Sony SL-810D Beta VCR Tech Question

[packrat79]

I service beta VCR's for resale, and right now I've got a Sony SL-810D that's driving me up the wall. When I acquired it, it was totally dead. Replacing the STK regulator on the power supply got it going, but the tape would only operate in reverse (even when it was supposed to be playing normally) and was also going way too fast. I replaced the capstan motor with a known-good one, this restored the correct bi-directional function, but the speed issue remains. When in normal playback mode, the tape runs at near fast-forward speed. When in slow-mo, it runs at just above normal playback speed. Pause, fast forward, and rewind work normally. The capstan free speed adjust potentiometer does have some effect, but doesn't bring the speed anywhere near within range.
The problem is present both during playback and record, and it correctly senses BII or BIII recordings, ruling out any issues with the A/C head or associated circuitry. I've spent countless hours pouring over the service diagrams, and have swapped numerous IC's from the ST-18 board with known-good ones from another VCR, all to no avail. I've also checked all the caps in that area for ESR.
Without a thorough knowledge of VCR design, all I can do at this point is swap parts. I thought I had enough skill, but I cannot grasp enough of this particular circuit design to effectively troubleshoot the problem. A good place to start would be knowing where to find the reference voltage or signal that determines the overall capstan speed.
Can anyone give me any ideas, or better yet, does anyone have another one of these units for parts?