I have an older Pioneer amp, which produces good audio, does not draw excessive current but it stops producing audio when I turn the volume up on my PC, which I use as a signal source. Audio stops, then it goes into this loop of trying to produce audio and cutting out until i turn the volume down again.
Amp does not have protection LED, power LED is lighted all the time during this problem but probing pins 2 and 15 of 494 shows that voltage on these pins is fluctuating and causing the amp to cycle between protect mode. Voltage is also fluctuating on pin 9 of PA2027A chip, which according to schematic is mute pin.
Waveform on PS FETs looks quite different from a nice square, could this cause the problem?
Attached are photo of scopes screen, when displaying signal on PS FETs (5V/div, 5us/div signal is the same for both banks) and schematic for the amp.
Amp does not have protection LED, power LED is lighted all the time during this problem but probing pins 2 and 15 of 494 shows that voltage on these pins is fluctuating and causing the amp to cycle between protect mode. Voltage is also fluctuating on pin 9 of PA2027A chip, which according to schematic is mute pin.
Waveform on PS FETs looks quite different from a nice square, could this cause the problem?
Attached are photo of scopes screen, when displaying signal on PS FETs (5V/div, 5us/div signal is the same for both banks) and schematic for the amp.
Attachments
Yes, lowest number which I saw was 10,9V. I thought that voltage from my supply may be dropping too low, but the guy who sold me this amp said that he tried the amp in a car, suspected bad grounding. Then he connected amps + and GND directly to the battery and got the same result. He did not think of measuring voltage. I will try the amp in car tomorrow.
Tried driving channels separately first. Left channel I can push as hard as I can, to clipping without amp shutting down. Right channel causes problems. When left channel was clipping, voltage on B+ and GND terminals was 10,5V. When right channel was causing amp to go to protection voltage on B+ and GND terminals did not go lower than 10,9V but amp went to protect. So PS is not the problem. When left channel is playing at low levels, output looks very clean on scopes display, no distortion. Voltages on output transisors are the same on both channels. DC offset on speaker terminals: both channels 4-5 mV.
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