I teach Electronics at a rural technical college in Tennessee, and we take outside customers broken electronics to use as student troubleshooting and repair training. So I had a student assigned to repair a sony receiver that is flashing "protect" on the front panel after about 5 seconds every time you start it up. We have the schematics and he went over it with a fine-toothed comb and cannot find any clear faults. None of the speaker terminals are shorted, all of the BJTs and amplifier circuits are testing fine, none of the emitter resistors are shorted or opened, no apparent problems. There was a possible short that turned out to be normal operation when we removed the BJT in question from the circuit and tested it, and compared it to the schematics. After several people doing visual inspections, no one has spotted a possible fault. I have stepped in to help on this one a bit because the students were hitting a wall, and I have to say I couldn't find a fault either with the time that I spent looking for it. Is there anyone who knows anything about the circuit protection circuit on these older sony receivers? Or could point me to a resource about them? It is my understanding that it is usually a speaker short that causes it to trip, and I have tried various other techniques to no avail. The owner did say they heard an audible pop when it went into protect mode, but there are no blown fuses, no burn marks or bad components... I figured the speakers had shorted and caused the problem, and it would reset after clearing the terminals and leaving it unplugged for 30 mins... but no dice. Any suggestions would be grately appreciated.

 

This may help?
http://www.justanswer.com/electronics/4bin4-str-d1011s-receiver-when-turn-message.html

 

yes this was helpful- It is pretty much confirming that it has to be an output transistor that is bad- although we can't seem to find one on the board which is bad! I suppose there could be a fault on one that is not an obvious short or open... We could just replace all of them- but there are 11 thermally connected to the main heat sink! I will probably have the student triple check the transistors again, and if they still can't find a bad one, just tell the owner that it is probably time to buy a new one!

 

yes this was helpful- It is pretty much confirming that it has to be an output transistor that is bad- although we can't seem to find one on the board which is bad! I suppose there could be a fault on one that is not an obvious short or open... We could just replace all of them- but there are 11 thermally connected to the main heat sink! I will probably have the student triple check the transistors again, and if they still can't find a bad one, just tell the owner that it is probably time to buy a new one!

If the transistors are paralleled up which it sounds like they are, detecting a short should be doable, but detecting an open circuit mat be very difficult in circuit.