The Fault

This is an all-in-one Bluetooth, radio, speaker, CD and cassette player made by Philips.

It arrived with it’s owner, faulty – no sound coming from the right speaker. This is something that could have probably been returned initially, but with the chaos of normal life getting in the way it just became something they put up with as time went on. When they discovered I offer repair services they were naturally curious, and as we’re well outside of the return and warranty period for this device, it seems worth a shot!

The Plan To Reach A Diagnosis

There’s a few places the fault could be coming from – we need to narrow it down so we can make repairs.

• Option 1 – The Input Stage:
  We can think of each playback option (CD, Bluetooth, Cassette etc. ) as separate devices. A failure in the right channel circuitry for any of these input devices could cause our fault.
• Option 2: – The Signal Processing Stage:
  Some appliances will process an audio signal to provide certain benefits. This could be to take the input and change it’s quality – maybe run it through an equalizer to get a better base response, or crossover to improve the sound-stage.
• Option 3: – The Amplification Stage:
  Audio input signals are usually very small! In order to make them powerful enough to drive a speaker and fill a room with sound, they need to be amplified.
• Option 4 – The Output Stage:
  The speakers and any cables between them and the amplifier.
  

Ruling Things Out Logically

The fault is exactly the same regardless of which playback mode the appliance is in. This means we can pretty much rule out the input stages entirely because if the fault was in one of them, we would only expect to see the issue when using that specific, faulty playback mode.

• Option 1 – The Input Stage

Ruling Things Out With Tests

I started by probing the speakers with a multimeter to check for continuity. All speakers, wires and connections are sound and do not change when moving the about More importantly they’re symmetrical – no difference between the left and right speakers. If you switch the left and right speakers with some jumpers, the fault doesn’t change either.

That’s good! It means out speakers are connected and show no abnormalities.

• Option 4 – The Output Stage

The Last Two Options

• Signal processing
• Amplifier

Looking at the main board on this device will give us some hints:

There’s really not a lot going on there. It’s a pretty sparsely populated PCB and I don’t see anything tied into the audio circuits which I can identify as a DAC, DSP or powerful enough microcontroller to be doing audio processing. In other words, apart from tying together the system functions of each input device and routing things to where they need to go, the motherboard doesn’t look like it does a huge amount. That doesn’t mean to say it doesn’t have faults! Even still, starting with the amplifier is going to be way easier to test, so that’s what we’ll do.

So how do we test the amplifier without the signal processing stage getting in the way? We bypass it entirely.

Bypassing All Circuits Before The Amp

We’re lucky with this one – the amp is more or less just a single chip with some simple circuitry to support it, and better still, the name is stamped on the side so we can find a datasheet:

The second image shows something interesting (see highlighted text). This suggests that the amp chip will not power up unless a couple of voltages are sent to specific pins on it. Frankly, I’m too lazy to build a way to do that, so we’re going to cheat. We’ll use some jumper wires to wire in those voltages directly from the motherboard. Then, rather than plugging in the motherboard like normal, I’ll solder an audio cable directly from my own audio player straight into the amp board, completely bypassing any faults that may be present in the earlier stages.

With the device rigged up like this, it will mostly behave normally. Once we power it up and select an audio mode (radio is easiest), it will automatically send the signals to the amp chip to power it up to play sound. What’s different though, is that now we have my audio cable rigged directly into the amp circuit, so instead of hearing the radio as the device expects, it will be playing music from my phone.

The Diagnosis

… Bingo! We’ve got our fault.

The stereo shows exactly the same fault as before, even in this modified state. Now this is significant: The fault STILL EXISTS, and yet, the rest of the device’s systems have been entirely bypassed apart from the amp. So assuming there aren’t multiple faults, the amp is where the fault is.

So there’s two possibilities for a repair:

• Replace the entire amp module.
• Repair the existing amp module.

Now sadly, these amp modules don’t seem to be available to buy, so a repair is going to be the only option (That will still work, I just have to use my brain more).

Repairing The Amplifier Module

First up, is a thorough visual inspection. I have a microscope for this exact reason which is perfect for going over each chip on the board and making sure nothing looks obviously blown, knocked or scraped.

Honestly? It looks pretty clean! And probing the obvious parts with a multimeter doesn’t show up any abnormalities either: the filter caps look alright, there’s no blown resistors or short circuits I could find. In some sense that’s a good thing, because we know nothing’s caught fire – there’s no serious damage to the PCB that would need extensive repair, but it doesn’t get us much further ahead.

Fortunately, it turns out these amp chips are really cheap. It’s about £4.00 for a replacement part.

At that price, I’m opting to just re-solder the amp chip to replace it with a new one and see what happens. It might not catch the fault, but there’s a very good chance and at that cost, the cost of the chip is likely worth less than my time to diagnose the issue.

While we’re waiting on the part, I’ll de-solder the existing chip and clean up the site:

After that, installing the new chip really only takes a minute.

Almost, But Not Quite

Now this is interesting. With the amplifier chip replaced we’re getting some sound from the right speaker, but it’s definitely not 100% – you can hear the bass very clearly, but all the mid and high tones are completely missing. Almost all audio circuits will have some passive components on the either side of the amp to filter out unwanted noise or voltages. Seeing as this is a relatively simple circuit and we’ve just replaced the main bulk of it, it’s safe to assume that these passives are where the rest of our issues lie.

We didn’t see the fault as it happened, but here’s my theory: When the amplifier IC failed, it overloaded some of the filtering components on either side and caused them to fail too.

Fully Working

• That’s it! With the filter capacitors replaced, this device is up and running with both speakers and a good sound-stage.

It’s strange for a fault like this to happen from factory – it could be down to so many things: poor process engineering, lack of testing, damage in transit or simply – bad luck.

The Result

So this was a quick look at a handful of diagnostics procedures which can be used for a relatively simple repair – I hope you’ve enjoyed it. Seeing a device come to me with very obvious faults and yet no obvious damage, marks or burns internally can sometimes be a nuisance, but this was a fun fault to diagnose! It’s always nice to rescue a device like this from the landfill.