Hi,
I have a proclem with a MC 1250 power amplifier. I disassembled the amplifier, fixed the soldering, replaced the electrolytes. I took a lot of photos during disassembly so as not to make a mistake when assembling. I checked everything 5 times before it was launched, but unfortunately it was not without problems. During the launch, resistors R1 and R2 heat up. There is a thermistor between them, which we heat through the above-mentioned resistors, slowly cutting off the power supply to the primary side of the transformer.
When the output stages are connected, the voltage of the +- 115V rail drops and draws about 60W from the mains. The output stages are disconnected, the thermistor is from 60 degrees Celsius if I remember correctly and draws about 25W from the mains. I thought that the problem was with the output stages, but I do not see any damage there. I measured all the semiconductors, checked the resistor values and everything is OK - according to the diagram. I didn't set the quiescent current, but if it was too high, the fans would turn on, which would suggest that the current is too high, nothing like that happens and I don't know what's going on here. 🙄
I have a proclem with a MC 1250 power amplifier. I disassembled the amplifier, fixed the soldering, replaced the electrolytes. I took a lot of photos during disassembly so as not to make a mistake when assembling. I checked everything 5 times before it was launched, but unfortunately it was not without problems. During the launch, resistors R1 and R2 heat up. There is a thermistor between them, which we heat through the above-mentioned resistors, slowly cutting off the power supply to the primary side of the transformer.
When the output stages are connected, the voltage of the +- 115V rail drops and draws about 60W from the mains. The output stages are disconnected, the thermistor is from 60 degrees Celsius if I remember correctly and draws about 25W from the mains. I thought that the problem was with the output stages, but I do not see any damage there. I measured all the semiconductors, checked the resistor values and everything is OK - according to the diagram. I didn't set the quiescent current, but if it was too high, the fans would turn on, which would suggest that the current is too high, nothing like that happens and I don't know what's going on here. 🙄
That is a soft start circuit to limit inrush current through the transformer primary at start up. The only reason the resistors would be getting hot is if the Relay does not energize after a short time delay, which shorts them out.
Also in the service manual you have attached - there is a caution note "The heatsinks are at +115v and -115V" . I must say that is downright deadly and dangerous. How could a manufacturer sell a product with lethal voltages on the heatsinks!!!!!!!
Also in the service manual you have attached - there is a caution note "The heatsinks are at +115v and -115V" . I must say that is downright deadly and dangerous. How could a manufacturer sell a product with lethal voltages on the heatsinks!!!!!!!
I first started this amplifier using a 150W bulb, because I was afraid that something had gone wrong and magic smoke would appear.
Are you talking about the pre-relay working at a specific moment? Or maybe it disconnects and that's why it's heating up. I have to investigate this and first of all assemble the amplifier, because it's completely disassembled.
When I disassembled the output stages, I had some suspicions as to why they were encased like that, and when I got to the power transistors to replace the thermal paste, I noticed the lack of insulating mica plates, and then I knew what the external plates were for and that there was a power rail on the radiators.
There are 5 relays in the whole device and they are all the same. I replaced it with new ones of the same type, new one on the left.