Audio Systems: Harman Kardon hk570i receiver weak left channel, volt ohm meter, power transistors

Question
I have a Harman Kardon hk570i receiver that is having a problem with the left channel. The left channel has weak volume when compared to the right channel. I use the balance control to compare both channels at a given volume and the left is definitely weaker. I am waiting on delivery of a service manual I purchased on ebay and I did some preliminary research and there seems to be a history of this problem. I have read about poor solder connections and bad power transistors. I was wondering if you could provide some suggestions for me (when I open it up) to check. My back ground is in motor control repair but I am not too worried about opening up the receiver despite a lack of audio experience. Thanks in advance! Karl  

Answer
Is it weaker if you use headphones?

The Physics of electronics is a bit different then in motor control. The servo part has similarities but still doesn't have the same states as audio.

I assume that the manual you are getting is the service manual and will show signal flow as well as B+ power to all stages of amplification. Ideally an oscillicope and generator is the best way to find the stage of amplification where the signal is weaker on the left channel then the right. Then using a VOM (volt ohm meter) you want to check the de-emphasis caps, in that stage, that block DC from the signal flow. You could have a bad cap or a poor connection in that stage. Usually the former because poor connections generally produce intermittent signal or no signal conditions.

Note: This is a tip but it's not in any way meant as a sure fire way to find the problem.