Dear members, the application of microcontrollers in stereo amplifiers has introduced a lot of problems. The microcontrollers sense and register all kind of paramters such as DC-offset at the speaker output, output current, heatsink temperature ultra high frequency input/output etc.
This technique was first introduced in surround sound receivers. Some even have several types of protection mode. Depending on the maximum current drawn from the power transistors, the period and slew rate of the maximum current decides whether the consumer can return to the normal mode by recycling power or that the unit shuts down forever. That is, until a service technician bypasses this mode by means of a secret (published in the service manual though) key-combination while powering on the unit.
The point is that most of the error-messages are of no use to the consumer because most of the times it is a fault caused by the amplifier itself! You would have to drive your speakers with lots of power for a long time until the protection mode kicks in. If it kicks in at moderate levels, your amplifier is at fault!
Often the DC-offset is exceeding a certain threshold and this is often due to the application of el cheapo Chinese made transistors at the input stage of each channel's power amp. A consumer can not solve this problem!
A-S3000 was delivered to my workshop today with the amp going into protection mode regularly. I already checked the service manual and the Japanese are crazy. They have more than 10 morse code like sequences of the flashing power LED in protection mode.
To run the diagnostic mode and to browse through the items one has to set rotary switches in certain positions....
One important thing to mention: In the service mode one can override the protection mode. This can cause additional damage, especially if the trouble cause is a high current running through the power amps. If this high current runs for a long period (it even runs when no speakers are connected) the power transistors wil be destroyed!!
Since this amp also seems to have bias-issues (heatsinks get too hot to touch), it may very well be that the bias is set so high that the amp will shut down immediately after it is switched on!!!
Bias circuits should be designed to be fail-safe. That means that if the bias trimmer's contacts get poor the bias should go to zero. Unfortunately some designers do not follow this scheme thus causing the bias to go to maximum current. This may destroy the power transistors.
The Japanese have the bad habbit to use bias trimmers that adjust very coarsly!! One just have to move the scredriver a little bit and the current goes through the roof!!
However, I do not believe that the adjustment is changed by vibrations during transport. I think it is just a matter of quality control. Also, bias adjustment takes time. The heatsinks have to warm up until one can set the right current and this may have to be repeated because if the heatsink's temperature drops, the bias current drops as well when we are talking about bipolar power transistors (it is different for power MOSFETs).
I will keep you informed about my reapir experience with the A-S3000 but I am aware the Yamaha uses plenty of SMD-componets. SMD-components, especially the complicated chips increase the failure rate of electronic equipment!
