Cambridge Soundworks Basscube 12S

[Logistics]

So I have a Basscube 12S with the usual problems--burned up around some power resistors, capacitors and diodes. I've seen several examples of this online. I don't recall anyone actually, coming up with an explanation for why it happens, other than poor component quality/placement/etc. So I'm all for reworking this thing as it's pretty simple, but I want to get your opinions on what to change, what to replace. I'm not keen on identifying diodes although, elsewhere on the net they are referred to as 15V Zener diodes.

Matthew

[budm]

Re: Cambridge Soundworks Basscube 12S

1) Bad brown glues need to be removed and then inspect the component's legs and copper traces to see if they are corroded, you can see corrosion on those tow 15V Zeners already.
2) Using resistors to drop 35 ~ 40VDC down to 15V is not good idea, the heat on the resistors has to be managed properly but since the boards are in enclosed cabinet so there is no ventilation to help remove the heat + bad parts placement in the hot areas. They could have used another low Voltage winding on the transformer to produce +/-15V instead.
3) So what Voltage measurement have you done so far?

[CapLeaker]

Re: Cambridge Soundworks Basscube 12S

I see a couple of leaking caps.

[Logistics]

Re: Cambridge Soundworks Basscube 12S

1) Bad brown glues need to be removed and then inspect the component's legs and copper traces to see if they are corroded, you can see corrosion on those tow 15V Zeners already.
2) Using resistors to drop 35 ~ 40VDC down to 15V is not good idea, the heat on the resistors has to be managed properly but since the boards are in enclosed cabinet so there is no ventilation to help remove the heat + bad parts placement in the hot areas. They could have used another low Voltage winding on the transformer to produce +/-15V instead.
3) So what Voltage measurement have you done so far?

Your suggestions for inspection and cleanup is what I'm gonna do. Someone cut the power cord off the unit so I can't power it up, yet. The way this unit was designed, the plate amp detaches from the back, and there is a large, plastic cup that sits around the perimeter of the plate amp, and the subwoofers' wires come through that cup, and are glued shut so the airspace is sealed. But because of this, I could space the plate amp out with spacers, and add a fan for longevity. Once I get to the point of replacing components, can you give me any tips on positioning? I mean the way some resistors are suspended far from the PCB, while others are not, the way the zener diodes are raised far from the PCB, and that large, white, cement resistor, could it be replaced with a comparable wirewound to improve matters?

Matthew

[momaka]

Re: Cambridge Soundworks Basscube 12S

Once I get to the point of replacing components, can you give me any tips on positioning? I mean the way some resistors are suspended far from the PCB, while others are not, the way the zener diodes are raised far from the PCB, and that large, white, cement resistor, could it be replaced with a comparable wirewound to improve matters?

I suggest just getting two linear 15-20V power adapters (the ones with actual line transformers in them) and connecting them to feed power into the +/-15V rail regulator inputs. You may even be able to get away with a 12V linear adapter, if the current draw is low enough. Most 15-16V adapters should output about 16-20V DC unloaded and go down with load closer towards 15V. That should be enough to feed the +/-15V regulator circuits.

An alternative to the above would be to use two 20V laptop adapters in series (with their grounds disconnected/isolated from each other and the wall). This will also give a nice supply for the +/-15V rails input regulators.

Either way, both of these would be a much more efficient design and result in less heat on the board. As budm pointed out, dropping 40V to produce 15V is a bit ridiculous. I've seen other amps do it too, and the PCB always looks cooked. This guarantees that any small caps (especially from crap brands) will get cooked and likely fail after the warranty has passed.