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.
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.