KilgoreCemetery
- Apr 12, 2017
- 258
- Joined
- Apr 12, 2017
- Messages
- 258
I've been trying to figure out this Kenwood KR-70 receiver. The guy I bought it from said that the volume drops to zero after you run it for a while and, although it hasn't done it to me yet, there are some other signs that something is wrong with the receiver. There's one power transistor that heats up way faster and way hotter than the other three. Also, I'm getting about 100mV at the speaker connects with the volume at zero. That being said, this receiver powers on just fine and will play music, but I'd like to fix whatever is going on before it becomes a bigger problem.
I've already spent quite a bit of time checking stuff over and making notes. I've swapped just about all of the transistors in the amp section from one channel to another with very little change. I've recapped the amp section after finding some that were way out of tolerance, and I've lifted the legs to test a bunch of resistors. I even tried marking and adjusting the trim pots. Nothing seems to affect the imbalance between the channels or prevent the one transistor from heating up faster than the others.
The full schematic pdf is too large to upload here, but I do have an enlarged picture of the amplifier section with a lot of my notes on it. My biggest concern is that no matter what transistor I swap into the Q5 spot (not to be confused with Qe5), it gets hot within a minute of being powered on even though it has a lower base and emitter voltage than the other channel. Q4 gets warm within that same time frame, but it may just be radiating heat from Q5. The other two transistors are cool to the touch pretty much all the time. It seems like there is good heat transfer to the heatsink as well based on how hot it gets.
The only transistors that I haven't swapped from one channel to the other are Qe1 and Qe2, which are the only ones that have near-identical voltages. All the electrolytic capacitors in the amplifier section have been replaced.
I really don't know what else to check at this point. Could somebody give me an idea of where to look?
I've already spent quite a bit of time checking stuff over and making notes. I've swapped just about all of the transistors in the amp section from one channel to another with very little change. I've recapped the amp section after finding some that were way out of tolerance, and I've lifted the legs to test a bunch of resistors. I even tried marking and adjusting the trim pots. Nothing seems to affect the imbalance between the channels or prevent the one transistor from heating up faster than the others.
The full schematic pdf is too large to upload here, but I do have an enlarged picture of the amplifier section with a lot of my notes on it. My biggest concern is that no matter what transistor I swap into the Q5 spot (not to be confused with Qe5), it gets hot within a minute of being powered on even though it has a lower base and emitter voltage than the other channel. Q4 gets warm within that same time frame, but it may just be radiating heat from Q5. The other two transistors are cool to the touch pretty much all the time. It seems like there is good heat transfer to the heatsink as well based on how hot it gets.
The only transistors that I haven't swapped from one channel to the other are Qe1 and Qe2, which are the only ones that have near-identical voltages. All the electrolytic capacitors in the amplifier section have been replaced.
I really don't know what else to check at this point. Could somebody give me an idea of where to look?