Harman Kardon 330B speaker output issues

[jacksa2]

Hello Everyone, tons of good info on this forum so I figured one of you audio wizards could help me out.

I am having an issue with my HK 330B and love the sound so I want to resuscitate it if at all possible. Recently the left channel speaker became very scratchy and popping. This can be heard on all inputs but not through headphones and It does not matter what speaker is hooked up.
I bought some deoxit and cleaned the connections and pots thoroughly, the receiver has been in a cabinet for the last 30 years unused so it was near spotless already. Unfortunately this did not help.
The tricky thing is sometimes it will sound great without scrathy noise for the first 5-20 minutes, but all of a sudden an audible pop happens and goes back to scratchy sound.

What could the issue be and how could I fix it?
I have searched extensively in the forums and saw many recommendations for upgrading some components so if i can kick the receiver up a notch even better! Is there a quick list of what would have the most bang for the buck?

Here are the schematics for the 330B
http://manuals.harman.com/hk/Service Manual/hk330B sm.pdf

Really appreciate any help!

 

[stillspin'n]

Since this happens a few minutes into the time the unit is turned on, sounds like something it warming up and failing. The first thing I would look for is cold solder joints, then if no luck maybe try some component cooler to see if you find the problem.

 

[jacksa2]

Thanks for the reply, is there a specific area I should be looking for cold solder joints or just try to cover everything?

 

[stillspin'n]

If it has problems on all functions, then probably preamp/ poweramp section, also did you clean the speakers switches as you said the headphones work but could be that they are taking less current and not warming the part that is failing up as quickly. Also check the pre/main jacks, maybe try some RCA cables there and see if that helps.