MHR
New Member
First of all, hi to everyone who’s interested to take time to read my problems and questions. This post is quite extensive, but it’s better to be complete, to rule out the obvious. At this point I had to swallow my pride and ask for help.
I’m not a complete idiot in the electronics department, as I successfully revised quite a few amplifiers up until this point.
Before I got my hands on this one the amp worked, but had the infamous crackle on the speaker-output caused by 2SA725’s gone bad on the power-amp board.
I decided the amp was well worth it to make it future proof so I have fully recapped my SA-8500, and all SA725/6’s are replaced by KSA992 and KSC1845. Also the 2SC1451 were replaced by KSC3503. The primary power-ELCO’s have been replaced from 50V/15000uF to 63V/15000uF.
I also did a balance and BIAS calibration.
Now, first of all let me explain the symptoms my SA-8500 now has after done all this work:
When fully hooked up, so speakers on terminals A and AUX in use from streamer, when i up the volume to like a quarter of the way, the relay switches off. When I decrease volume the relay jumps back on and the sound comes back. When I unhook everything, I can crank the volume up to max if I wanted to but the relay does not switch off.
So I did a little investigating myself:
Note: All these tests I did behind a dim-bulb-tester, just to be safe.
1. I first put on a signal generator on the right channel with a 1kHz/300mV sinus wave. Then I hooked up a 200W 8 Ohm power-resistor to the terminals to mimic speaker load. I hooked up my oscilloscope to the power resistor to visualize output.
Result: Same symptom on both left and right channel, no matter what input, Tuner, TAPE1/2. The little sinus wave I saw was good, no weird artifacts in the wave.
2. Then I checked the voltages on EQ-assembly (Volume, input selector and Phono stage) as well as Control amp (tone-control board) and primary DC input from just behind the rectifier from the Switch circuit board.
Result: All voltages were in-spec.
3. Now… I did the same test when hooking the AUX input, put a 1kHz/300mV sinus signal and the power resistor tot the speaker terminal. There were no dramatic voltage drops across any of the known measure points, as can be found in the manual.
4. Then I noticed some very faint hum coming off the transformer (ATT-233B EU-model). I realized that I never heard hum coming from a transformer in a power-amplifier before. So I decided to investigate on Ohm’s and continuity within and between phases.
Result: Nothing out of the ordinary, resistance well between 0 and 10 Ohms. No contact between phases (no short)
OK, this bit may be the important bit:
5. OK… Then I decided to bypass the pre-amp section by removing the RCA short bridges and put the same signal on there. Because I had to rule out which one is faulty, the pre-amp section or the power-amp section. I put the same signal on the pre-out as before turned on the amp, no issue. I cranked the signal up to 1.6V and I measured 20.4V when the amp started clipping, so with 8 Ohm this equals (20.4x20.4) / 8 = 52 Watts. Specs are 60Watts/channel so I didn’t know why it was under performing. But… of course, the dim- bulb-tester. I took the amp off the dim-bulb and tested again. However this time after switching on the amplifier with the signal already on the relay switch didn’t come on. So I turned off the signal and the relay clicked back on. So I reduced the signal too 100mV, then the relay turned on, when I upped the signal amplitude to 200mV it switched off again.
So I hooked the amplifier back on to the dim-bulb tester, which had a 300W light bulb in it and started the procedure again. I noticed that when I got above 1V the bulb started to brighten. So I’m thinking there is a fault in the power-amp section drawing excessive current, thus maybe an intermittent fault in one of the components causing a short above a certain power-level.
At this point, this is as far my knowledge goes. Maybe that’s the reason the hum of the transformer is audible, because it has to work hard to overcome some threshold and eventually gives up and the whole amp goes in protection mode.
If hou have managed to read my story all the way up to here, I commend you for it
If anyone has a suggestion how to proceed with determining what component might be at fault I’d be happy to receive any suggestion you might have. I'd be happy to provide pictures if necessary and any additional information you might need.
I’m not a complete idiot in the electronics department, as I successfully revised quite a few amplifiers up until this point.
Before I got my hands on this one the amp worked, but had the infamous crackle on the speaker-output caused by 2SA725’s gone bad on the power-amp board.
I decided the amp was well worth it to make it future proof so I have fully recapped my SA-8500, and all SA725/6’s are replaced by KSA992 and KSC1845. Also the 2SC1451 were replaced by KSC3503. The primary power-ELCO’s have been replaced from 50V/15000uF to 63V/15000uF.
I also did a balance and BIAS calibration.
Now, first of all let me explain the symptoms my SA-8500 now has after done all this work:
When fully hooked up, so speakers on terminals A and AUX in use from streamer, when i up the volume to like a quarter of the way, the relay switches off. When I decrease volume the relay jumps back on and the sound comes back. When I unhook everything, I can crank the volume up to max if I wanted to but the relay does not switch off.
So I did a little investigating myself:
Note: All these tests I did behind a dim-bulb-tester, just to be safe.
1. I first put on a signal generator on the right channel with a 1kHz/300mV sinus wave. Then I hooked up a 200W 8 Ohm power-resistor to the terminals to mimic speaker load. I hooked up my oscilloscope to the power resistor to visualize output.
Result: Same symptom on both left and right channel, no matter what input, Tuner, TAPE1/2. The little sinus wave I saw was good, no weird artifacts in the wave.
2. Then I checked the voltages on EQ-assembly (Volume, input selector and Phono stage) as well as Control amp (tone-control board) and primary DC input from just behind the rectifier from the Switch circuit board.
Result: All voltages were in-spec.
3. Now… I did the same test when hooking the AUX input, put a 1kHz/300mV sinus signal and the power resistor tot the speaker terminal. There were no dramatic voltage drops across any of the known measure points, as can be found in the manual.
4. Then I noticed some very faint hum coming off the transformer (ATT-233B EU-model). I realized that I never heard hum coming from a transformer in a power-amplifier before. So I decided to investigate on Ohm’s and continuity within and between phases.
Result: Nothing out of the ordinary, resistance well between 0 and 10 Ohms. No contact between phases (no short)
OK, this bit may be the important bit:
5. OK… Then I decided to bypass the pre-amp section by removing the RCA short bridges and put the same signal on there. Because I had to rule out which one is faulty, the pre-amp section or the power-amp section. I put the same signal on the pre-out as before turned on the amp, no issue. I cranked the signal up to 1.6V and I measured 20.4V when the amp started clipping, so with 8 Ohm this equals (20.4x20.4) / 8 = 52 Watts. Specs are 60Watts/channel so I didn’t know why it was under performing. But… of course, the dim- bulb-tester. I took the amp off the dim-bulb and tested again. However this time after switching on the amplifier with the signal already on the relay switch didn’t come on. So I turned off the signal and the relay clicked back on. So I reduced the signal too 100mV, then the relay turned on, when I upped the signal amplitude to 200mV it switched off again.
So I hooked the amplifier back on to the dim-bulb tester, which had a 300W light bulb in it and started the procedure again. I noticed that when I got above 1V the bulb started to brighten. So I’m thinking there is a fault in the power-amp section drawing excessive current, thus maybe an intermittent fault in one of the components causing a short above a certain power-level.
At this point, this is as far my knowledge goes. Maybe that’s the reason the hum of the transformer is audible, because it has to work hard to overcome some threshold and eventually gives up and the whole amp goes in protection mode.
If hou have managed to read my story all the way up to here, I commend you for it
If anyone has a suggestion how to proceed with determining what component might be at fault I’d be happy to receive any suggestion you might have. I'd be happy to provide pictures if necessary and any additional information you might need.
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