Trying to repair an old Triad subwoofer amplifier - diyAudio
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Old 4th December 2010, 06:45 AM   #1
spooney is offline spooney  United States
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Default Trying to repair an old Triad subwoofer amplifier

Just picked up an older Triad 10" powered subwoofer for 5 bucks off of craigslist. Previous owner said that the subwoofer has no output other than an annoying hum through the subwoofer driver. I confirmed this to be true . Regardless of input the sub only makes the annoying hum. Subwoofer driver itself appears to be fine. There is about 4 or 5 volts dc present on the speaker wires with the driver unhooked. I did not find any shorted output transistors. Not sure if a home amplifier is like a car amplifier but the rail voltage looks a little funny to me. I have +63 volts and -40 volts on the rails. Does that sound right? I know in a car amp that those usually match but I really have no idea what to look for in a home amplifier. I would like to get this thing running strong again as it looks like a decent little unit but I could really use a hand being pointed in some directions to help me along with this repair. If anybody could be of some help I would greatly appreciate it.
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Old 4th December 2010, 11:51 AM   #2
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It sounds like you have a faulty power supply filter capacitor that isn't doing its job properly and allowing mains hum onto the power rails. This would also explain why the rail voltages are wildly different too.

The filter capacitors are usually the biggest on the PCB and will probably have a rating of 4,700uF upwards and a voltage of >60V going on the power rail voltages you stated.

At least the subwoofer driver is OK due to the fact that it still passes sound.

It would help if you could provide a model number as then there is a possibility of finding some service information.
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Old 4th December 2010, 01:28 PM   #3
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Unfortunately there is not a model number anywhere on the sub. The amplifier itself has a sticker that reads "Triad 9.1 Amplifier" on the circuit board but so far I haven't been able to turn up any info . One of the filter capacitors was bulging just slightly at the top. They are rated for 63 volts. I will try swapping those out and see if that changes anything. Would these caps being faulty also keep the amp from playing audio?
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Old 4th December 2010, 02:45 PM   #4
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These caps, depending on how much their value has changed may prevent the amp from playing audio properly because the various circuits aren't getting a clean power supply. If only one of them is bulging, this explains why the one rail voltage is so different to the other and usually indicates a buildup of pressure inside the can and therefore a problem with the capacitor.

When replacing them, you can use ones with exactly the same voltage and capacitance rating or ones with a higher voltage rating. Using capacitors rated at a higher voltage will help in terms of reliability.
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Old 4th December 2010, 03:22 PM   #5
spooney is offline spooney  United States
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Yeah I was hoping to get some 100 volt rated caps to replace them with and possibly do some work on the enclosure. They have these huge foam blocks that lay right against the amplifier. That extra heat can't be good for the caps or anything else in there for that matter. You can see indentations in the foam from where the caps have been laying against it for 10 plus years. I will swap out these caps and see what happens from there. Maybe I will swap the caps positions and see if the voltage changes from rail to rail just to verify that they are causing the issue
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Old 4th December 2010, 03:30 PM   #6
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Another way to test the caps would be to connect them to a multimeter set to measure resistance and then check the reading. If it steadily climbs for a while and carries on going then the cap is good. If it climbs and then stays constant, the cap is bad.

By foam blocks, what do you mean? These may be there to dampen any vibrations that are caused by the low frequencies of the subwoofer and stop any rattling so it might not be a good idea to modify them.
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Old 4th December 2010, 03:34 PM   #7
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The foam blocks are roughly 4 to 5 inches thick and are similar in stiffness to lets say a couch cushion. I think I could take a little bit of the thickness out (just enough to let the amp breathe) and still have a majority of the foam in there. I will get the amp up and running and then go from there. Perhaps there is a better way to get some breathing room for the amplifier
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Old 4th December 2010, 03:36 PM   #8
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If I were to remove the caps and power the amp up would that cause any problems? Would that isolate the power supply from the output side of the amp? I know pulling the rectifiers in a car amp would do that but I am unsure if the caps would accomplish the same thing.
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Old 4th December 2010, 03:37 PM   #9
spooney is offline spooney  United States
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And thank you for your help thus far
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Old 4th December 2010, 03:45 PM   #10
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Running the power supply without the capacitors will be dangerous and could possibly destroy something. Even though the bridge rectifier converts AC to DC it only converts it to PULSED DC so the capacitors are still needed to smooth out those pulses into a steady current.

The hum that you are hearing is more than likely because these capacitors are not doing their job properly and allowing some of the AC pulses onto the power rails. When the power transistors are turned on, instead of DC flowing through, there is DC with some AC that is transferred to the speaker. This is the hum that you hear.

Yes, removing the bridge rectifier in a car amp but also any amp will isolate the power supply from the amplifier (apart from the ground connection which doesn't have any voltage on it anyway). This is pointless though as to test the amplifier you still need power connected and testing the PSU caps in-circuit may bring up false readings because of connections elsewhere in the circuit.

Regarding the foam, I would get the amp repaired first and then think about the enclosure.
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