I picked up an Onkyo TX-SR607 AVR listed as faulty to see if I could get it to work. I’d like to use this as a low-investment opportunity to learn electronics repair. I know someone ‘tried to get it to work’ before I took it off them, and I’ve done a couple of things to it which in hindsight were misguided. The fault is that, when first turned on, after about 30 seconds it enters protection mode, and then enters protection mode after a couple of seconds thereafter. There was some water damage to the top of the case. The two resistors on the speaker output board for FL and FR shows signs of heat damage on the PCB, but measure fine. The output rails, +-B1, measure around 58V, not the 53.8V indicated on the schematic. I am in the UK and wonder if this is due to an assumption of 220V mains.
One of the channels (SR) has a few hundred millivolts on it when probing the central point of R6104 with no signal and no load. After testing all transistors and in-circuit resistances across all resistors and capacitors and comparing them to another channel I have not found anything which indicates why this would be. I suspect this is what is causing the amp to enter protection mode.
While 'playing' with this, I first shorted the output transistors on the SL channel (Q6053 & Q6063) while probing, unplugged but without discharging the capacitors, resulting in a bit of a bang and the transistors being shorted. I've replaced these with a pair from Aliexpress, I don't believe they are matched despite the listing, although think they are likely used genuine parts.
I also tried to remove (a few 10's of millivolts) offset from the FR channel's speaker terminals by adjusting the bios pot. Reading up on class AB amplifiers, this seems to have been misguided, given that it is setting the bias point, not modifying the position of the zero crossing! Anyway, while turning the pot to one end of travel one of the output transistors popped and burst into flames! The pair are currently removed from the board, and the rest of driver board is obviously not okay. Is this degree of destruction expected from enthusiastic pot usage, or was there likely a fault in the driver board already?
Many thanks for any help you can give me with this, it'd be nice to save this AVR from the scrap heap but appreciate this will be quite a learning curve for me.
One of the channels (SR) has a few hundred millivolts on it when probing the central point of R6104 with no signal and no load. After testing all transistors and in-circuit resistances across all resistors and capacitors and comparing them to another channel I have not found anything which indicates why this would be. I suspect this is what is causing the amp to enter protection mode.
While 'playing' with this, I first shorted the output transistors on the SL channel (Q6053 & Q6063) while probing, unplugged but without discharging the capacitors, resulting in a bit of a bang and the transistors being shorted. I've replaced these with a pair from Aliexpress, I don't believe they are matched despite the listing, although think they are likely used genuine parts.
I also tried to remove (a few 10's of millivolts) offset from the FR channel's speaker terminals by adjusting the bios pot. Reading up on class AB amplifiers, this seems to have been misguided, given that it is setting the bias point, not modifying the position of the zero crossing! Anyway, while turning the pot to one end of travel one of the output transistors popped and burst into flames! The pair are currently removed from the board, and the rest of driver board is obviously not okay. Is this degree of destruction expected from enthusiastic pot usage, or was there likely a fault in the driver board already?
Many thanks for any help you can give me with this, it'd be nice to save this AVR from the scrap heap but appreciate this will be quite a learning curve for me.
