For many of us it is very famous problem. Reversed DC polarity and …
There were many discussions on this topic. Here I would like to discuss two possible solutions, with simple TO-DO for everyone
Replace whole converter by modern type, sold here and there on eBay. You can find various 5V to 12V converters, or even anything to 12V converters. They’re small, much smaller than the original one. They’re more efficient. They can deliver more power. So why not to use them easily?
The answer is simple – they inerfere with internal circuits of D6C. You need to build ground shielding around it. More, their output voltage isn’t “clean” enough and it requires another filtering. Impossible? No, it just requires some time spent. Does anyone have experiences?
Repair the old one. Is it easy? Yes and no. The most difficult part is to remove the converter from PCB and open the original shield. It is difficult but everyone will know how to do it as soon as he tries.
I will rather describe how to troubleshoot the circuit that is inside.
Here is crop from service manual. It is just symbolic circuit and it has some basic mistakes. But it is very good start point.
Transformer and transistor on the left side create oscillator. Diode on the top is rectifier. Transistor and diodes on the right create stabilizer. When you reverse polarity, three parts can be burned – the ones marked with red letters and transformer. However, the wire on the transformer is much thicker than on the 33uH coil, that’s why it will survive. In most cases, it is necessary to replace both red parts:
Instead of hard to get 2SD1048 you can use BC817 as 1:1 replacement. Well, the older one is little stronger (700mA vs 500mA) but in this application it doesn’t matter that much.
It is good to have right soldering station to be able to solder SMD parts. Or ask someone who can.
I hope you find this article useful. Any comments are welcome.