Studer A807 distortion problem
Something screwy happened to my A807 over the course of doing some upgrades the audio boards, but I'm having trouble figuring out what it is. Here's some background. I upgraded op amps and caps on one board, tested it and it all worked brilliantly with no problems. I upgraded the second board, installed it, and then experienced some level and distortion problems (like the sound of op amp clipping) on both boards. So I tried cleaning contacts and re-seating the boards. At first I got strange pops, level jumps, fuzzy sounds, and then it all went away. Problem fixed I thought.
Fast forward to today. I went to use the deck and noticed the same buzzy clipping distortion on both channels. I again tried re-seating the boards. No luck. Here's what's happening.
1. Machine is in input mode and I'm feeding it a mix from the DAW. I'm monitoring the output of the Studer in the DAW. Both channels are set to cal mode.
2. The monitor level (on the deck) is turned all the way down but there is broken distorted sound coming out of the monitor speaker on the tape deck.
3. I switch off the deck and switch it back on. Initially the sound is clear and distortion free. There is no "broken" sound leaking out of the small monitor speaker.
4. After about 6-8 seconds the sound becomes distorted again and all of a sudden I have distorted sound coming out of the monitor speaker despite the fact that the volume is turned all the way down.
I realize it could be any number of problems, but I'm hoping to eliminate a few variables. Because I'm monitoring the input I know the source of the problem is either in the line input preamp stage, or the output stage. The delayed onset of distortion indicates to me that some capacitor is charging up...or discharging. In the attached schematic of the output circuit, you can see C120 in the upper right, highlighted. I changed that from 470uF to 1000uF. By doing that did I screw up the transistor voltages? I had the board working fine initially with the 1000uF cap in there, but I just need a sanity check.
What else might cause the sound to go from clean and clear to distorted and buzzy after a finite amount of time when the machine is powered up? And what's causing the sound to leak out of the monitor speaker even though it's turned off? The two phenomena are definitely related and it's driving me nuts.