In an effort to keep these machines healthy I've noticed some slight differences in the capstan circuit board.
There are some which have 3 IC's soldered directly onto the board (the large board that's beneath the capstan and extending to the right where the power/pitch/speed/timer switches are soldered to it).
The others only have 1 IC soldered directly to that board and have a small daughter board with a screw attaching it to the chassis at the back. One of the IC's on this small board is different from the one that's directly soldered to capstan board.
The easy way to tell the two apart is simply to take off the back of the machine (two screws and the four feet) and look to see if your 1500/1506 has the small board which has wires leading back to the capstan board. When you take off the back, the small board is attached to the top of a crossmember just to the right of center (almost touches the bottom shield of the capstan motor).
If you could post your machine type (1500/1506) and the serial number which is on a badge on the back cover you just removed I'd greatly appreciate it.
Thanks.
BTW I have one of each.