Accusound OM-600a Subwoofer repair - very low volume

[BoringName]

Hi,

A mate was going to junk this but I offered to try and fix it. I'm a bit of a novice and trying it out for experience.

The initial symptoms were no sound and when first turned on the LED would take ages to come on and it would make grumbling noises for about 30 seconds before settling down.

I tested all the capacitors with an ESR meter and a few of them weren't good. Particularly 2 x 100uf 16 volts in line with 2 x 1k ohm resistors not far from the 2 big capacitors on the power supply board. I had seen on another thread about a different accusound sub model that these generally fail and to replace them with 220uf 25 volt capacitors, so I did. There is a dark patch on the board between the these capacitors and the 2 resistors. The resistors look a little dark but are still testing at 1k ohms. There is also 2 zener diodes (I think) near this dark patch.

I also replaced every other capacitor as a few others were stuffed and I figured I may as well do them all. All the same uF values but most had higher voltage values eg) 16 replaced with 25, 50 replaced with 63. I didn't replace the 2 large capacitors on the power supply as they tested fine.

After replacing the caps, the subwoofer now turns on, the LED is red for about 1 second and goes green. I'm not sure if it is supposed to do this without a source present. The grumbling noise is gone.

I plugged it into my amplifier (where my subwoofer works fine) and did a test tone and could only hear it if I put my ear right on the sub, it was extremely quiet. Using a 3.5m jack to RCA adapter, I plugged my phone into the sub and played some music. If I turned my phone volume to max I can hear music coming out of the sub but it is extremely quiet. I played around with the volume knob on the sub and that seems to be working ok but I haven't tested the resistance of the potentiometer. I had the crossover set to max.

When taking measurement it's made a few loud thumps and I can hear music so I'm sure the speaker is fine.

There is 3 boards
Power Supply - Connected to the amplifier board via a ribbon cable. The speaker is wired to this board.
Amplifier board - Connected to the 3 plugs on the back of the board (volume, crossover, phase) and connected to the Input/out board with 3 wires. Red/white/black.
Input/output board - Connected to the input/outputs on the back of the subwoofer.

Next to the ribbon cable on the power supply board it states 12 volts supply. I've tested the amplifier board and it is receiving 12 volts. There are 3 x RC4558P (amplifier chips) on this board and they are all measuring around 12 and -12 volts on vcc+ and vcc-.

On the power supply board there are 2 big transistors C5198 and A1941. These failed testing but that's with them still attached to the board so it probably doesn't mean anything. I don't have any solder wick presently to remove them. Would one or both of these being busted explain the low volume? I followed the trace from the speaker and it seems like these guys are what supply higher voltages to the speaker?

Any other ideas would be greatly appreciated, I feel like I'm so close to getting it fixed!

I have a basic understanding of different components but still getting my head around how they all work together in a circuit and what to test for.

Thanks.

[BoringName]

I removed the 2 transistors (C5198) and (A1941). They tested ok.

I have now moved on to the volume potentiometer as I am getting some strange readings, hopefully someone can help with that.

It's currently in circuit. With 3 pins, lets call them A, B, C

If I measure A/C it registers as 22k ohms constant. So I assume they are the wipers. Turning the knob alters the value very slightly. 0.01-0.02 of an ohm.

If I measure between either A/B or B/C I get wild readings in the mega ohms ranging from  5 to 30, it darts around a bit and once it settles it slowly counts up or down. If I turn the volume right down it slowly counts down, about 1M ohm every 20 seconds. It stops around 7M ohms. If I turn the volume knob up it slowly counts up. I let it get to 20M ohms and stopped measuring.

It's stuffed right or could be because it's still in the circuit? There was no power in the circuit at the time.

The crossover and Phase potentiometers behave as expected.

It's going to be a fiddly prick to get out so I just want to make sure before I risk breaking something.

[cprogrammer]

I have now moved on to the volume potentiometer as I am getting some strange readings, hopefully someone can help with that.

It's currently in circuit. With 3 pins, lets call them A, B, C

If I measure A/C it registers as 22k ohms constant. So I assume they are the wipers. Turning the knob alters the value very slightly. 0.01-0.02 of an ohm.

The outer two terminals of a potentiometer will register a fixed resistance. They are called the lugs.

If I measure between either A/B or B/C I get wild readings in the mega ohms ranging from  5 to 30, it darts around a bit and once it settles it slowly counts up or down. If I turn the volume right down it slowly counts down, about 1M ohm every 20 seconds. It stops around 7M ohms. If I turn the volume knob up it slowly counts up. I let it get to 20M ohms and stopped measuring.

It means the wiper is not making contact with the resistance strip. Most probably you will have to replace it. Try deoxit to clean it. There is a easy way to test if your subwoofer is working. If you touch the B terminal with a small piece of wire, or a screw driver with the metal part touching the terminal B, you should get a hum.

[BoringName]

The outer two terminals of a potentiometer will register a fixed resistance. They are called the lugs.

Right, I'd watched a video on how to test them and the person on the video called them wipers and I was thinking of that when I posted. I realised that was incorrect when I quickly checked for replies before bed yesterday. I figured everyone would know what I meant.

Try deoxit to clean it. There is a easy way to test if your subwoofer is working. If you touch the B terminal with a small piece of wire, or a screw driver with the metal part touching the terminal B, you should get a hum.

It looks like a sealed unit (little green box). I'll take it off and see if there is any openings to clean it. I'm going to Jaycar later anyway so I might just pick up a new one. Touching terminal B with a screwdriver gives a hum so I'm pretty happy about that.

Thanks. I'll post back with the results.

[BoringName]

We have a winner. In hindsight, it seems obvious the volume knob might effect the volume  I spent quite a bit of time testing everything else.... good experience though.

Very happy! Thanks for the help.