The 300d is one of my favorite bodies.
Somebody gave me a few day ago a very strange one. If you inserted the battery, the body was making only one shot automatically, then nothing else – but the first stage of the exposure button was fully functioning.
Hmmmm. At the first look, I told to the owner, it must be a damaged button, which makes a short circuit. But actually, I’ve never seen a button, where the second stage is damaged, and not the first one…
Talking a little bit with the owner, she told me, that she was trying to supply the 300d externally with a power supply. what???? the 300d has no plug for that think, there is only one possibility – through a dummy battery. But she has no dummy. She has inserted the supply plug IN THE PLACE OF THE REMOTE CONTROL.
On the surgery desktop I’ve found no short circuit on the button – the button was absolutely perfect. There was no short circuit on the remote entry either.
But. I’ve found, that one entry has approx 2 Mohm resistance, the other only 12 kohms reported to the GND. Something is not OK there.
Here you see the backside of the 2,5 mm jack socket. There is a small part there, marking: 10A (3 pins).
If desoldered it, after then the body was working well. I’ve found on the net the smd marking code. It seems, that it is a double Zener diode – I think to protect the entry. Unfortunatelly, if the voltage comes reversed, the zener will burn out…
I had another sacrified 300d, so I soldered in a new “10A” part on the PCB – now the 300d is ready for the next adventures.