trainwreck58
New Member
I have a JVC JA-S71 with an odd reaction to audio input. I haven't found any posts that relate specifically about it, so I'll post mine here.
The amp seems to set off the speaker protection circuit when a strong bass passage is played through any of the aux or tape monitor inputs. It does it whether it is connected to the preamp or amp directly. To check further, I hooked up an audio signal generator and a dummy load with wattage meter to the amp and discovered that the signals below 80hz required little voltage to trip the protection, resulting in a 10w or less output in that frequency range. The lower the frequency, the less input voltage needed to trip the circuit, and the less wattage read at the output.
However, when inserting a signal above 200hz, the full 80 watt output is possible. I have tried all different frequencies up to 20khz and have achieved full 80w output.
So my question is... what the heck is wrong with this thing? I have used a DVM with a diode tester to check the output transistors and the 2 sets of four seem to be getting very close readings. None of them are obviously different than the others.
I have set the offset and bias voltages to the range indicated in the service manual. I have replaced all of the electrolytic caps, except for the large power caps and the ELNA low leak electrolytics.
I have only had this amp for a month and do not know its history, however, it will play all day long at the threshold of shutoff (about 30w output) without getting very warm or tripping the protection circuit.
Anybody have any ideas? Driver trannies, output trannies, etc.?
The amp seems to set off the speaker protection circuit when a strong bass passage is played through any of the aux or tape monitor inputs. It does it whether it is connected to the preamp or amp directly. To check further, I hooked up an audio signal generator and a dummy load with wattage meter to the amp and discovered that the signals below 80hz required little voltage to trip the protection, resulting in a 10w or less output in that frequency range. The lower the frequency, the less input voltage needed to trip the circuit, and the less wattage read at the output.
However, when inserting a signal above 200hz, the full 80 watt output is possible. I have tried all different frequencies up to 20khz and have achieved full 80w output.
So my question is... what the heck is wrong with this thing? I have used a DVM with a diode tester to check the output transistors and the 2 sets of four seem to be getting very close readings. None of them are obviously different than the others.
I have set the offset and bias voltages to the range indicated in the service manual. I have replaced all of the electrolytic caps, except for the large power caps and the ELNA low leak electrolytics.
I have only had this amp for a month and do not know its history, however, it will play all day long at the threshold of shutoff (about 30w output) without getting very warm or tripping the protection circuit.
Anybody have any ideas? Driver trannies, output trannies, etc.?