This unit, s/n 420432, was purchased to help me figure out what is wrong with another unit I own. This unit was untouched prior to me having it, however, it tripped the protection circuit at low levels, less than a couple of watts. After replacing one xistor and a cap in the protection circuit it now trips at higher levels but well short of maximum. However, on sine waves from a sig gen it can attain full+ output. Any thoughts on why it trips on musical low notes but not sine waves across the audio range. I have noticed that the pre-amp section audibly resonates at around 1kHz. In an earlier post it was suggested that pf caps across the outputs (B-C) might alleviate this problem. It did not. Also the r chan runs considerable hotter than the left even though the offset and bias settings are very close to each other and the spec. Thanks for all the help in the past and for what may be offered now.