RCherrington
New Member
So I have this old Technics SA151. Nothing special about it, but its a little bit special to me and I'd like to keep it.
About a month of so back I was listening to some music fairly loud, the volume slider was just shy of 4 (of 10), but with the little infinity speakers, it was pretty loud. I listened to one album (Peter Gabriel Scratch or possibly Security, I can't recall which) and shut it off. The next night I go to turn it on again and I get nothing but a low buzz/humm from one channel.
I connected to to a signal generator 1KHz @1v and traced the signal through the main board and it seemed to end at the STK4141 amplifier, after that I just got a 60Hz signal, I think, or perhaps I was picking up the fluorescent lights?
Thinking that the amp was bad, I replaced it with one from ebay. Unfortunately I have the same problem with the new one. Now, I realize the possibility I got a bad chip from the seller, but how likely is it that it came with the exact same problem? Then I noticed that resistor 621 is discoloring and smoking slightly. Turned the power off and checked the resistance and its about 208 (s/b 220) so that's pretty much ok. Then I checked the incoming voltages on the STK4141 (IC601 on the schematic) and pin 11 was at +35v with the neg probe on the chassis and the pos probe on the pin. (Diagram says that should have been -39v, but I chalked that up to my crappy multimeter auto-correcting or something) Then I looked at pin 12 - which seems to be fed directly from pin 11 through that 220 ohm resistor and its reading 15v. Schemaitc has it labeled at -36!
My guess was that c619 has shorted and is pulling down that rail, but when I put an ohm meter across its legs, I get OL and I don't see any obvious deterioration on it, legs are clean, no leakage and the can isn't swollen that I can tell. (this is without removing the cap)
Anyone have any thoughts what else it could be, or something else I should be looking at? With that resistor cooking itself (though still reading good) I'm hesitant to let the power stay on longer than a second or to to get a reading, then I shut it right off.
About a month of so back I was listening to some music fairly loud, the volume slider was just shy of 4 (of 10), but with the little infinity speakers, it was pretty loud. I listened to one album (Peter Gabriel Scratch or possibly Security, I can't recall which) and shut it off. The next night I go to turn it on again and I get nothing but a low buzz/humm from one channel.
I connected to to a signal generator 1KHz @1v and traced the signal through the main board and it seemed to end at the STK4141 amplifier, after that I just got a 60Hz signal, I think, or perhaps I was picking up the fluorescent lights?
Thinking that the amp was bad, I replaced it with one from ebay. Unfortunately I have the same problem with the new one. Now, I realize the possibility I got a bad chip from the seller, but how likely is it that it came with the exact same problem? Then I noticed that resistor 621 is discoloring and smoking slightly. Turned the power off and checked the resistance and its about 208 (s/b 220) so that's pretty much ok. Then I checked the incoming voltages on the STK4141 (IC601 on the schematic) and pin 11 was at +35v with the neg probe on the chassis and the pos probe on the pin. (Diagram says that should have been -39v, but I chalked that up to my crappy multimeter auto-correcting or something) Then I looked at pin 12 - which seems to be fed directly from pin 11 through that 220 ohm resistor and its reading 15v. Schemaitc has it labeled at -36!
My guess was that c619 has shorted and is pulling down that rail, but when I put an ohm meter across its legs, I get OL and I don't see any obvious deterioration on it, legs are clean, no leakage and the can isn't swollen that I can tell. (this is without removing the cap)
Anyone have any thoughts what else it could be, or something else I should be looking at? With that resistor cooking itself (though still reading good) I'm hesitant to let the power stay on longer than a second or to to get a reading, then I shut it right off.