If you have a history of back pain, you have to be very careful for it can tend to re-occur if care is not taken. Posture is a very important thing while doing any activity, even on the cycle which is going to be used as helping you alleviate your health problems.
While on the cycle, please ensure that you are not bent too far forward from the shoulders, rather that you should bend from the hips so that your upper back is not rounded. Then again, your knees should not be excessively bent beyond 90 degrees to your thighs.
If you are doing the rowing action make sure your movement is from the hips and torso rather than too much from the arms. This will ensure that your upper body is moving as one unit and will cause less stress on your weak joints.