Any thoughts on what is causing this blip on the waveform?

My dad asked me to take a look at a little Rotel receiver that he has. It is a RX-1052. I have the service manual for it and was able to use it to replace a surface mount resistor that had torched. There was also an electrolytic in the same circuit that needed replaced. I’ve gone through in spot, checked several of the capacitors, and some of them have somewhat high ESR and some of them are OK. I have not checked any of the capacitors on the amplifier board as it’s mounted to the heat sink and I wanna wait until I pull it out. I am able to get output from the amp now, which I wasn’t able to before, but on the bottom of the waveform on both channels, there is this small clip. It doesn’t happen every time on the bottom. Seems like maybe every other time the wave goes down it happens.

IMG_7477.jpeg


Apologies for the blurry pic. I’m going to pull the amplifier board and heat sink and then go through and check the capacitors on the board, especially the ones close to hot resistors. I am also going to go through and check some of the transistors like the drivers and just make sure they are OK. Since I am seeing output, I am nearly certain they are just fine, but with the anomaly in both channels, I’m guessing it might be power supply related??? As I turn the volume up the clip gets bigger.

Thought I would ask here to see if the collective great minds might be able to give me some suggestions on what to look at.

Thank you,
Dan
 
Moderator
Joined 2011
Maybe current limiting. See the part of the protection circuit with Q702, 703.
Suppose you could try first replacing the filter C624 in case it's bad.
Maybe there's a great mind that can decipher all this.
 

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Several steps are required to help narrow down the problem.
1. Remove LOAD, if the notch is gone then the CURRENT LIMITER is working.
2. Short / Jumper C624. this disables the CURRENT LIMITER. is the notch still active?
To find the real component failure if the CURRENT LIMITER is active can be caused by several things like open, higher values or burned resistors.

I can't go through all of the various failures modes with limited tests.
Duke
 
Just to update everybody on this, since I was offered so much help, which I really appreciate. The main filter caps measure perfectly fine, but there were some other caps in the power supply that did not. Some of the capacitors that I measured in circuit and measured OK were actually not good after I pulled them from the circuit nearly every single capacitor measured with an ESR of 20 ohms or higher. Other We’re actually not good after I pulled them from the circuit nearly every single capacitor measured with an ESR of 20 ohms or higher. Other than the main filter caps they were probably only 10 capacitors that I would have been, that’s okay to stay. I replaced those regardless. After replacing all of the capacitors, those blips are gone. I’m surprised I was getting much of anything, there were at least two dozen that were completely open.

Dan