Apologies in advance for the wall of text!
Let me start by just saying I've looked over many similar threads here and it seems that most of them just die out before a resolution is found, so in spite of the number of similar threads, none of them I was able to find actually resulted in a resolution.
I promise to anyone that decides to help, if I decide I've had enough and give up on the project, I'll say so in the thread rather than just disappearing!
I have a Yamaha RX-A1000 receiver that I've owned since new. It worked flawlessly for ~10 years before it was put in storage for two years. When it was pulled out of storage, it still worked mostly ok, but one of the HDMI outputs was putting out garbage. All of the others worked fine. After using it for another 18 months or so, it suddenly stopped working. I came home to find it powered off, and attempting to turn it on results in it going into the power protection mode almost instantly. This happens regardless of what connections are made on the rear panel, including with no connections at all.
I was able to find the service manual online and boot into the diagnostic mode, which indicated a "DC_PRT 0L" message, which the manual describes as
"Power amplifier DC (DC voltage) output is detected. The voltage at pin 1 (DC_PRT) of IC902 is displayed. Normal value: 35-68 (Reference voltage 3.3v = 255)"Several forum posts I saw recommended testing the output transistors. I assumed this meant the massive pairs of darlington transistors for each output channel (Sanken STD01N/STD01P), which are a real pain in the neck to get to with this unit due to their position combined with the board to board wire routing, but I gave it a shot anyway. I took apart the the receiver carefully, board by board, to get to those transistors. Unfortunately due to their location it's essentially impossible to get to them with the receiver assembled and all the boards in place.
Once all the boards were out of the way, I tested the transistors as best I could determine how in circuit with my multimeter. I checked for:
- Dead shorts from the gate pin to any other pin
- Diode forward drop of the N channel pair between pins 1 and 2.
- Forward drop of the diodes on the P channel pair between pins 4 and 5.
- Forward drop of the P channel pair between pins 1 and 3.
In retrospect I should have probably checked more, but everything here checked out, and I put it all back together. Honestly, I don't know if these are even the transistors I was supposed to be checking, but it seemed like the right place to start -- because it was the most difficult, probably.
Since reassembling it, it still shuts off with a DC protection error, but now it shows a high value rather than 0. On the display for P1-1 it shows "DC: 84" at present, but as I watch it, it jumps around between about 79 to 85.
I took pictures of every board front and back with and without my phone camera flash enabled as I was putting them back in after looking them over for obvious damage, without finding any.
This imgur album has all the photos. The ones taken of the individual boards were just before reassembly, the ones inside the receiver were taken as I took it apart, so I knew which cables went where.
https://imgur.com/a/KnwSsHeI do basic circuit design and microcontroller stuff, but it's just a hobby, I'm a developer by trade. I'm at my wits end trying to figure out what went wrong here, which is a shame, because as old as it is this receiver has been great since the day I bought it, and it seems like this should be an "easy" fix -- I just don't know where to go from here. Please try a dash of ELI5 if you can, clearly when some post says "check the transistors" I may not know which ones are meant or how I should test them. Mostly I just build things and when I let the smoke out, I throw it away and build again. I don't usually have to troubleshoot in this way.
So thanks in advance for any help in figuring this out.
EDIT:
Oh, there are a few closeups of discolored areas of two of the boards. Just looked like normal heat discoloration to me, but other opinions are obviously welcome.
EDIT2:
Results of onboard diagnostic pages, normal values from the service manual.
P1-1 - DC: 100 (normal 35-68)
P1-2 - 130/120 (normal 84-141 / 81-153)
P1-3 - TM: 120/122 (normal 116-255 for both)
P1-4 - OUTLVL: 255 (normal
. 255 = 3.3v. Pg. 62)
P1-5 - LMTCNT: 140 (normal
. 255 = 3.3v. Pg. 63)