Akai 4000 ds make sound issue


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Sound seems low and slightly muffled on this system. It's been stood for a while. Replaced the belt. Gave it a clean. But the sound seems low and muffled.
Cleaned the heads and ran a couple of reels through. Seems to be getting slightly better but any ideas on how to improve or fix this TIA
It does rather depend on so many things, but the first thing to check is that the recorder doesn't have an audio problem, so connect a known good sound source to the line ins, like an mp3 player, your phone - that sort of thing. Feed it in, select the right routing so the input comes out of the output and check that the audio you are hearing is clean and not muffled. That done, you've proven the electronics from in to out are trouble free. I suppose we're also assuming it doesn't have worn heads. If the output is low and muffled, so much so you spotted it easily, then we need to work out of the problem is on record or replay, if you have cleaned it properly. Do you have multiple tapes with something already on? Are all these muffled and quiet? Could they just be terribly recorded? Can you record on them from your known good sources? Is that muffled?

Is there any chance somebody has been tweaking the alignment? Bias and other presets, like replay EQ can cause these things. You're going to have to go through the alignment - is this something you can do?
I did think about alignment. But couldn't find any info regarding this. It's been a few years since the unit has been used. I have tried recording and playback is still muffled on 2 separate hifi outputs.
No you missed the point. Does it sound muffled with the sound just going through the recorder, not via recording. Alignment just puts the machine back to standard. Do you have the ability to do it? See if you can download a service manual and see if the instructions make sense. You probably neee to buy a demag too.
Yes it sounds low and muffled just on playback of tapes. I have the service manual as I replaced the belt. Could you explain what the demag does please. Also thank you for your responses so far
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.
The machine used to play and record fine on the tapes I have. It's just been stood for a while and not been used. The belt needed replacing which I ordered and replaced but the sound of a pre recorded tape and fresh recorded and playback was muffled. Will look into the demag. Not sure what you mean by the bias though?
The record system uses a high frequency tone - way above the audio spectrum to make the transfer to the tape take place on a linear section of the frequency curve. Different tape needs different bias current, but it impacts recording only. If it is wrong, then recordings can sound very dull or very bright on replay. If old, previously good tapes now sound dull, then it's probably dirty heads, but I'm sure you've cleaned them properly, or magnetised heads, or worst case, electronics that have 'gone off' during the down period. Capacitors can dry out, and these usually impact on the frequency response. Look at guitars, the one little capacitor in the tone pot does such a lot.