension tube, but reversed? Hmm.

So I took it apart again and noticed two shims that limited the depth of the rear element. I unscrewed the rear element slightly and tested again. Same result.

On reopening I discovered it had three shims! So I took two out and tried again. Hey that looks better!

By now it was 10pm so I was shooting indoor tests. Worse still, I was set on a telephoto focal length for internal IS was set to 135mm! I tried both the 24 and 28mm lenses with a much improved shake reduction value, and I believe that both lenses are now giving quite decent results! 

If spare time is available in daylight, I will check it again. Good night and high hopes for tomorrow's tests!!

Vivitar 24, smc-a 28, 24-85g +ftz

Update: sunlight tests show the 24/2.8 is working much better. However, it is not as good as the 24-85g at f/4. That isn't a great shock, as the prime failed to wow me before the adjustment failed; it's simply not a great copy and it's perhaps 30? years older. Also: the 24-85g/VR is that good! 

Testing my Pentax-A 28/2.8 against the zoom shows a pretty close contest between them at f/4 (zoom is f/3.8 at 28mm). I didn't get rigorous as the lenses don't really compete so Team Pentax will have their turns! As for autofocus kit, it means the wide shots are well covered with 24-85 and 40/2.

As noted elsewhere, I've released the 20mm for now (plus other gear) to acquire the Tamron 70-300 rxd. I really like the idea of having an 18-20mm prime, but its usefulness is not great. When I'm certain of a long hike above treeline, something small in that range will be valuable - but such a trip is not in the works. Perhaps by then a few other wide primes will be available!