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I think it's going to be a struggle to diagnose without an oscilloscope.

The TDA7348 appears to have 3 identical stereo inputs. Pins 8 & 12 for the radio, 9 & 13 for the tape, and 10 & 14 for 'Aux' which I'm guessing is the CD in this instance.

The tape head pre-amp looks like it's a dual op amp - either an RC4558 or LM833. However, there is a mute circuit between the output of the preamp and the tape input of the TDA7348 which is controlled by the microprocessor. If the mute is being turned on and off rapidly it could sound like distortion and would affect both L & R channels.

So basically I'd want to 'scope the output of the preamp, the input to the TDA7348, and the mute circuitry. It should be obvious from these where the distortion is occurring.

How are the computers connected?

You won't get near a gigabit unless you have a cabled connection to the hub.

hi .just checked yes sorry its a 7348 ic !.  im on a similar way of thinking and did power the drive motor seperate from a 9v battery ! still distorted!
this is why its driving me mad.if the input to the 7348 works fine (i swapped this chip from other unit) and the output before it is good when fed into another amplifier then the only common link there is the coupling caps but swapped them too . its theoretically working perfect on paper but not in reality .   
ai is totally useless on questions like this too as it tells me i could have a faulty tape,dirty or misaligned head ! all basic stuff which are eliminated by swapping deck complete .
please dont let this get into your head too and drive you mad as well ! its just something that shouldnt be happening and is common to a few units now so once i find the offending part it should mend them all !

Perhaps it is starting to find it's  mojo.

Ookla just showed 1018mbps down and 906mbps up.

Install went well and the engineer performed a speed test that showed more than 1000mbps both up down.

Thing is, when I do a speed test with my computer I average around 230mbps both up and down, and only about 15mbps on my tablet that has a free VPN.

Mrs Opti's computer only shows around 75mbps up and down. Although her computer is 10 years old.

So where do they get the 1000mbps figure from?

Ok, survived Clarks village - just.

Thinking about it on the drive home, it seems unlikely that you would get distortion on both left and right channels due to a single fault if the fault is in a capacitor or resistor. I mean all the circuits I've looked at the signal path is basically separate for L and R, so a single capacitor in the signal path going duff might distort one channel or t'other but not both. The chances of both L&R capacitors failing at the same time is tiny, and the chances of the same thing happening on multiple radios? Nahh, Occam's razor says it's something else.

You mention a TDA4748. Are you sure of that part number - or could it be a TDA7348? TDA7348 appears to be an audio processor & switch, switching the input between radio and tape inputs, and is used in lots of Blaupunkt stuff. TDA4748 is an obsolete switch mode power supply chip, and I can't see it on any of the circuit diagrams I've found.

The other possibility is power supply. When the tape deck motor is running the unit will probably be drawing 1A-2A more that when it's running on radio or CD. I'm wondering if this extra power is enough to cause the supply to drop and/or get noisy, enough to upset the supplies to the analogue sections of the tape and speaker drive circuitry. That would be enough to cause distortion on the 'tape' inputs, but allow the radio and CD to operate normally. It would also affect both L & R of the tape equally. It might also explain why if you pick off the tape signals and feed them into an external amp there is no distortion - the speaker drivers will also take a lot of power - perhaps there is only enough power to rune either the amps or the motors, and not both?

To test that theory I'd locate the supply pin to the tape deck - probably +12V - and power it externally from a bench supply. Need to be careful though - you don't want to blow up the cassette motors.

May he Rest in Peace,  driving home at Christmas such a fantastic song at this time of year he certainly won't be forgotten.

I’m in the midlands at moment. Just had a pleasant drive around a housing estate looking at peoples lights.

Yeah. Why bother yourself when you can just look at everyone else's 

hi .no its definately not a deck issue ive swapped decks ,that was the very first thing i did years ago and now as well .i dont have a signal generator or oscilloscope but yes if i seperate the sound circuit after the head preamps and the main input ic (at the coupling caps) then i get a good sound out AND a good sound if i feed into the main ic so this should be impossible .one way should be faulty but it isnt .im back on it now and looking at dome smd caps see if changing them makes a difference !

I’m in the midlands at moment. Just had a pleasant drive around a housing estate looking at peoples lights.