Hi Patrick,
Welcome to the forum.
The first thing I'd do, is try using a source other than the internal oscillator (maybe a cd player) and go 2 channels at a time recording and then playing it back. Just use the same length of tape if you can. It could just be the connections to the oscillator.
One very common problem with tape machines is the switches get oxidized or dirty. Even a machine that's fresh out of the box but is 20 years old will have this problem. Try working the switches back and forth to
clean a contact patch so the signal doesn't have to fight to cross the contacts.
It's a good idea to get yourself a manual and familiarize yourself with the way the signal is routed through the different components (record amp, mixer, playback amp, VU meters etc). That way, you can figure out the different switches and pots that the signal goes through (routing). Without knowing that, you're just guessing.
You should be able to download a manual here;
http://www.analogrules.com/So, give the CD a try and please be as detailed as you can be regarding the symptoms.
