Cambridge Basscube 12S

[patlaw]

Has anyone located a schematic for one of these? I haven't had any luck finding one.

[The Dude]

Email Support

Maybe try contacting them directly.

[patlaw]

Email sent. I'm not holding my breath.

[patlaw]

Their response:

Thank you for your inquiry. We do not provide or can we access any schematics for legacy products and our current products. We regret that we are no longer able to provide support, replacement parts, or repair services for this product. We do try to support products for a period of time after they go out of production, but this product has been discontinued for several years, and we ended support for it some time ago.

[The Dude]

Well, isn't that helpful of them.

Have you peeked under the hood? It may be something you can figure out without a schematic.

[patlaw]

Have you peeked under the hood? It may be something you can figure out without a schematic.

That's always a possibility. Yes, I have it here. It's not that complicated, but life is sure easier with a schematic.

[patlaw]

I have just had the worst luck trying to repair this piece of junk. It's not blowing fuses any more, but it's also not passing audio to the output. It would be a simple matter to do signal tracing with a schematic, which I don't have.

It has several 1 watt, 2 watt and 5 watt resistors. They're obviously that size because they need to dissipate power. However, without having seen the amp in good working order, it's impossible to know how hot is too hot. The circuit board traces are quite fragile.

It's amps like these that make me wonder if I have a clue about fixing amps. It's just not that complicated. The output transistors are not blown, so I may take another run at it. Since it's mine, I may just deep six it.

[DrGonz78]

Did you check all the lower voltages for the preamp? Is there a way to confirm the output is working and if the problem lies in the preamp section? What type of gain structure is the preamp using, chips or transistors? Using the data sheet for any number of components will help figure where signal should be in the amp. Also, those data sheets will help figure out your power supply rails. Probably not the most fun amp to work on in the first place and without schematic makes it even more a drag. Still many people have fixed problems like this even when the darn thing was not worth it, I know I have done it before. Sometimes you just do it for the challenge and then the compulsion kicks in too. Best of luck!

[J M Fahey]

A little amp porn might help.

Show a couple PCB clean readable pictures, where we can see the transistors or ICs used, filter caps, etc.

FWIW it has an "auto power on" system, which usually isn't so, but simply a mute so the amp is not humming/hissing at 3 a.m. whan you are trying to sleep.

Detecting audio signal at the input (you start playing music on your main system) , it unmutes.

No need to say that if amp is in working order but fails to unmute you will hear nothing.

Very common in "auxiliary audio" systems, such as .... subwoofers .... home theater, etc.

[patlaw]

I'll probably dig into it again and post some pictures. There is a line input (RCA jacks) on the front, which is where I'm feeding the input from the signal generator. The tone is getting through most of the preamp, but I can't tell where it's disappearing.

EDIT: It appears to be fixed. In addition to bad caps and burned resistors, the relay failed. The filter caps need to be replaced. I'm trying to decide whether to buy from eBay or Mouser. The eBay parts fit better, but Mouser is more trustworthy.

[J M Fahey]

Juyst for reference on what you might find inside, one of the "JBL Cinema" subwoofers (which I guess everybody clones or at least is "inspired by") and its in-famous "Auto ON-OFF" circuit, as I said before not much more than a mute (or they might have a relay open speaker out):



The point is that probably your amp works, if you find its input (the amp itself should be quite conventional) you may first inject some audio straight there to confirm and then trace back to bypass the mute, if it annoys you or is hard to repair.

Google "jbl cinema amplifier schematic" and enjoy, you might find a very similar one

One quirk of JBL is that many of their amps had class B biased power transistors.

I guess being in subwoofers crossover distortion is irrelevant.

[patlaw]

I'm feeding audio into it, and it sounds fine. The question is whether any component values have changed and, if so, whether those changed values caused the original fireworks. I had to use my best guess for the value of the burned resistor. The relay actually switches the plus and minus 45 volts to the amplifier. One of the contacts was welded shut, and the other one never made contact.

The power supply uses some 2W and 5W resistors and zener diodes to reduce 45 volts to 15 volts. As I mentioned earlier, it's hard to know how hot power resistors are supposed to be. They're very hot to the touch. It's tempting to slap a couple of 7815/7816 regulators in there, but it's not worth the trouble. For the record, this amp is Chinese garbage.

[Jazz P Bass]

"Chinese garbage".

Yummy.
Dig in.

[J M Fahey]

IThe power supply uses some 2W and 5W resistors and zener diodes to reduce 45 volts to 15 volts. As I mentioned earlier, it's hard to know how hot power resistors are supposed to be. They're very hot to the touch.

Not at all.

Power dissipated is voltage across it times current through it, or voltage across it squared, divided by its resistance.

Voltage is easy to measure and resistance is printed on the body.

How hot is normal?

If power dissipated is above power rating ( say 1.5W in a 1W rated resistor) then we have a problem.

If you dissipate 4W in a 5W resistor, it's within ratings.

It may be hot, unbearably hot, but if within ratings, fine.

"Chinese garbage".

Yummy.

Pigs forever have been garbage-into-meat-converters, that's why over a Billion people on Earth (Moslems and Jews mainly) do not eat them, not even if cooked (which in theory would destroy bacteria).

Can't say I don't understand their point .

By the way, they also consider dogs dirty ... same thing.

Chinese? ... they'll eat anything:

That pig is clean and safe compare to frying grease pulled from ... sewers.

https://www.youtube.com/watch?v=zrv78nG9R04

It's tempting to slap a couple of 7815/7816 regulators in there, but it's not worth the trouble. For the record, this amp is Chinese garbage.