The Yamaha RX-A740 was my first real electrical challenge that wasn’t already published online. It took the longest to fix, and I ended up buying 2 additional Yamaha receivers for parts. Purchase price was $30. Technically: +$10 +$20 for the additional receivers but more on that later…
This receiver was hit and miss – it could work for an unknown amount of time or not work at all. To diagnose it, I made sure it was off and drained of power, reconnect power cable, hold Tone Control and Info, then press power/main zone. This displayed “PS3 PRT:131 L”, then turned off. After consulting the service manual (very easy to find).
- PS3 = The last shutdown was caused by the 5.5+ rail.
- PRT = It doesn’t realy explain what PRT means, possibly Protection Relay…<something>?
- 131 L = The value recorded was lower than an exceptable value, so the unit was shutdown to protect against power issues. “Cause: DC output of the power amplifier is abnormal.”
Sometimes I’d get 5-10 minutes before it died. I ran heat test to see if it made it worse. It, infact did not, and when I heated the main power PCB it would last longer. Now it would last hours. I left a probe on the 5.5v rail and normally it would be 3.4-4v, and you could see the voltage increase when heating the board. This threw me for the longest time as I thought it was just a poor/dry solder joint, so I went over the entire board more than once.
At this point I stupidly bought 2 additional receivers to pull parts from. The electronics between the boards were too different to be of any use for the long term. I did how ever get the PSU from one of them to power this receiver, and it was working OK for testing.
I went back and tested every capacitor on the PSU PCB and the second last one was the root cause, I don’t have the exact capacitor number from the service manual, but I do remember it was common and one of the smaller through hole caps. I’ll update this if I have to open it up again.
From memory the capacitor was on the bottom left of this image.
Cheers