The heads gradually get magnetised and the effect is that they start to get a little dull on replay, and can even start to reduce the level of what is on the tape with each pass. They're really simple - essentially a coil wrapped around an iron core that pokes out of the housing - they're held in the hand, quite small really. The 50/60Hz AC mains creates a very strong magnetic field. You bring the tip in close to almost, but not quite touch the head, where the gap is, then you slowly increase the distance until you are 2-3m away, then switch it off. You saturate the head with the new strong field, then it gradually reduces and you end up with a head that is unmagnetised. If you switched the power off whiler still close, then the head would be left magnetised, so it was vital to reduce the field strength slowly by using distance. A demag and head clean was important for any recording that was important. I used to do it once a week or so, but at work, it was every other day. As your problem is the machine, I'd certainly order one - they're not expensive, but it could be that the machine isn't set up for the type of tape you have? Was the machine and the tape ever OK, as in, has something changed. Bias could need checking - You need a scope but I think Akai used 100KHz and 10V, something like that. There are a couple of preset pots that adjust the bias - the aim is to get identical output from the tape on low and high frequency tones. If this adjustment is out, you get a loss of HF normally, and of course it depends on the tape you are going to use. You will need a scope for these checks - a normal meter isn't much use.
