Pioneer CT-F1250 Wont stay in play mode - Its NOT the motor or belts.

Hello fellow Hi-Fier's!

I am lucky enough to have a beautiful, and highly problematic CT-F1250 cassette deck that I have been working on the last month restoring. I have rebuilt the entire transport with new grease, new belts, new pinch rollers (thank you czech guy on the webs!), new idler tire, and yes, the infamous RXM-047 has been rebuilt (twice, since it didn't quite take on the first rebuild).

My problem is the typical one with this deck, it will Fast Forward, Rewind, stop, pause, the slac take up circuit works beautifully, basically this deck is working perfect, except for the playback function.

With a tape in the deck, play being hit, it will play, then stop in less than a second, every time. I took a test tape that had a low amount of tape on it, gave the take up reel side a bunch of slack, hit play, and the deck will operate in play mode as expected, until the slack gets taken up, then it stops.

I've checked voltages on the motor, in FF & RW it operates at around 14.44vdc CCW/CW, in play mode with a tape in the deck, it operates anywhere from 8.9-9.1v, without a tape, it operates at around 12v steady. Voltage is actually very consistent on the motor. I've tried different belt diameters as well to see if one thats slightly more loose or tight would make a difference, which it did not.

I'm almost 100% sure its not the motor based off of its operating voltages (unless I glossed over that part in one of the rebuilt threads online), nor the belts.

This is leading me to think it is something electronic, which is what I've come here to ask about, since the playback issue on this deck is almost always the reel motors fault, according to the internet. I have not been able to find anyone with this issue, that wasn't related to the transport. (IE not mechanical, but an electronic issue). So I come here, in hopes that I can get some guidance, since I'm at a loss and I am desperately wanting to revive this deck to place it with my Pioneer Series 20 system.

Any help at all, is of course, appreciated.

It's been more than 30 years since I worked on one of those, but... it sounds like a problem in the Tape Slack Removal or Auto Stop circuits. Some of the components are common to both. The basic functionality and circuit descriptions are on pages 20 and 21 of the service manual. You'll probably need a scope to check for pulses generated by the photo interrupter and other voltages that change quickly when the deck switches off.