Hi Level, thank you for your kind offer, and opinion on this. Just a small update, (which is not good news). Last night I fitted a new drive belt to the 688, which was a bit tedious, but only because I had not read instructions properly. Once I had removed a few extra screws it was actually simple. Anyhow, after putting it all back together, it will go into Play/Record on the tape deck, Stop/pause/ Fast forward. Fast rewind seems to have stopped working. I am sure it worked before I did this, so maybe I dislodged a contact or something? Not sure. I am going to get a battery today, which is a simple thing to change, to see if it brings its audio signals back to life. I think that you might be correct about it being needed to select inputs. I am not sure about the Rewind problem. Maybe it uses separate motors for these functions? One of the motors did have a bit of an odd looking appearance on the outside of it, like melted black "plastic" down one side of it, which may have been a wire . i did not examine that any further , as I was concentrating in getting the belt changed. I will let you know how this proceeds, as it may be useful to you in the future. There are some very good posts about how to disassemble the 688, which makes the task easy. I think they were on this forum, or perhaps on 'GearSlutz" forum. My 8 track tapes were also tied into Cubase Midi files, using an old Atari omputer( which I no longer have), so are probably not worth the hassle. I was just more curious to get this machine functional again, as it seems to have some value! Thanks again for reply.
John
PS the member who posted the helpful instructions is called heatmiser, on this forum. Thread Tascam688 diassembly( and hopefullyrepair)