Hi guys I have a VLP1500 amplifier here - one channels works fine and the other channel distorts even at low volume settings
Protection and Clip LED are off
There is something wrong with the BIAS voltages around T16 and T18 (in particular) on the bad channel
I tested all diodes and transistors in this area in circuit with diode mode - both channels seem identical. Same in resistance mode across all the resistors here.
The output DC voltages on pin1 and pin8 of the 5532D dual op amp at the input of each amplifier module are the same, and the audio waveform looks OK here, it goes wrong after that. Playing some audio I can see on my scope that the waveform around T11 and T12 is not 'centered' on the bad channel, it is larger in the negative direction - on the good channel it is symmetrical. I also tried swapping the amplifier modules over, the fault is definitely on the amplifier, not front panel, rear panel or wiring to it.
The yellow trimmer pots are both good and set the same on both channels. The white trimmer pots are good and set slightly different on each channel. I tried setting the bad channel white trimmer pot same as the good one but it did not help so I set it back again.
Also I hear a click on the speaker if I put my meter probe on base of T16 or T18 on the good channel but only on the base of T16 on the bad channel.
Any suggestions and does anyone have a schematic?
See pics and voltage readings
Thanks
Richard
Protection and Clip LED are off
There is something wrong with the BIAS voltages around T16 and T18 (in particular) on the bad channel
I tested all diodes and transistors in this area in circuit with diode mode - both channels seem identical. Same in resistance mode across all the resistors here.
The output DC voltages on pin1 and pin8 of the 5532D dual op amp at the input of each amplifier module are the same, and the audio waveform looks OK here, it goes wrong after that. Playing some audio I can see on my scope that the waveform around T11 and T12 is not 'centered' on the bad channel, it is larger in the negative direction - on the good channel it is symmetrical. I also tried swapping the amplifier modules over, the fault is definitely on the amplifier, not front panel, rear panel or wiring to it.
The yellow trimmer pots are both good and set the same on both channels. The white trimmer pots are good and set slightly different on each channel. I tried setting the bad channel white trimmer pot same as the good one but it did not help so I set it back again.
Also I hear a click on the speaker if I put my meter probe on base of T16 or T18 on the good channel but only on the base of T16 on the bad channel.
Any suggestions and does anyone have a schematic?
See pics and voltage readings
Thanks
Richard
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