Ok I finally managed to fix one of these nice subwoofers. There is something I dont fully understand. Amplifier is capable of 800Watts RMS power. However, the power cord is rated 2.5Amps. The input RFI choke is rated 2.5Amps. Mains transformer has very thin secondary windings, something like 0.5mm diam. How do these withstand such a load?
Power supply is very simple self oscillating SMPS (ZCS/ZVS type?), no controller IC's, only few discrete components. Output voltage is +/-112Volts. Amplifier is also made out of discrete components, Class TD topology.
Yamaha claims:
Power supply has superior load characteristics, minimal noise, higher efficiency and stability.
Power supply holds the voltage and current levels to zero while the decice turns on and off, this reduces noise.
Other advantages include superb sability against power fluctuations.
Yet, so simple smps circuit? Well, it failed and there has been many YST-sw800's having the same issues, main amplifier failed and psu failed.
What I did notice, is that there was many bad solder joints especially on amplifier pcb. One driver (Q1A) transistor was shorted but output fets and other semiconductors were all ok. I changed the driver transistor and fixed all bad solder joints. When I turned the amp on, that very same driver transistor immediately got very hot. Bias current was out of spec, so it had to be failed bias trimmer which I soon changed. Problem solved, amplifier is now very stable and is running cool.
So the original cause of this failure was possibly some bad solderings and/or bad bias trimmer. This caused the amplifier run out of control and, possibly burn the SMPS.
SMPS main transistors (2SC4140) were shorted. Both drivers were shorted and two resistors was burned. I changed these and it fixed the smps. I used BUX98AQ as a replacement type, these are more robust than original 2SC4140's.
So, there is no short circuit or over load protection for SMPS?! Did they choose the cheapest way to make SMPS? 😀
Schematic:
Power supply is very simple self oscillating SMPS (ZCS/ZVS type?), no controller IC's, only few discrete components. Output voltage is +/-112Volts. Amplifier is also made out of discrete components, Class TD topology.
Yamaha claims:
Power supply has superior load characteristics, minimal noise, higher efficiency and stability.
Power supply holds the voltage and current levels to zero while the decice turns on and off, this reduces noise.
Other advantages include superb sability against power fluctuations.
Yet, so simple smps circuit? Well, it failed and there has been many YST-sw800's having the same issues, main amplifier failed and psu failed.
What I did notice, is that there was many bad solder joints especially on amplifier pcb. One driver (Q1A) transistor was shorted but output fets and other semiconductors were all ok. I changed the driver transistor and fixed all bad solder joints. When I turned the amp on, that very same driver transistor immediately got very hot. Bias current was out of spec, so it had to be failed bias trimmer which I soon changed. Problem solved, amplifier is now very stable and is running cool.
So the original cause of this failure was possibly some bad solderings and/or bad bias trimmer. This caused the amplifier run out of control and, possibly burn the SMPS.
SMPS main transistors (2SC4140) were shorted. Both drivers were shorted and two resistors was burned. I changed these and it fixed the smps. I used BUX98AQ as a replacement type, these are more robust than original 2SC4140's.
So, there is no short circuit or over load protection for SMPS?! Did they choose the cheapest way to make SMPS? 😀
Schematic: