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Author Topic: Lab Series L5 distorted background notes  (Read 353 times)

tiziano

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Lab Series L5 distorted background notes
« on: December 27, 2019, 04:51:47 PM »
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... :o

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.

Enzo

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Re: Lab Series L5 distorted background notes
« Reply #1 on: December 27, 2019, 09:13:43 PM »
OK, you state the problem only occurs on one channel.  The power amp and filter caps have no idea which channel you are in nor any way to only affect one.

If you apply a sine wave can you hear this?  That would be easier to scope through the channel.

g1

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Re: Lab Series L5 distorted background notes
« Reply #2 on: December 28, 2019, 01:32:22 PM »
  So it isn't coming from the input stage... :o
Not sure why you are saying this?  The problem is only with Ch.1, so that eliminates everything else.  You verified this by plugging into the pwr. amp in and finding no problem.  So power amp, bias, etc. are ok.
You have narrowed down the problem to ch.1 and the most likely suspects would be A101 or A102 IC's.  You said the treble has an effect on the noise so that makes A101 the most likely.
That is the input stage.

tiziano

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Re: Lab Series L5 distorted background notes
« Reply #3 on: December 29, 2019, 03:58:42 AM »
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: