After reading all 33 pages of this forum, I don't believe I have seen anyone reporting the problem I'm encountering now, which is, my Sony DTC-670 will not fast forward but it will play and fast rewind. I have downloaded a service manual, so what I describe here is what I read from the service manual. When I press fast forward, it appears the F/R lever assy (part #369) won't engage with the take-up reel assy (part #366), consequently it will produce loud grinding sound as if the two gears are trying to engage in order to fast forward. I have taken the mechanism apart several times, remove all the sticky grease, cleaned all the mechanism, and had recheck everything but couldn't find anything else that needs to be fixed or adjusted.
Tonight, after studying the service manual, I noticed that on my machine, I don't see a washer (part #367) installed on the very top of the take up reel, so whenever I press fast forward, it will somehow push up the take up reel a bit and consequently the gear from the take up reel won't engage with the F/R lever assy, thus preventing the mechanism to perform the fast forward function. If I tap/hit the machine a bit while in fast forward mode, the gears will engage and it will fast forward. By the way, when I said it doesn't have a washer on the very top of the take up reel, I don't mean the washer pop out or that it's missing, I meant the take up reel's rod doesn't have an indent on the top to fit a washer. I find this highly unusual but I assumed it somehow worked for many years with out the washer from the previous owner. I'm quite sure if there is a way to install a washer to hold down the take up reel, it would solve this problem. First of all, has any of you heard of this before -- I would call it manuf. defect? If so, or if not so, how would one go about modifying the take up reel's rod to fit a washer on the very top?
There is one more unusual thing about this machine. The service manual indicate that there is a back tension solenoid spring (part #390) installed near the back tension solenoid but my machine doesn't have the spring. I suppose this can also cause the fast forward not functioning properly. If so, is there any where I can buy this spring?