Tascam 414 Mk1 - one channel recording very low volume

[unguitar]

Hello, my recorder records on 3 tracks correctly.
The first one delivers a very low signal.
While recording the VU meters are running fine, showing the correct signal strength which, on the other channels delivers the right volume when recorded.
Of course I've gone through all the double checks I could think of.
Anything to suggest me?
Many thanks

 

[SkywaveTDR]

Clean the heads- the other day a guy was asking me what to do on a unit and I said what did you clean the heads with and he said he had not done that yet. What is the reason to jump over this?