I am trying (with a lot of limitations) to understand what's wrong with this DCC decoder. If I switch it on for a fraction of time, I am able to see in the infrared camera a problematic section (marked in the foto). The heat flow is that first lights the resistor and secondly the 1F IC. These components seem good if I test with the multimeter...........however I will remove them from the PCB and let's see if I find something faulty.
Also, I removed what it might be 3 Mosfets (2 of them where shorted). Their SMD code was BAPLZ,
The remaining one behaved (out of the PCB) like a N channel mosfet..........but who knows. I was replacing them with A09T (a N channel mosfet).
I suspect that pins 3,4 and 5 of the 1Ft transistor constitute the NPN pass transistor in a 5V regulator. The F0 diode is probably a 5.6V Zener, eg BZX585-C5V6. In other words, I think the area inside the marked box is a linear 5V regulator. The cathode of the "14" diode would be the positive rectified AC input, and the 6V capacitor would be consistent with a 5V output. I think it would be normal for the resistor to become warm during the initial charging of the capacitor. Thereafter, the current draw would be minimal, so things should settle down.
This seems to be a general layout for your decoder:
Finally I've managed to fix it!! :-)
- I removed the 1F (The suspicious one I had in the red rectange) by a 1B.
- The 3 removed BAPLZ's by 3 N-Channel mosfet also seemed to work.
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