Fixing a Maplin EAP450 Car amplifier

This amplifier is used for parties at The Oak Tree where we have no power on site – a 12V leisure battery is good for about 7 hours. I’d noticed one of the pots getting scratchy so one of the four channels was low. This is a four-channel amplifier, and I use it bridged 2×2 as my speakers are 8 ohm. The amp works but I’m losing power, experience tells me this sort of fault doesn’t fix itself.

The go-to site for car audio tech is Basic Car Audio Electronics so if you’ve blown your amplifier and are looking to fix it then that’s a good place to start. The tl;dr version is it’s probably not worth your while, get a new one. The trouble with blowing the amp is not only do you have to find and change the blown part, but also establish the rest of the parts are okay. I’m just fixing a passive part in an otherwise normally functioning amplifier. It’s probably still not worth my time but it’s criminal to junk the amp for the sake of £2 worth of parts.

I did get a meter on one section of the pot and established that it is intermittent and ratty. So I didn’t need the schematic. It’s nice to have, though, but knowing Maplin I figure it’s probably not worth calling them up and asking them to send a PDF or a dead-tree version through the post. I’m still sore about the way they washed their hands of a portable TV that failed after 15 months. On investigation it has clearly not worked right and been incompetently hand-repaired at the factory leading to the screen failure.

Inspection of the circuit board and a bit of Googling turns up some leads. I observe the circuit board is marked APA-450 Rev 2 and Googling APA-450 tells me that Clarion used to make one of these, and the control panel looks suspiciously similar. So now I know what a Maplin EAP450 is – presumably they bought the rights to rebadge this discontinued Clarion amp in a different housing. Or perhaps there’s some noname Chinese company in Shenzhen hawking their production line to whoever will pay 🙂

I got the service manual and schematic from those nice guys at Elektrotanya and established that the part numbers tally with the board.

plan A – changing the pot

Ebay was my friend in sourcing the pot, as it happens also from China. One of the great things about Ebay is the variety – it’s important to get the length of the splined shaft right so it doesn’t stick out too far and this is a slightly oddball length. A 6mm spline shaft is close enough.

 

I got that right, but I was caught out by the pitch of the pins being smaller than normal. I’d already dismantled the amp before I spotted that. D’oh.

Take a photo before you dismantle the amp

You can’t have too many decent photos of the gear before you take it apart . Once I had taken out the screws I got to see the PCB – it’s clearly not the finest reflow job I’ve ever seen

 

plan B – fix the original pot

Time for Plan B – dismantle the offending wafer of the pot.

 

By carefully bending the tabs I could get the wafer out and look for the problem – there’s a faint patch of shiny something not right at the top left of this, which I cleaned off with some isopropyl alcohol. Plan C was going to be replace this pot and the other one with two fixed resistors per wafer. There’s no real need for this sort of control on a power amp. Ideally I want to line the amp up for full-scale output at +4dBu – there’s far too much gain and it needs pulling well back. the line level input sensitivity is specified as 200mV to 6V

final test – success

 

This is the amplifier at just over 40V p-p showing the two bridge amplifier outputs which are out of phase (I am too cheap to get the probes to set the read out right, so it shows 1/10 of the real sensitivity)

 

and the final trace shows both bridged sides going into clipping at the same time.