Hi,
"127 L" means the supply voltage is too low.
If you measured it using a Digital multimeter, then you should know that on a fluctuating DC voltage, most of the DMM's measure the peak voltage.
Measure it with an oscilloscope to see if there is significant ripple voltage on it. Some ripple is normal, as it is an unregulted supply voltage.
You may visually check that the electrolytic caps that are filtering the +5.6V on the OPERATION (3) PCB are not bulged. C3714, C3715, C3718. Even it they are not bulged, they may have lost significant capacitance.
If the...

Anyone got the firmware for yamaha rx-v571? Not available on the internet.
I've got a defective RX-V571. Could be due to a failed update, but the only function I can invoke is "USB UPDATE".
Normal power on, Diagnostic mode or Protection Cancel mode don't work.​

Hi, my name is Istvan. I repair AV receivers as hobby.