Sharp GF-4949 stupid deck design!

Hi all, I've decided it's time to get all the decks working on my boxes, so I started with a nice simple box, the Sharp GF-4949.

The deck is a plastic mechanism with one square belt. It rewinds perfectly, but doesn't seem strong enough to play or fast forward. On closer inspection, I've found that the mechanism moves one way for rewind and the other way for play and fast forward. When it moves in the direction to rewind, it stretches the belt very tight, thus making rewind extremely powerful. But when it moves in the other direction to play or fast forward, it slackens the belt, so it hasn't got enough grip to perform those functions. So for the belt to be tight enough to play or fast forward, it would end up being extremely tight indeed when rewinding! Why doesn't it keep a similar tightness for all functions? What a stupid design! Has anyone else noticed this type of thing when rebelting boxes? I have flat belts but no square ones so I can't fit a tighter one just yet, but I'm ordering some.

Another thing, when the deck runs properly (with no cassette in), is there a particular reason why play runs as quickly as fast forward? (The motor doesn't have an adjustment screw on the back.)

I'm really interested in people's thoughts on these points, particularly the design issue!

Typically, the tape take up reel runs fast to ensure all the slack is taken up. The actual speed is throttled by the tape being run through the pinch roller & capstan. Without a tape, there is no throttling so the take up reel will not run at the normal operating speed.