Wednesday, January 27, 2016

AEG MC 4455 repair and mod

I bought this unit as broken for what I thought it was a good price, ~10 EUR while the retail price is around 50 EUR. Since I usually buy stuff that's DOA (Dead on Arrival) the fix turns out to be most of the times a cold soldering joint or a blown capacitor.

But first, in case you haven't bought one, here's a

Quick review


I'm surprised that in this day and age manufacturers still produce units without an auxiliary input, such as the one pictured below.


It claims to have 10 Watt output (maximum), infrared remote, can play MP3s from a CD or USB stick and has a radio. Let's review all of that, again:

The 10 Watt audio output is produced by a SJ2038 chip which can provided a maximum of 2x3.5W into 4 ohms at 5 volts with a THD of 10%.

The speakers are 6 ohms which would mean probably around 2x2.8W output at 10% distortion. Probably less than 4W for decent listening.

To be fair, for such a low power the speakers sound decent, much better than all of the cheap Bluetooth portable speakers out there.
The minimum volume is too loud for nighttime listening, as all Amazon reviewers are complaining. Also the blue backlight is annoying at night.

There is audible hum and hiss during quiet sections at all volume levels. I thought I could also hear quantization noise from the DAC, but I'm not such an audiophile.

While playing MP3 the entire disc or stick needs to be scanned which can take almost one minute. That's the case will all cheap system-on-a-chip MP3 players. During songs there is a significant seek and buffering pause. Before and after the seek phase above, the amplifier is disconnected and reconnected, resulting in a loud pop.

I haven't received an IR remote with my unit but I was able to 'create' one on my Galaxy S5 using the MyRemocon app and the Samsung LX-6000 preset. There's a lot of functionality that's only accessible using the remote control.

The radio has decent reception but is slow to scan and has again an audible pop while switching between stations.

Conclusion: there are much better units out there, probably for even less money. It does look nice though. Save your money and buy something else.

Repair


My unit came with the CD drive not functional, missing remote and it also would not read USB sticks. The USB port was as well not receiving any power.



Left side is the front panel with LCD, on the right is the main PCB, power supply at the bottom, amplifier at the top, not visible because of the MP3 decoder and CD driver carrier board sitting on top of it.

Notice the white 2-pin large connector in the middle-left part of the main board - it's the regulated power output for the entire unit, providing 5V. In a comical twist, the red wire is the ground while the black one is positive. You can see the carrier connector (center and above) has the correct labels.

Since I don't want to mess with 220V the first thing to do is to solder some wires that provide a safe 5V supply. Green is positive and blue is negative for the extra thrill of mismatching the polarity:






 The all-in-one A2495F chip does most of the work, has no datasheet but at least a description from Avid:

CD Boombox Servo , CD/MP3/WMA , CD/CD-R/CD-RW, ESP : CD 15/60/120sec , MP3 30/120/240sec(128bit) , WMA 60/240/480sec 

It seems to have a matching partner that is able to read USB drives, the A6001L: USB Host , Peripheral Part of Digital Boombox.

There's a Hynix memory chip that's probably used for buffering and skipping protection and a D5888S chip that drives the CD servos.

Underneath sits the 'power' amplifier:





The 12MHz crystal in the middle-right part provides the clock for the USB host part.


The first thing to do when something is broken is to check for voltages. I could see that all the voltages were correct but they all dipped heavily under load.
To a trained eye the cause might seem obvious but it took me almost one hour to figure it out:


The ground lead was not crimped and pushed properly into the connector. Obviously it seemed fully inserted, the picture was taken after I started pulling on wires and tapping on parts.

After fixing this, the CD player started to almost work but the USB port was still unpowered.


On the underside of the carrier board sits the radio IC, Beken BK1080 alongside a 32Khz clock crystal. I'm not sure how they do AM reception with an FM chip.

Grab the datasheet while you can since it's marked as 'proprietary and confidential'.







Looking more at the bottom side of the carrier board we can see some unpopulated pads, traceable to the LIN and RIN pads. I assume this was supposed to be the amplifier frontend for the auxiliary input, which, for cost reasons (0.25$), was not added by the manufacturer.
I suspect enabling it requires some pin from the A2495F to be set low or high, but without a datasheet it would take forever to find out which pin.
Probably once the AUX is marked as available, hitting the INPUT button on the front panel would toggle between FM, CD, USB and AUX.

On the bottom side of the main board we have an APM9435 30V 5.3A P-Channel MOSFET.




In another corner we have the matching USB host sibling for the 'boombox chip', A6001S:




I could not see any power going to it - so I just took a shortcut and shorted the MOSFET drain and source with a wire.
The transistor was supposed to switch a 3.3V supply on and off.
After this, the USB interface started working.

Having the wire there at all times might explain why the unit sometimes makes pop noises or other weird sounds.
There is no additional heating or power consumption involved but the USB port is only able to supply ~100mA. Either the 5V power supply is too weak or there are more messed connectors/joints.

Tweaking the CD player


With the unit mostly fixed I noticed that not all CDs were being read.

Not sure if that was from the random bodge wire coming from the front panel that got detached just by looking at it:

















However, we do have some adjustment pots on the CD player servo board, there's nothing to lose by messing with that, except overdriving the laser.



After a few minutes of tweaking the player seemed to reliably play badly scratched CDs. It even played a bad 90s euro-techno megamix:



The power supply voltage had to be raised as the voltage drop across the wires was too big at peaks of 1.7A consumption.

Modding


Well, the unit was working fine now but was pretty useless. You could not use it to charge your phone and nobody listens to CDs anymore. Time to add an auxiliary input jack.



A headphone cable was stripped off of its drivers and the wires were soldered to the terminals going to CN6 (see power amp picture way above) marked PH-R, PH-L and GND,

Also, a switch was placed between the SEST (?) terminal and 3.3V. By scoping around I've determined that this is the chip that's responsible for turning on (actually un-muting) the amplifier. The switch can stay on at all times but it will mix the aux-in on top of whatever is playing (FM). I think that's a feature, but it has the disadvantage of introducing noise.

The switch was affixed to the back cover with hot glue. Near it you can see the headphone wire with its strain relief. The strain relief is there to make it harder to yank out the wire by mistake. There's also significant strain relief and hot glue inside the unit.



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