Unusual "scratches" on the wood with LT70

[CX3]

I'd say you have a flat spot on a bearing. It is definitely in the large bandwheels. Something isn't coming around square and flat.

[YellowHammer]

I noticed you checked quite a few things, and lots of good suggestions made.  however, I have observed that most times it is due to high speed blade bounce, sometimes due to harmonics, sometimes due to foreign matter on the band wheels, sometimes due to incorrect tension, or even not removing tension after sawing and slightly flat spotting the tires.

Generally, this is easy to spot because if there is blade bounce it will manifest itself as the characteristic ridges as soon as the band enters the cut, especially with a soft log and sharp band.  Also, as soon as enough sawing force is applied to the band (speeding up), the bounce will be dampened out and the ridges will smooth it.  Slow down, they will reappear.  

If this is suspected, then it will be bouncing or vibrating well before a cut is started, so it's easy to spot in advance.  Engage the band full speed and look at the band as it's running.  It should look as stable and flat as a laser beam, no shaking, boucing, fluttering, etc.  If the band is running and sitting dead quiet, then the cuts will be dead smooth when entering a log.  Of its doing a high speed flutter or bounce, then that flutter will cause ridges in the cut and the blade vibrates up and down.  Sometimes simply applying more band tension will help round out of mask whatever is causing the vibration.  Sometimes other things cause it like sawdust packing between the band and wheels, or under the wheels.  Either way, most times it's a symptom that is detectable and fixable before the first cut.