This lens has a broken aperture flex cable. As there is not possible to buy it, I decided to purchase a flex for something else, and binding them by soldering (I had to cut it at 2 places, so 2×6 solderings on the flex, and glueing with 2-component glue).
After replacing the cable the aperture was ok – but no autofocus. Disassembling, assembling several times – nothing.
I decided to replace the motor drive IC on the PCB. Is a LB1836, normally used for stepper motor, here for the DC motor. I desoldered it from another lens, soldered in here – no change.
Playing with the motor unit, I tried to rotate the axis a little (this is possible only by forcing it). After this, the AF worked.!!!
I measured on the motor, 0.4 ohm. This is a very low value. I measured on the old sigma, 4 ohm….
After rotating the motors spindle, the resistance changed to 5 ohm! Rotating more, 0.5 ohm again…..
It seems that the DC motor has a shot circuit on one of it’s rotor coils, and if it is stopped in a such position, the IC will stop the current – self protection.
Than I have taken out the complete motor unit, I soldered a short instead of the AF/MF switch. Now the lens works fine in manual mode, and as it is a macro lens, used in macro mode this is not a problem – in macro mode the focusing You have to make anyway with changing the object/camera distance, (the lens camera distance changes mainly the magnifying)
On the focusing barrel I have found one broken teeth – somebody tried to force the motor by hand through the barrel. Be careful – this is not possible because the structure of the gear!