Problem with 2120 Right Channel

[bmfl]

Hi

I'm having an issue with my right channel in my 2120. I probed around with my scope and am not sure what to make of some of the traces and would appreciate any comments. Below are the speaker outputs for the left channel and right channels for a 1 kHz square wave.

Left Channel

Right channel


I have the Power Guard switched to off/out at the back of the unit. I'm not sure if the issue is somewhere with the LDR or the Power Guard opamp. If I compare the scope trace between pin 7 (Power Guard Input) on the molex connector between the left and right channels, I see the following:

Left channel


Right channel


Also different is the output of the power guard opamp (pin 6). Below are the left and right channels of pin 6 of the opamp. Input Pins 2 and 3 of the PG opamp look like normal square waves for both left and right channels.

Left channel


Right channel


Below is the schematic of the right channel power output board

 

[Bob the Tech]

If I'm following you right it looks to me like your R channel is clipping and oscillating. The only thing that the PG in/out switch does is provide a path to ground for the input signal if the LDR is activated by PG. With it set to "out" your PG circuit is still activating, it's just not able to mute the amp inputs like it's designed to do. I didn't look at all your traces closely, but it seems they might just be what they normally look like when the unit wants to go into PG. I think you should shift your attention to the output stages and find out what's wrong and causing the clipping and oscillation.

Is the PG lamp lighting?

If you do the same test and leave the PG enabled, does the PG lamp light and the channel go quiet?

Bob the Tech.

 

[bmfl]

Problem may have been the molex connectors and broken solder joints somewhere. I soldered all the pins again on the PC boards and it is working for now. Thanks for your help. Below is both outputs at 20 kHz and no issues.