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Tascam TSR-8 recording issues

Tascam TSR-8 recording issues

Hello everybody, I hope somebody can help me

I recently got a Tascam TSR-8 in very good physical conditions, the previous owner told me that he was a DJ and bought the machine brand new just to mess around but never used as a multi-track recorder.

I own a home studio with basic equipment and I have been recording stuff for several years, althought this is my first open reel recorder, so I started reading the manual, studing the basic set up, I got a brand new tape and finally made my first recording las week.

I first recorded a known song from my CD player in every pair of channels (1&2, 3&4, 5&6, 7&8). The signals at the peak level meters were just coherent (around + 3 dB) while I was actually monitoring on reel time every recording from the outputs of every channel and everything seemed just fine. But when I played back the song I had recorder, I found a weak signal (around -3 dB) and the sound is like trembling or vibrating in all the channels.
After that I made a new performance, this time recording a 1 KHz tone (got it from my computer as a test signal) on every channel, and the same trouble appears again. althought the periodic vibrating effect with the pure tone is more obvious.

I know it must be a calibration issue, I have electronics knowledge, so I already got the maintenance manual and I´ll get ready to do it myself as I live too far away from any service center but before I wanted to ask if any one has experience with this machine or this kind of trouble?

Any answer would be helpfull