[milesy]

HI Guys

i have a vivitar 100-300mm lens that i was given some time back. All seems to work ok with this on my K-y except i cannot get it to recognise when the lens is set to the A setting on the aperture ring.

Anyone have any idea why this would be _ it was orginaly my dads lens that they used on their old pentax bodies (cant remember the model even but film cameras)

appreciate any ideas here

Adrian

[Adam]

The lens might be broken- does the A setting work on another body?

[milesy]

thanks Adam the thought did occur to me but i dont have another body to try.......does anybody know how the A setting actually works??

[Douglas_of_Sweden]

Try reading Features and Operation of the Ka Mount

[drougge]

When you set the aperture ring to A one of the contacts on the mount is connected to the rest of the mount. Usually (but not always) this is done by a "pin" (looks more like a ball) popping out of the lens. If this is stuck, the A settings won't work. (The other pins are used too, but this is the part that seems like it might break.)

It's the pin that mates with the (only) recessed contact on the mount. (That is, the only little circle that isn't the top of a pin on the body, but rather a shallow pit.)

Edit: And I'm slow. Of course.

[milesy]

thanks Douglas ill have a read of that and see if it tells me anything

Drougge- i was sort of after how it actually works - is it based on resistance or something similar?

[drougge]

It's just connected or not. Binary, if you prefer. One pin for "A mode enabled", and the others to encode the aperture range.

Edit: See http://www.robertstech.com/matrix.htm for encoding.

[milesy]

thanks Drougge.......ok starting to understand this.

It appears in this case that the A pin (the one that pops out from the lens when set to A) carries 3.3V from the camera body to the lens. This 3.3V must be then supplied to the actually lens mount through a resistor where it connects to other contacts on the camera body (or not connects depending on the lens). In this instance it seems that the connection between the A pin on the lens and the mount on the lens is open circuit so the body doesnt see it as an A lens.

I have taken the mount off the lens and think i can see whats wrong but will need to take it to work and have a look under the microscope to be sure. Ill do that and post a pic or two for those who are interested.

Adrian

[milesy]

ok well as promised here is an update for anyone who is interested.

so if you have a look at the following picture it shows the back of the lens mount



and here is a closer view so you can see the action



so at location marked 1 is the pin that connects to the 'A' connection on the camera - this provides 3.3V from the camera to this pin. My first assumption was that the voltage carried along the track from location 1 to location 2 and then onto 3 where it 3 created the connection to the lens mount - but this turned out to be incorrect. There is a split in the track essentially at location 2 - and i now think that this break in the track is to seperate the Pentax A connection and the Ricoh A connection. (Ricoh pin at location 3)

So what was the problem well if you look at location 2 you can see an extra leaf connection sitting above the main track - this is supposed to be pushed down to make connection to the main track by the little nodule shown in the following photo



it seems that the nodule has worn down (it is only plastic on the aperture ring) to the point that it no longer makes the leaf spring touch the main track.

So the circuit actually works 'A' pin supplying 3.3 volts - runs along the track and when the aperture ring is the A setting the voltage goes through the leaf spring shown (at 2 above) and to the lens mount through the mounting screw that holds that leaf spring in place.

So what do i do to fix it - well i dont have much choice here - i will put a dab of solder between the extra leaf spring (at 2) and make it a permanent connection which means it will always be in auto aperture mode - suits me anyway.

If nothing else i learnt something and i hope someone else does too.

If anyone wants more info let me know and ill be happy to explain further

Adrian

[milesy]

and again for anyone interested i have taken a couple of shots with the Auto Ap working

first one wide open



and second one at F22

[milesy]

for anyone interested - i am very pleased with the shots taken from this lens now set in the A position - i was a little concerned that this old lens might not produce the best shots but they are quite pleasing