Question Half channels from Anthem MRX 700 receiver at low volume. How to troubleshoot/fix?

[kryonas]

Hi everyone, thanks for reading. I have an anthem MRX 700 receiver which I have used for years. Recently, I noticed the volume for my front right, center, back left, and surround left channels were at a much lower volume (not distorted or entirely silent just ~18db lower than the rest). I'm trying to troubleshoot this and see if it is fixable myself since I dont have the opportunity to replace it or send it in for official service right now. I do have some experience in electrical engineering so hopefully that will be possible. I don't have a service manual unfortunately(let me know if you do please!). I've done the basic troubleshooting (swapping cables/speakers/outputs) and have narrowed down the issue to the receiver itself. The channel outputs look clean at the connection. I took off the outer case and can see some corrosion around the hdmi board but nothing else. I did notice that the channels in question are physically in a line next to each other at least for part of the amp board near the transistors as well as on the preamp board and bus. That can be seen here:

Any idea what would be some good steps to potentially troubleshoot or fix this? Thank you again!

 

[kryonas]

Some new info: voltage output for each channel with no audio is 0.029V. when doing noise output for level calibration FR goes up to 0.7V SR goes up to 0.15V BSR goes up to 0.15V as well, on the low volume speakers voltage increases to 0.039 on each when noise is output to their channels. connection from the analog board to the amp shows 0.024V on the pre signal for the channel with noise 0v otherwise, same with the connection from analog board to pre-amp.

 

[lumpylipton]

Hi, my MRX700 had the exact same symptoms. I sent it to Anthem for repair and they replaced the pre-amp board. So while that doesn't tell you how to fix it yourself, it narrows it down a bit to the preamp board. The repair was $40 parts, $160 labour Canadian dollars from Anthem. Too bad you can't just get the part yourself, it would be a cheap repair! Good luck.
Warren