| Electronics > Beginners |
| Does this circuit even make sense? |
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| hermitengineer:
Hello all, I am attempting to repair an old game console, just for the challenge of it. The first major challenge, though, was a complete lack of schematics. So I had to reverse engineer the whole thing. Anyway, there is a particular circuit in here that does not seem to make sense. I've pulled the parts and inspected both sides of the board just to verify that I got it correct, and it appears so. But then... is the circuit even useful? From what I learned in trade school, the BJT only allows current flow in one direction, right? So C249 can charge when it turns on, but I see no path for it to discharge. Is there something I'm missing? The only possibilities I can think of are: * I'm blind and can't see what should be an obvious trace * Some troglodyte replaced the transistor with a wrong partWhat I do know is that the part number of the current BJT is clearly printed on it. |
| Mechatrommer:
one possibility is to adjustable capacitance between 4.7nF (Q207 turned off) and 4.7047uF (Q207 turned on) |
| oPossum:
Collector to emitter current flow is typically better in one direction than the other, but it does go both directions. Some transistors are optimized for bidirectional operation. An audio muting transistor is one example of that. |
| hermitengineer:
--- Quote from: Mechatrommer on October 26, 2019, 06:18:24 am ---one possibility is to adjustable capacitance between 4.7nF (Q207 turned off) and 4.7047uF (Q207 turned on) --- End quote --- I agree about the intent, but without a usable drain path, I can't see how the 4.7uF matters in this circuit. I wonder if it's supposed to be a JFET maybe? --- Quote from: oPossum on October 26, 2019, 06:27:28 am ---Collector to emitter current flow is typically better in one direction than the other, but it does go both directions. Some transistors are optimized for bidirectional operation. An audio muting transistor is one example of that. --- End quote --- For BJT's, I've always heard that reverse leakage will be in the nano-amp range though. This circuit is part of the video output generation, and its input is the horizontal sync signal. So nano-amps through a 4.7uF capacitor probably won't drain it by any noticeable amount. |
| oPossum:
Reverse leakage would be for BE and BC, not CE. |
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