magicsound1
New Member
Hello All,
I'm working on my 2nd 7900, struggling with a subtle bias problem. The differential signal rails that also set the idle current are supposed to sit around plus and minus 1.9 volts. One channel is fine, the other measures +2.28v and -1.35v. As a result, the bias current is 0 and can't be adjusted on that channel, although it passes audio with no DC offset at the output, and nothing is getting hot.
The small internal voltage offset seems to propagate all the way back from the front-end dual transistor. I replaced it and the associated electrolytic with no change. All the resistors and diodes measure good. I'm stumped!
Here's the simplified schematic from the manual, with voltage measurements (100 v.a.c. variac input). The good channel's measurements are in parentheses for comparison.
Thanks for any advice.
Alan / Magic Sound
I'm working on my 2nd 7900, struggling with a subtle bias problem. The differential signal rails that also set the idle current are supposed to sit around plus and minus 1.9 volts. One channel is fine, the other measures +2.28v and -1.35v. As a result, the bias current is 0 and can't be adjusted on that channel, although it passes audio with no DC offset at the output, and nothing is getting hot.
The small internal voltage offset seems to propagate all the way back from the front-end dual transistor. I replaced it and the associated electrolytic with no change. All the resistors and diodes measure good. I'm stumped!
Here's the simplified schematic from the manual, with voltage measurements (100 v.a.c. variac input). The good channel's measurements are in parentheses for comparison.
Thanks for any advice.
Alan / Magic Sound
