kintave
New Member
I got a free KR-7200 with a scratchy, intermittent right channel and some hiss. I replaced nearly all electrolytic capacitors in the entire unit (pretty much all of them were leaking). I cleaned the relay and the contact pins on the main amp board, and I replaced all the amp resistors (Rq1-4). I replaced a 2SC983 on the main amp (Qe6) with an NTE 287. (In the process of checking voltages, I also managed to accidentally bridge a connection with a probe, blowing Qe7, which I replaced with an NTE 175).)
The right channel is no longer scratchy, but now it is much louder than the left channel, with what sounds like some distortion. The schematics indicate what voltages should be at the base and collector of Qe5 and Qe6, which are in the L and R channels respectively (-44V and base and -0.62V at collector). The voltages going into Qe5 (L channel) are fine, but Qe6 (R channel) is showing -1.7V at the collector. When I check the DC bias, I see 9 mV in the right channel, and 40 mV in the left channel (you'd expect it to be the other way around given the distortion). I'm having trouble adjusting that with the bias trim pots (VRe 1-2), but they seemed fine when I tested them.
There is also a ground hum in the L channel. Is it possible that there's a short somewhere causing it to lose voltage? I can't see anything obvious on the board, and the voltages seem to be correct. All the transistors seem to be fine. The resistors seem to be reading at spec, and I replaced all the electrolytic capacitors on this board.
Basically, I have no idea what's going on, and I'm at a loss for how to diagnose it. I wouldn't expect the loud, somewhat distorted channel to have OK DC offset, and the quiet, cleaner channel to be the higher one. I'm willing to check every component thoroughly, but does this seem to point to anything I should be checking first?
There's also still some hiss. I didn't replace the big 6800uF capacitors (C301 and C302 on the schematic) because they are annoyingly connected to everything by wire wrap, but should I assume they're responsible for the hiss I'm still hearing?
Thank you in advance. This is my first repair project, and I've tried to read as much as I could on my own, but this is really confusing me.
The right channel is no longer scratchy, but now it is much louder than the left channel, with what sounds like some distortion. The schematics indicate what voltages should be at the base and collector of Qe5 and Qe6, which are in the L and R channels respectively (-44V and base and -0.62V at collector). The voltages going into Qe5 (L channel) are fine, but Qe6 (R channel) is showing -1.7V at the collector. When I check the DC bias, I see 9 mV in the right channel, and 40 mV in the left channel (you'd expect it to be the other way around given the distortion). I'm having trouble adjusting that with the bias trim pots (VRe 1-2), but they seemed fine when I tested them.
There is also a ground hum in the L channel. Is it possible that there's a short somewhere causing it to lose voltage? I can't see anything obvious on the board, and the voltages seem to be correct. All the transistors seem to be fine. The resistors seem to be reading at spec, and I replaced all the electrolytic capacitors on this board.
Basically, I have no idea what's going on, and I'm at a loss for how to diagnose it. I wouldn't expect the loud, somewhat distorted channel to have OK DC offset, and the quiet, cleaner channel to be the higher one. I'm willing to check every component thoroughly, but does this seem to point to anything I should be checking first?
There's also still some hiss. I didn't replace the big 6800uF capacitors (C301 and C302 on the schematic) because they are annoyingly connected to everything by wire wrap, but should I assume they're responsible for the hiss I'm still hearing?
Thank you in advance. This is my first repair project, and I've tried to read as much as I could on my own, but this is really confusing me.
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