make notes as you go....
some specific notes- which tracks are affected, given a fixed input signal, what viewmeter readings do you get....
if the tape head cleaning, new tape, external connections don't cure it, it could be the channel cards - there are 8 seperate but identical cards, accessed thru the bottom, each has a relay on it, and some adjustments for bias, etc. sometimes the relays stick or get contact corrosion. Cycle the unit on-off, and record to playback, to exercise the relays. Did any of the channels clear, or get better. Try pulling and swapping around the channel cards and see if the weak / dead channels follow the cards.
I had a similar problem when I first got my 38, alot of exercising the relays cleared it up.
good luck - they are a great and fun machine. Do hope you got a manual - very important, and a lot of good info in there.
b-h