Originally Posted by CT_Wiebe M Code
-- That's interesting, and very unusual. That means that the transformer can quit, without actually failing (open circuit breaker, only). Since excessively high temperature is the primary cause of electronics failure, they may chose to do that to protect the rest of the electronics in the units. Using a non-resettable thermal breaker is unusual for Yamaha equipment (which I have always associated with quality designs).
Setting the temperature limit to 75 degrees C is very low, for a transformer. Commercial measurement equipment, for example, have operating temperature ratings on the order of 80 to 90 degrees C (IIRC).
A temperature of 75 degrees C is 167 degrees F, which is very hot, though, for home electronics. But, as you said, it does point out the need for good ventilation around AVRs (and I've seen pictures of some installations that make me shudder -- 2" or less over the top of an AVR). If the power transformer is running near 75 degrees C, the power amps (transistors or FETs) have to be running over 100 degrees C, which is way too hot for reliable operation.
My Pioneer AVRs have always stated that they need at least 12" to 18" of clearance over the top of their units (they don't use cooling fans). With adequate clearance, they run very cool (relatively).
EZ to understand..
The power transformer is the single most expensive
component within the AVR, and Yamaha (and other brands) have made the decision to slim down the transformer to save $...
Note that the AVR mite still put out the rated power as the brand will simply add more windings but of a thinner
End-result is that the transformer will saturate sooner when pushed hard and then its output will sag..
Yamaha in its lower priced AVRs (SRP <$999) are not considered a high-current design, as for the last 3 generations they have been purposely been limiting
max available power supply voltage. Additionally things like output device specs have been decreasing
as well as heat sink area.. Therefore as the power amplifier circuit is pushed hard driving lower impedance speakers, all of the related components/circuits start to heat up
and it then the temperature increases very rapidly. Also as previously mentioned
, the warnings about air circulation are usually ignored
Regarding the internal thermal fuse within the power transformer this is a mandatory
UL/CSA requirement. However since power transformer has been sized down it is more likely for the thermal fuse to become open...
Keep in mind..
Certain materials such as copper (transformer), steel (transformer, chassis), aluminum (heat sink) have tripled in price
over the last 2 years..
Yet some consumers still expect
the price of AVRs to decrease like PCs so the brands try to slim down on internal components cost..
Just my $0.01..