Nakamichi ZX-7 Cassette Deck - - - Dubbing Problem

So I just had the pleasure (hopefully) of purchasing a fairly high-end system from a well respected bandleader/composer 2 weeks ago and have been busy in my limited free time testing out all the equipment. Unfortunately, I have come up with a number of issues through the testing process. This question relates to a Nakamichi zx-7 cassette deck.

The play function of the deck works perfectly, however, dubbing is a bit of another story. When dubbing vinyl yesterday, I came across a problem with left channel being a bit quieter upon playback than the right channel. The levels are the same when listening through the “source”, however, when playing back the recorded tape, the left channel was noticeable quieter than the right by about 10 decibels. I tweeked the “sensitivity levels”, turning up the left side and down the right, but this time when I played it back, the left channel was completely dead, the right was fine. I then recorded again and didn’t move the setting at all and upon playback, the left channel had come back but was still a bit quieter than the right.

It seems to be a bit of an intermittent issue, wondering if anyone has any ideas on what might be causing this problem.
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If its intermittent, does it ever work completely right? (no 10 decibel loss). A couple of things you may want to try. Just to confirm, did you listen to the tape itself after it was recorded or just while it was being recorded? Also, it wouldn't be a bad idea to try one of the recording on another deck, if you have access to one.

It would probably be a good idea to clean the heads. While doing that, I would make sure the azimuth adjustment is working probably. I'm pretty sure the ZX-7 has one.

Other than that, you may want to take the top off and look the unit over really well. Something like a cracked soldier connection could be causing your problem. You'll just have to use your best judgement and look for anything out of place.

One last thing I just remembered. Tapes go bad all the time; espically if they are old. Even unused blank ones. I would definitely try several different tapes.
What Zd542 said is all good advice, but, it could be your sensitivity control pots are dirty. In that case a good quality contact cleaner (like Caig Deoxit D5) could help cure something like that, if you can have good enough access to the pots.
all good suggestions. I would also try another tape and made the recording again. If memory serves me correct, you can fine tune the deck for each tape used.
I can't say what is causing that issue, but Nak decks are complicated devices, a ZX-7 is pretty old, and yours needs a professional calibration, at the very least. send it to Willy Hermann.
I second Lloydc's suggestion. Send it for proper qualified servicing.
You will then have a really nice deck to be proud of!!