Yes, electronic service for consumer units has largely died. The W&F measurement you have seems good to me. We would measure it on high end units that had mechanical drives and listen to a bit of a record we were familiar with on any unit before it was shipped. The speed drift you observe was not too uncommon, one of the problems if the machine had a strobe as it would report to the owner speed variations they wouldn't notice otherwise... something I had to explain a time or two. I have a belt driven Dual 1245 that speeds up just a touch from a cold start to warm, a few minutes. I either ignore it or let it warm up a couple of minutes. It's not uncommon with the mechanical speed control of the Dual, noticed them on the bench... it's a small variation but it is there.. I keep it adjusted so it just crawls ahead when cold and when warm it's practically still. Quartz locked should be right on if the quartz hasn't drifted... but if it has, so long as it stays, the speed will lock to it, for good or bad. When quartz lock first came out it was a switchable function, but soon that was no longer standard and one could not tune the speed, at least not from the top... I'm sure the benefits outweigh any drawback but I always liked to be able to adjust the speed a bit. I wouldn't even begin to suggest a reason for the speed drift on your SL-1500 but I'm sure it's something to do with the temperature of some component in the circuit.. depending on how much it is drifting upward it could be doing that and be within it's specifications. One thing I might say is be sure the center spindle bushing is clean and lightly lubricated, sometimes old lubrication will change it's characteristics over a few minutes of use and affect speed. On a quartz locked table, they hardly ever drift... might be slow or fast but hardly ever drift.