Repairing a Cambridge Soundworks Model 12 speaker system

Note: I’ve never fixed anything like this before, this article (or articles) is going to be as much as a learning journey for me as you, the reader.

Managed to score myself an old Cambridge Soundworks Model 12 in intact but untested condition. For cheaps, of course. Everything seemed to be there, but it was a bit dirty. I couldn’t wait to plug it in.

The brand, Cambridge Soundworks

A now defunct brand, Cambridge Soundworks was started in the 80s by an experienced audio engineer wanting to bring high end sound systems and speakers to the mainstream. Their products flew off the shelves and they rapidly expanded.

They achieved the lower price points via good engineering with cheaper digital components and outsourcing manufacturing.

If you can find any of their equipment then they’re still quality products today, although as this example is about to show, they’re probably failing (as a result of the digital components) but easy to diagnose and fix (probably, I haven’t actually repaired it yet).

The model, Model 12

The Model 12 is a pretty non-descript looking item, as you can see above. It just looks like a plastic case. However it’s branded the BassCase and that’s because, if you flip it around, there’s a subwoofer on the back.

It’s actually a very cool unit, the Model 12 is a 2.1 channel portable sound system, with the speaker and amplifier stored inside the case itself, with the empty case acting as the subwoofer box when assembled.

Its dimensions are designed to fit under the seat of a plane, and is the revision of the Model 11, which carried a similar idea but in a much larger suitcase.

It’s designed to run on a 12V source and includes an AC to DC power brick, although you could also run it off a 12V battery (like a car battery) making it ideal for travelling musicians. It would probably be awesome for a busker.

This machine itself

This machine itself is completely intact and undented. It was obviously looked after by a previous owner, but based on the dirt and fly waste on it I’m guessing was left in the garage, with intent to be fixed, when one of the channels started giving static.

Excepting the dirt, it’s otherwise in excellent condition.

The Only Problem (so far) – Static on the Left Channel

After putting it together, and firing it up, there was interference and static on the left channel. This could have possibly been the channel, or the speaker plugged in. So I switched the speakers over, same outcome on the other speaker.

This indicates the unit is at fault.

Fact Finding – Cursory Googling

  • Several web forum threads indicated that static on a channel on a digital audio system is failed/failing capacitor(s).
  • Capacitors should last around 10 years if they’re not abused
  • This unit came out in 1996 (25 years ago), so any working capacitor is beyond expectations
  • Normal tolerance on a capacitor is about 20% (unless otherwise specified), so a 10uF capacitor could be 8-12 with no concerns
  • As capacitors age, they fall out of tolerance (eg 50% different from their rating, or 100%)

So basically I’m operating under the assumption that capacitors have probably failed. I’ll reiterate I’ve never done anything like this before, so may be completely wrong, but that’s okay. I’m learning a lot.

So, first things first, lets crack it open.

Five screws later and the top pops off easily, and I was able to identify three capacitor types after further Googling:

The blue ones are Electrolytic Capacitors, which are polarised, the green ones are Polypropylene Capacitors which are not polarised, and the small orange ones are Ceramic Capacitors, also not polarised.

All capacitors have some sort of label on them so you can determine their specifications, with the Electrolytic capacitors being the most informative right on the unit itself (the other two just have a model number, like N 22 or 223J, although they give good information itself by giving the capacitance, where to find the decimal point and the order of magnitude. So 222J is 2.2NF, while 223J is 22NF and 224J is 220NF).

The big old chunky capacitor, for example, says it’s 10000 microfarad and can take up to 25V and handle up to 85’C. On the Electrolytic Capacitors, the black line under the arrow says which terminal is negative.

I have the spec, now what?

I need to know whether the capacitors have failed. After close inspection of the board, I can’t see any visible signs of failed components, which is good news, as the non-capacitor components probably haven’t failed. But I also can’t see any failed capacitors, which tend to bulge when they’re completely dead.

So, maybe they’re just partially dead? I’m going to have to test the capacitors, as I don’t really want to replace all of them and find they’re all fine and it’s something else. To achieve that end I returned to Google and found you can buy multi-meters that can test capacitance (my standard generic yellow multi-meter doesn’t have that function).

Off to AliExpress, and I find a smart multimeter (model S11, under several brands) that can not only test capacitance, but it can automatically switch between modes, has a light on the back and a lithium ion battery (so I needn’t keep replacing 9V batteries).

$30 later, and about 4 weeks, and it arrived and absolutely lived up to its hype, very easy to use, turn on, touch the terminals and get anything I want without being in the wrong mode. It’s very cool.

So now I can test my capacitors. I flick through each one and have hit the most difficult challenge so far, balancing the multimeter on the capacitors so I can both test capacitance and take a photo.

Four hundred photos later, I achieve it, the first capacitor, which is rated for 10 microfarads:

10.8 microfarads! Well within the 20% tolerance, it’s a good capacitor. I work my way down the list, find a couple 15 microfarad capacitors that are rated for 10, which is out of tolerance and should probably be replaced.

And then hit this bad boy, which is a 10 microfarad capacitor:

300 microfarad! That’s a little bit out of the 20% tolerance of 10 microfarad. So that confirms it, definite 100% capacitor problem. It may turn out to not be the only problem, but regardless it is definitely a problem.

Waiting for parts

So now I know I will definitely need to replace capacitors before moving on to seeing if there’s further issues.

I found as many different models of each capacitor type as I could identify on the board and just ordered kits like this from AliExpress:

They’re cheap (510 capacitors for about $15, or even less, 960 ceramic capacitors for $10) and I’ll be able to use them for future repairs. It also means that I’m equipped to replace every capacitor on the board if I need to, but have already identified many that don’t need replacing.

From the top of the board, I was unable to test the ceramic and polypropylene capacitors, so I’ll need to test them from the bottom, something I was hesitant to do before determining the problem.

So next steps are to wait a few weeks for the new capacitors to arrive from China, in the meantime I’ll take the board out, map all of the capacitors on the top, then flip the board over and test all of the capacitors from the bottom and identify every capacitor that is out of spec.

Once I know which capacitors need replacing, it will be a simple solder out solder in job once the capacitors arrive then test the sound output.

To be continued…

2 thoughts on “Repairing a Cambridge Soundworks Model 12 speaker system

  1. What happened with this? I’ve got one that doesn’t power on. I was things caps. First thing I did was check the output of the power supply and it was showing 17V! That’s about 20% high based on it’s rated 13.5V. So that leads me to believe something burnt out after running hot.

    1. Heh, sorry, I took a map, was ready to do the work then moved house and its presently buried in a box somewhere in the garage.

      Possible, anything visibly burnt out?

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