Pentax 6x7 TTL meter. Maybe the PCB failed, Help.
I have done a lot of try and error, but end up here for the final hope.
A month ago, my Pentax 6x7 TTL meter is dead. So I have done some research and try to fix it. I do not want to bother you with the long story, so I will just make it short and talk about what I have learn/find.
The meter has no response, it always stay at "+", which is where it stay when the meter power off. I suspect some of the contact might go bad, which may need some cleaning or re-soldering.
So, I have check the power and the shutter speed + aperture with a Multimeter, they all seem fine. So I go through the repair manual (which you can found it on google, free. I am not sure if I have the right the repost it, so I will not put the whole file here, sorry .)
The repair manual say that there should be 90mV go though the ammeter needle, when the needle go to the zero position. So I check the voltage of the ammeter(the needle)(blue arrow in the picture) , and find it max go to 17~19 mV. That why the needle always stay at "+" position, because there is not enough voltage for it. The good news, there is power get to PCB.
Here comes the interesting part.
Then I suspect this might be calibrate by the VR on the PCB. I know they had been calibrated by the factory, but at least I want to push up the voltage and see want we get. So when I turn VR6, there is a 50% chance the meter will back to life and the needle voltage will go to around 90mV, depend on the shutter and aperture setting . Now, if turn the VR6 it work for what is should be. (with power ON)
However, if I power off the TTL meter and on, the meter is dead again, and the needle voltage will back to 17~19 mV. This is repeatable with turning VR6 randomly.
There are something I also tried.
VR5 is close(see in the circuit map) to the ammeter(the needle), so I try to put it at the lowest resistance, and the needle voltage will go up to about 60 mV max, but still won’t push the needle. I also solder it off for check, it has a "22k" mark at the back, it should be response for it max resistance 22k Ohm. And the lowest resistance it can go is 3k Ohm.
Now If I do the VR6 trick, the measured voltage can add up, but it will go way too hight for the needle.
I think there is something wrong with the IC or related parts (the red square in the pic.). And according to the circuit map there is a transformer(?) which link the shutter and aperture resister to the IC , and there are some capacitors, not quite sure what they do. (sadly the repair manual do not state any spec. for the parts.)
I have borrow another pentax6x7 body from a friend for test, I am sure it is not the camera body problem.
Hope anybody might have some idea for this situation. I love the Pentax 6x7 and hope it can work again.
Thank you for reading this question. Good luck, stay heathy and stay happy.
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