I have a Kenwood KA-6000 on my bench that outputs a clean 1kHz sine wave, but when I go to a 400Hz square wave, there a problem in both channels.
This looks similar to what I expect to see with full bass attenuation and max treble. And in fact, listening to music through the Kenwood, it sounds exactly like that--no bass and tipped up treble.
I separated the pre and amp sections, and the issue definitely begins with the driver board. (The bias and DC center voltage were checked and set.) Scoping the board, I found that I have a good square wave up until R927, and then right after that resistor, the pattern changes to what you see above. The resistor itself measures fine in both channels, though it's 4.7k here and 2.2k in other versions of the amp.
I thought perhaps bad coupling caps at the output, but I swapped in a new 3300uF 50V cap in one channel, and no change.
This looks similar to what I expect to see with full bass attenuation and max treble. And in fact, listening to music through the Kenwood, it sounds exactly like that--no bass and tipped up treble.
I separated the pre and amp sections, and the issue definitely begins with the driver board. (The bias and DC center voltage were checked and set.) Scoping the board, I found that I have a good square wave up until R927, and then right after that resistor, the pattern changes to what you see above. The resistor itself measures fine in both channels, though it's 4.7k here and 2.2k in other versions of the amp.
I thought perhaps bad coupling caps at the output, but I swapped in a new 3300uF 50V cap in one channel, and no change.
Turned out that C919 and C920 were both bad. Replaced and the bass and gain are back. Thanks Craig.