Could be a lot of things:
1. Blown soft start resistor (replace with more robust part if so - it's usually the inrush current that kills them, more so than the actual long-term dissipation)
2. Electrolytic caps breaking down below full supply
3. Faulty rectifier diode
4. Output stage meltdown w/ shorted devices
It's nothing that a competent local repair guy shouldn't be able to at least diagnose / narrow down. A soft start resistor shouldn't be too hard to find, they're generally 10 W or higher rated parts, not too many of those in there. Rectifiers can generally be checked for shorts quite easily (multimeter diode test), as can power transistors. Diagnosing excessive current draw tends to require a dim bulb tester; the rest depends on how easily the output stages can be disconnected from the power supply.
1. Blown soft start resistor (replace with more robust part if so - it's usually the inrush current that kills them, more so than the actual long-term dissipation)
2. Electrolytic caps breaking down below full supply
3. Faulty rectifier diode
4. Output stage meltdown w/ shorted devices
It's nothing that a competent local repair guy shouldn't be able to at least diagnose / narrow down. A soft start resistor shouldn't be too hard to find, they're generally 10 W or higher rated parts, not too many of those in there. Rectifiers can generally be checked for shorts quite easily (multimeter diode test), as can power transistors. Diagnosing excessive current draw tends to require a dim bulb tester; the rest depends on how easily the output stages can be disconnected from the power supply.
Hi Armed, any luck?
A friend has asked me to look at his 350Mc which has a very similar fault. The amp doesn't power up at all (nor the front LEDs) then the rear breaker will trip about 5 seconds after. I assume its the soft start board, as there appears to be numerous postings saying these are known troubles.
A friend has asked me to look at his 350Mc which has a very similar fault. The amp doesn't power up at all (nor the front LEDs) then the rear breaker will trip about 5 seconds after. I assume its the soft start board, as there appears to be numerous postings saying these are known troubles.
Great to hear, I bet that was a relief! 
When I first opened this one, nothing was obvious so i checked the fuses on the main amplification board and both appear fine (0 ohms).
Reading this thread I’ve first focused on the Slow Start board.
The two 470uf Caps measured fine in-circuit however I removed them and found one leaking. I replaced with some quality parts. The tracks underneath were corroded a bit, and the nearby 15030 NPN's pins appeared covered in residue, and perhaps showing signs of heat. I cleaned all the tracks and removed the 15030, measuring it at Hfe 45, so once everything cleaned up, put it back.
I have also removed and checked the KBP10M rectifier bridge which tests fine, the 10nF input cap removed and checked, and all small resistors appear to measure within value in-circuit.
The large 20W 7Ohm wire wound resistors *appear* to measure fine in-circuit, four in parallel measuring 1.8Ohms, however the white ceramic bodies are cracked. I have NOT yet replaced these.
I have not yet checked/replaced any of the 3 optocouplers or the relay, which are pretty much the only other components on this board. I also haven’t yet probed around live to check voltages.
Unfortunately testing again has NOT resolved the issue after replacing the leaking cap. When power first is applied I see the relay contacts move, then maybe 5 seconds later the breaker trips. I don’t recall the relay changing state at this time but will check again.
I might carefully measure the voltages and see if anything is quickly obvious, although fear voltages might not give an indication. Perhaps I could swap around the slow start board from the other (working) channel amp which might help isolate the fault?
How did you work out it was the power transistor? Was either of the fuses on the main board blowing? (I assume it’s similar and has these).
It seems a pretty major job to strip it back that far - maybe more than I’m comfortable to do as a favour on someone else’s amp!
When I first opened this one, nothing was obvious so i checked the fuses on the main amplification board and both appear fine (0 ohms).
Reading this thread I’ve first focused on the Slow Start board.
The two 470uf Caps measured fine in-circuit however I removed them and found one leaking. I replaced with some quality parts. The tracks underneath were corroded a bit, and the nearby 15030 NPN's pins appeared covered in residue, and perhaps showing signs of heat. I cleaned all the tracks and removed the 15030, measuring it at Hfe 45, so once everything cleaned up, put it back.
I have also removed and checked the KBP10M rectifier bridge which tests fine, the 10nF input cap removed and checked, and all small resistors appear to measure within value in-circuit.
The large 20W 7Ohm wire wound resistors *appear* to measure fine in-circuit, four in parallel measuring 1.8Ohms, however the white ceramic bodies are cracked. I have NOT yet replaced these.
I have not yet checked/replaced any of the 3 optocouplers or the relay, which are pretty much the only other components on this board. I also haven’t yet probed around live to check voltages.
Unfortunately testing again has NOT resolved the issue after replacing the leaking cap. When power first is applied I see the relay contacts move, then maybe 5 seconds later the breaker trips. I don’t recall the relay changing state at this time but will check again.
I might carefully measure the voltages and see if anything is quickly obvious, although fear voltages might not give an indication. Perhaps I could swap around the slow start board from the other (working) channel amp which might help isolate the fault?
How did you work out it was the power transistor? Was either of the fuses on the main board blowing? (I assume it’s similar and has these).
It seems a pretty major job to strip it back that far - maybe more than I’m comfortable to do as a favour on someone else’s amp!
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