Yamaha CR-400 Power amp noise circuit debug help needed
- Thread starter rocknroller
- Start date
rocknroller
Active Member
Certainly a possibility, though I should clarify this problem was occurring before I ever cracked the case open and it's had no prior service. The likelihood of a cap being in backwards from the factory is far more remote then me putting it in backwards (and I've been know to do some stupid things before!) Still, I double checked all the caps on the power supply/amp board just now - all in correctly as noted and while I didn't voltage measure EVERY cap there, just the bigger ones, they are are oriented properly.
Since there doesn't seem to be any obvious source of this rouge voltage (as i'll call it) that anyone has identified yet, perhaps there are some parts that can be removed to see if one makes it go away? I'm reaching for anything that will help narrow down the cause obviously.
Pretty good mystery you've got here. Just to be clear, is the noise different on both channels, IOW is the noise in stereo or mono. If mono than it's something common to both channels. If stereo, it's something bad on both, but not correlated between the two channels. If you AC couple the scope and look at the supply lines, can you see it there? If you look at the speaker return ground, can you see it there? Can you upload an audio file of it? That might tell people in an instant what the cause is, if not where.
rocknroller
Active Member
I uploaded a video with the noise here
It's too difficult for me to tell if the noise is identical between channels because of the volume differences. Today noise is heavier on left channel vs right but in past it has been opposite. It is certainly the same TYPE of noise on both chanels for sure though. When you say AC scope the power supply I need to understand that better. I assume that means switch my scope to the AC setting in the lower left (see pic) but anything else I need to know other than dialing it in? Also clarification about the speaker return ground. Are you saying probe the actual negative terminal of the speaker jack vs chassis ground? Looking for ac or dc there? (had already confirmed DC voltage between positive/neg terminal with multimeter)
Attachments
![IMG_20170608_155910[1].jpg](/forums/data/attachments/820/820380-ed22b7bc6ee2ae32b54cc6d44cd8b0e0.jpg)
Dang, that's not a noise I've heard before and I thought I'd heard 'em all. It's not the typical sound of a bad transistor. It's more like a bad ground or cracked trace or something. Let's see if anybody else has a clue.
When you switch your scope to AC you can look at small amounts of AC riding on large DC voltages. No real trick, just switch to AC and turn up the sensitivity until you see something. Expect it to take several seconds for the trace to drift back on the screen when you do this. For the speaker ground, I'm assuming chassis ground is "good" and if you probe the speaker ground you might see something, though it can work both ways.
When you switch your scope to AC you can look at small amounts of AC riding on large DC voltages. No real trick, just switch to AC and turn up the sensitivity until you see something. Expect it to take several seconds for the trace to drift back on the screen when you do this. For the speaker ground, I'm assuming chassis ground is "good" and if you probe the speaker ground you might see something, though it can work both ways.
rocknroller
Active Member
Sorry for the delay in response - I haven't made much progress (any). Mostly due to my ignorance of where I should be looking, what I'm looking for, and how to use the scope properly. I did confirm there is nothing found between Chassis ground and speaker ground and the AC setting. Did confirm the AC setting on the scope is working, checking it at the fuses (only could see part since I didn't have a 10x probe) and checking at a couple spots on the board edge where they where labeled for 20VAC, 28VAC (seeing about 31.6 on a multimeter there), and 11VAC (11.9 there) and I see sine waves, though not perfect. No indication of any odd movement on any of those when noise occurs. Since the power supply and amp are on the same board, I wasn't really sure where else to probe down and listen. I saw something strange on R15/R16, a triangle wave on one side, audio type wave on the other and when the popping occurred there where momentary full signal spikes on that side. I also thought I saw signals there sort "invert" for a brief fraction of a second when the popping occurred. But this made just be whatever random setting I had on the scope as I was playing around a lot with the range and time to try to see something happen and in detail. Of course, this was one of the days when the noise/popping not as bad/loud or consistent, so I couldn't really get handle on a pattern. I also uploaded a fuller view of the schematic with the power supply section shown.
Attachments
rocknroller
Active Member
That's R615/617 I meant. This is the same place I hear noise easily with audio tester.
Scott Bierly
New Member
Hello out there...I have the same model and seemingly the same symptoms (haven't cracked the case yet). Eagerly read through this only to find the thread ended 2 years ago without comment...how did this turn out? Did I end up buying your broken unit?! 
LOL....Hello out there...I have the same model and seemingly the same symptoms (haven't cracked the case yet). Eagerly read through this only to find the thread ended 2 years ago without comment...how did this turn out? Did I end up buying your broken unit?!
Welcome to AK..
