Hey guys, sorry about that -- didn't know anyone was waiting for a fix. Actually, mine is stil in pieces. I found the problem though -- there is a solenoid that keeps the FF and RW from releasing. The fix will actually take some troubleshooting and then waiting for parts. Without a service manual, it makes it a bit harder since I can't do any troubleshooting from the desktop. Rather, I have to actually look at the PCB and try to reverse engineer the intentions. The problem: Space. I need a workspace that will allow this guy to be disassembled for at least a week or two while I wait for parts. This guy doesn't have a internal chassis like other boomboxes so once apart, it will be all over the place.
My guess: There is a power transistor that powers the solenoid. I haven't found it yet. My guess is that it's shorted. This solenoid is for the AMSS search feature. During the search function, it actuates to hold the FF and RW in place. When it deactivates, the FF or RW will stop and allow the normal play function to resume. Since it's on all the time, the stop function does not work.
Other possibility: This one is more problematic but less likely since this circuit uses low signal levels instead of the power transistor circuitry which for obvious reasons, is more prone to failure. I'm talking about the AMSS search feature circuitry. Without a manual, there is no way to diagnose or fix this, if it indeed is the problem. But if it is constantly sending an activation status signal to the solenoid, then obviously, the solenoid will always be engaged and prevent the stop function from working correctly.
The WORK-AROUND: Good news. If your SK-550/SK-750 is having this problem, merely disconnecting the solenoid will permit normal tape functions. Obviously, the search feature will not work. But at least you can use your boombox normally otherwise.
Try the workaround Glucifer and let me know if this works. Someday, I will find out which transistor is causing the problem and if I find a solution, I'll let you know.