What is the bias control's function?
The Bias control is used during recording. If you turn down the bias,
you will get more highs, but the distortion will increase, and then
start to increase dramatically if you back off the bias more.
As far as playing tapes from different decks goes.......
1..Does the tape that you recorded on your deck sound OK on your deck ?
2..Does the tape that you recorded on your deck sound OK on other decks ?
If 1 and 2 are true, then your deck is OK.
If the tapes recorded on a specific deck sound bar on yours, but 1 and 2 are true, then the other deck is faulty.
When a deck leaves the factory, the head is aligned to a " standard "
tape. By standard tape I mean a very special tape recorded in such a
way, that a tech can tell if the head is out of align and "tweek " the
physical alignment of the tape head..
All decks are aligned just a little differently, but fairly close to each other.
first do the mechanicals....is the head of your deck spotlessly clean ?.
Use 99% pure isopropanol alcohol and a cotton swab. Never use anything
metal to clean the tape head. If you scratch the head, its toast.
If your deck is clean, plays its own recordings well, and those
recordings play well on other decks, then your deck is not the problem.
Hope this helps,