Here's a tip that might help a few people.
This cheap (£50) midi CD/DAB which I suspect has the same innards as a popular Crown model sold in supermarkets, had hum on quiet passages and the laser sled occasionally got confused - which I put down to mains transients.
So I'd resolved to upgrade the smoothing and filtering if I ever had good reason to open it up. A couple of days ago one channel failed so there was my excuse.
First of all the dead channel - the soldering looked as good as can be expected for lead free solder and continuity checks on the lead from the PCB connector to the back panel were OK, the connector to the h/phone socket may have been suspect but couldn't be proved so I removed the amplifier chip (AA8227PH-E1 - data sheet is on the web) and re-fitted it with real solder.
Now that both channels worked I set about the smoothing/filtering, the
25V/3300uF capacitor is grossly inadequate so I added a 2200uF to the rectifier leads which took care of the hum, the mains in lead is looped through a half decent toroid, so I wedged a 0.47uF chip capacitor between the pads where the transformer secondary meet the PCB and added a couple of 1uF mylar caps in parallel on the solder pads of the 3300uF.I've never considered myself one of the "golden ear brigade" but the sound quality seemed noticeably improved - the music definitely has more punch. The jury is still out on its susceptibility to mains transients but the laser sled hasn't got confused so far since the modification.