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This SWR 750x is producing repetitive 'snits' on the output, very present when there is input signal, though I can still hear it faintly with nothing plugged in, or better with a 100k termination plug inserted into the Passive input jack. I had briefly heard the same snit on the first of the 3 750x amps that I've been doing preventative maintenance on, and, after replacing the one preamp tube on that one, I never heard it again. Not so on this one, though.
I've at least isolated it to being somewhere on the main PCB, as patching into the Effects Return jack, running the test signal thru the power amp is clean, free of this annoying snit.
750x Preamp Rev D Sch.pdf
750x Speaker Output Rev A Sch.pdf
750x Power Amp Rev E Sch.pdf
I've been probing thru all of the signal path, looking for the source. As it's a narrow pulse, it's been hard to catch. I hear it best using very low frequency tone, or LF random noise from my B & K generator, sine-random with a 10Hz BW, tuned down around 30Hz. On the scope photos, I replaced that with 20Hz sine, and had to fiddle with the storage writing controls to finally catch the repetitive transient.
I thought at one point, I had caught the sucker on the power supply leads of the input to the +/- 5V regulators, which power up the Subwave circuit. I was seeing something very odd there, but it wasn't the this snit....not sure what that one is. It almost sounds like a cap breaking down in the power supply. I am seeing something odd at the input side of both +/- 5V regulators. With the LF signal applied to the amp, I'm seeing this 3.5V P-P signal (20Hz sine input to the amp) at the junction of either R78/C46 or R81/C53...the inputs to the +/- 5V IC regulators. When I remove the input signal, that waveform is gone. With the signal applied, and looking at the output of the regulators, there's no signal, so the regulators are doing their ripple rejection job. I haven't yet pulled the main PCB out and swapped out any of the LV supply caps to see if there's something breaking down in there. There's just no relative sync to the snit rep rate and the ripple frequency.
I've run out of space for the other photos on this regulator series.
Any suggestions?
I've at least isolated it to being somewhere on the main PCB, as patching into the Effects Return jack, running the test signal thru the power amp is clean, free of this annoying snit.
750x Preamp Rev D Sch.pdf
750x Speaker Output Rev A Sch.pdf
750x Power Amp Rev E Sch.pdf
I've been probing thru all of the signal path, looking for the source. As it's a narrow pulse, it's been hard to catch. I hear it best using very low frequency tone, or LF random noise from my B & K generator, sine-random with a 10Hz BW, tuned down around 30Hz. On the scope photos, I replaced that with 20Hz sine, and had to fiddle with the storage writing controls to finally catch the repetitive transient.
I thought at one point, I had caught the sucker on the power supply leads of the input to the +/- 5V regulators, which power up the Subwave circuit. I was seeing something very odd there, but it wasn't the this snit....not sure what that one is. It almost sounds like a cap breaking down in the power supply. I am seeing something odd at the input side of both +/- 5V regulators. With the LF signal applied to the amp, I'm seeing this 3.5V P-P signal (20Hz sine input to the amp) at the junction of either R78/C46 or R81/C53...the inputs to the +/- 5V IC regulators. When I remove the input signal, that waveform is gone. With the signal applied, and looking at the output of the regulators, there's no signal, so the regulators are doing their ripple rejection job. I haven't yet pulled the main PCB out and swapped out any of the LV supply caps to see if there's something breaking down in there. There's just no relative sync to the snit rep rate and the ripple frequency.
I've run out of space for the other photos on this regulator series.
Any suggestions?
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