Thanks for the quick responses! My lack of experience with SS amps made me doubt myself. I don't have spare ICs but I replaced A101 and A103 from position and the fault is nog on channel II. So the suspect component is A101, just as @g1 pointed out. Thanks for the tip!

I didn't know that ICs can produce such a fault. I won't forget about it soon... :cheesy:

Hi,

This is literally my first post on this forum. I have a Lab series amplifier on my bench. I've been troubleshooting the amp all day long and while the internet for solutions, I discovered this website. I hope I can contribute in the future, but for now I would appreciate the help.

the problem occurs on the left channel only. I cleaned all pots (pots checked ok) and replaced a faulty coupling capacitor. However I still hear a fuzzy background noise when the notes fade away. The treble pot seems to have an effect on the noise and when I plug the guitar in the power amp input, the background noise disappears. I double checked the whole preamp board, but couldn't find the problem. So it isn't coming from the input stage...

Then I thought this could be some sort of bias issue. But I don't have much experience with solid state amplifiers. I measured the power transistors for DC voltage. (I just placed the + probe against the transistor with the - touching ground) and the first 5 transistors measure around 60v. The 5 most to the right (Q315, Q316, Q317, Q318) read almost 0. The associated emitter resistors (R345, R346, R347, R348) measure -60v. But again... I'm not used to work on SS amplifiers so this can be a stupid comment... Is this how push-pull works on SS?

I also checked all the filter caps and they seems to measure within specs and have low ESR. I didn't upload the schematics because I noticed someone else already uploaded them. Anyhow... I'm interested hearing other people's thoughts on the matter.