This Sony 5.1 receiver came with lots of signs of humidity inside, rusting in bottom frame, components legs, etc. I thought that this could be the fault's root, so before testing anything else I cleaned it thoroughly and even replaced some wire bridges that were too rusty and about to break.
Reassemble and again "PROTECT". No luck with just cleaning. Nothing short in final transistors. No DC in any output (before Relays). PROTECTOR line reads 1V instead of expected 3.3V according to schematics. So (once again this week) it seems a bad protection detection. I observe that D733 (Rohm 1SS133 diode) is not behaving like its brothers D734 and D731, it has 0,8V on Anode, while others have 0V. Strange. I decide to disconnect it and, voila, no protection anymore. The diode is not short and it passes normal diode test. Weird but not so much: this happens. I replace it with a scrapped one and it seems fixed.
Not so fast. SP.OFF shows on display, regardless if headphones jack is inserted or not.
This means that it won't activate speakers relays. To my relief, at least it sounds through headphones.
My first FAIL: During initial cleaning, I forgot to connect ground wire from Headphone Board. This is required for CPU to detect that headphone jack is empty. After fixing this, SP.OFF persists!!
My second FAIL: the cause is that I was operating it wihout the back metal panel. It doesn't seem essential, but it is: it's a ground bridge between parts of the circuit. After screwing it, headphone detection works fine and speaker relays are activated normally.
We tend to reassemble just the minimum needed to test the equipment under repair. Only when the repair is confirmed we proceed with full reassembly. But this can lead to unexpected failures due to unexpected ground paths or other dependencies between boards.
Reassemble and again "PROTECT". No luck with just cleaning. Nothing short in final transistors. No DC in any output (before Relays). PROTECTOR line reads 1V instead of expected 3.3V according to schematics. So (once again this week) it seems a bad protection detection. I observe that D733 (Rohm 1SS133 diode) is not behaving like its brothers D734 and D731, it has 0,8V on Anode, while others have 0V. Strange. I decide to disconnect it and, voila, no protection anymore. The diode is not short and it passes normal diode test. Weird but not so much: this happens. I replace it with a scrapped one and it seems fixed.
Not so fast. SP.OFF shows on display, regardless if headphones jack is inserted or not.
This means that it won't activate speakers relays. To my relief, at least it sounds through headphones.My first FAIL: During initial cleaning, I forgot to connect ground wire from Headphone Board. This is required for CPU to detect that headphone jack is empty. After fixing this, SP.OFF persists!!
My second FAIL: the cause is that I was operating it wihout the back metal panel. It doesn't seem essential, but it is: it's a ground bridge between parts of the circuit. After screwing it, headphone detection works fine and speaker relays are activated normally.
We tend to reassemble just the minimum needed to test the equipment under repair. Only when the repair is confirmed we proceed with full reassembly. But this can lead to unexpected failures due to unexpected ground paths or other dependencies between boards.