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Old 1st February 2012, 05:36 PM   #1
Kei is offline Kei  Wales
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Default White noise problem

I turned on my L-220 today and got white noise out the right channel (regardless of volume and input, the volume knob does not affect the loudness of the noise) Had the same noise once yesterday, but put it down to the connecting cable A8 between the amp and tone control having a fractured wire. (initially i thought it was the diodes as they were all i had changed between switch ons) I chopped the damaged section of the cable out and re-soldered the connections and it worked fine. This cable is part of the negative feedback. (can just see the split in the white wire next to the red)
Click the image to open in full size.

I changed the rectifying diodes yesterday and it was working fine all through the evening. These were the original diodes and these are the replacements.

The only other parts to have changed are the amp input caps, the caps on the function switch and a pair on the tone control. (Most of these were electrolytic and are now polypropylene film. C118a/b, C301a/b, C302a/b, C207a/b on the schematic below) These were done first thing yesterday and have been fine for the last few switch ons.

This schematic covers most of the amp. (there are some small differences)

Last edited by Kei; 1st February 2012 at 05:54 PM.
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Old 1st February 2012, 06:43 PM   #2
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None of the inputs, or even backfeeding into a tape output should make noise with the volume down. Make sure the board with the volume control has a good ground path. Try shorting the amp inputs from the volume control to ground. It looks like the amp has muting FETs at the input of both power amps. You might make sure there isn't anything strange on the control line to those FETs. Nothing in the power amp circuits should be able to affect both channels. (do verify it really is both channels - stereo/mono switch to stereo, speakers not accidentally wired between left and right instead of to ground etc) Problems like that could not come from the rectifiers. If the problem started after changing something, look at that area most closely first. if pressure was applied to the circuit board, make sure the volume control didn't get cracked carbons on the ground side (same effect as no ground). You might want to mention if inputs work normally in addition to the noise, and how loud the noise is. Is it more of a hiss like the sound of a phono section, FM radio noise or a crackle? I wouldn't expect much noise from an open cap across the +24V supply Zener, but I'd probably still check that (feeds amp input stages).
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Old 1st February 2012, 07:23 PM   #3
Kei is offline Kei  Wales
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Just got back from diagnosing the problem. Turned out to be the amp input cap C118. The floating arrangement for the replacement film caps had put sufficient strain on the joint to damage the track. Tapping the cap caused the channel to crackle and the noise to come and go. I've taken the film caps back out as i couldn't hear a difference with them in. I've repaired the track and put the electrolytic caps back in place.
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Old 1st February 2012, 07:24 PM   #4
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Also check that those caps are the right size if you removed the originals. Those values are very very large for polypropylene film. You might have nF where you need uF, 1000 times too small. Usually when electrolytics aren't quite good enough at high frequencies other smaller caps are ADDED in parallel. Sometimes people like to increase the size of some electrolytic coupling/feedback-leg caps to get better 20 Hz squarewave response (less bass loss and phase shift). That helps things like the snare drum sound tighter.
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Old 1st February 2012, 07:35 PM   #5
Kei is offline Kei  Wales
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They were 3.3uF film caps, which meant that they were very big. This is as was before i started work...
Click the image to open in full size.

and this was with all the changes.
Click the image to open in full size.
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