Hi people, I am currently working on the amplifier from a Mackie SWA1801 powered subwoofer that had a meltdown, blew both its driver transistors and the bias transistor and a few resistors. I have gone through the rest of the circuit and tested all the other transistors, diodes, replaced all capacitors etc but it is not working. It passes audio but it is very distorted and looks like only one half of the sine wave is being output.
If I test with the scope on the emitters of Q1 and Q2 I get a clean signal, but on the collectors of those transistors the wave form is heavily distorted.
I have included a photo of my scope showing the waveform output, both with and without load. If there is no load connected the output is a sinewave with slight distortion (second photo) but as soon as I connect a speaker then I get massive distortion and a dramatic drop in level (first photo). I have also attached a copy of the amp schematic. If any one has any ideas and is able to help Id be very grateful for any ideas.
**Problem is now fixed (Murphy's law, only find the problem after running out of ideas and asking for help). Rookie mistake, R56 was open circuit and I didn't even think to check it, thinking I'd tested it earlier in the initial diagnosis, despite it being an obvious starting point for bias related problems haha. Amp now runs sweet!**
If I test with the scope on the emitters of Q1 and Q2 I get a clean signal, but on the collectors of those transistors the wave form is heavily distorted.
I have included a photo of my scope showing the waveform output, both with and without load. If there is no load connected the output is a sinewave with slight distortion (second photo) but as soon as I connect a speaker then I get massive distortion and a dramatic drop in level (first photo). I have also attached a copy of the amp schematic. If any one has any ideas and is able to help Id be very grateful for any ideas.
**Problem is now fixed (Murphy's law, only find the problem after running out of ideas and asking for help). Rookie mistake, R56 was open circuit and I didn't even think to check it, thinking I'd tested it earlier in the initial diagnosis, despite it being an obvious starting point for bias related problems haha. Amp now runs sweet!**
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May I make a suggestion?
EVERY SINGLE ONE of those amps that have come through my shop had cracked solder on the small speaker connector board. Unless yours is a later version, does the wires from the speaker connect to a small board in the corner, board maybe 75mm square? Said board plugs onto four flat spades on the main amplifier board?
If so, check close the solder on the flat spade connectors for cracks.
EVERY SINGLE ONE of those amps that have come through my shop had cracked solder on the small speaker connector board. Unless yours is a later version, does the wires from the speaker connect to a small board in the corner, board maybe 75mm square? Said board plugs onto four flat spades on the main amplifier board?
If so, check close the solder on the flat spade connectors for cracks.
Has the loudspeaker got a shorted turn in the voice coil if Enzo's fix doesn't work? You could also have a faulty OPT (slight reverse leakage) hard to spot with a meter. This can also occur with small transistors as well and stuff up the feedback loop.Check the rail fuses and holders for an intermittent OC assuming its got them.
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