Tascam 112B - capstan runs continuously while tape detected

I got a busted Tascam 112B where the capstan runs continuously while tape detected.

I tested without a tape by pressing the detection switch..

Rewind: works normally, tape heads down. Will loft/return to stop mode if button released.

Fast forward: Capstan and takeup spindle runs fast, sometimes tape heads will rise

Stop: Tape heads lowered, capstan spins

Play: tape heads rise momentarily then go back down. Capstan runs, taleup spindle runs fast.

I'm thinking something is shorted causing the capstan motor to run continually instead of as needed. Any thoughts where to start with this one?

It's an adventure to be sure!

 

The capstan on the Tascam 112B are meant to run all the time while the unit is on by design, it is the same on my Nakamichi CR-7. They are meant to run all the time, turn off the unit if you are not using it.

All my kit are left on all the time, except my turntable and tape deck.

 

Fair enough. I actually realized that after posting this.

At this point I'm thinking it might be mixing up modes/going into ff when it's on play and going into play when it's in stop mode, etc.

I noticed there are pots labeled with the different modes. I was thinking of looking at the service manual to see what is up with those and learn how that all works on this deck.

In the meantime here's a video:
https://imgur.com/a/rMGh8tx

 

Ok according to the manual the pots are to adjust the height of the play heads/pinch rollers, etc. good to know. I'll keep digging.

Edit: gonna take my meter to the mode chip and see what I can see because the more I think of it the more I feel like it's mixing up modes.

 

It's entirely possible, though the fact that the tape head position is being erratic makes me a little worried.

I put in a recordable test tape and it ate the friggin tape in the stop mode of all things, since the tape head was high enough to keep the tape against the capstan while the takeup wasn't running.

There's a procedure for adjusting the position of that assembly on a per mode basis but I am wondering if I should make sure I am in the right mode first.

I took voltages at the mode chip but I think the signal is ac not dc so I need to double-check since currently it looks like all input pins have the same vdc on them which clearly isn't right because changing modes changes the behavior and outputs.

So back to metering, mostly because it's a good exercise that doesn't damage anything.

 

It's almost like the tape head part is backwards between stop and play while the takeup spindle is running as expected for the modes

 

I think I figured it out! There's a white piece connected to a potentiometer that the deck uses to detect the head position. It's got a couple prongs and should be over a piece on a black gear but it's to the side, not over the ridge on the gear that is meant to hold it in place. I bet that's the problem, time to fix it and find out!

 

Ok now the tape heads aren't moving at all even though moving the black gear moves them. Time to make sure I didn't knock the wire off the motor or anything stupid like that

 

Yep knocked a wire off like a doofus. Time to fix that then hopefully I might be good.

I suspect the previous owner changed the belts but didn't get it back together right, then sold it rather than figuring out what went awry.

 

Update: I put it aside overnight, went to solder it this morning and the other wire for the motor that shifts the tape heads came off.

Is there any safe way to verify which way around they go? One terminal has a marking beside it but I don't know whether that indicates positive or gnd. (Or orange vs black in this case.)

 

Found a shot of motor wiring on a youtube video, took a screenshot, soldered her up and we're good!

That did the trick! Tape heads act normally when modes change now!

I ended up futzing with the tape head alignment pots so I'll have to grab my calipers and adjust them back to the standards outlined in the guide, but she runs!

HECK YEAH!

So for future folks, if you ever get erratic tape head behavior on a Tascam 112B, check to make sure the white prong is over the top of the ridge on the black gear.

 

Picture of the piece in question

 

I would advise you to stop now and take it to a tech to fix it for you, before you do some serious damage.

 

They shouldn't be too far out of their original position. I tried to adjust with a non-conductive screwdriver as per the service manual and noticed that after changing the mode back, they didn't go back into the right position for the mode so I stopped touching them pretty quickly, as such they shouldn't be very far out of position.

Currently the deck functions and visually the tape heads look like they're in or close to being in the right position for their modes. That said, you make a good point, it might be a good idea, though I'd also like to learn how to do this sort of thing for myself as I would like to become a tech.

The trick with taking it to a tech is finding a tech in the area who can do it.

 

Being close to the original position means nothing, it can be off a tiny bit off and the sound will be way off.

If you want to learn, you may want to find a tech that will allow you to sit with them and hopefully they can teach you what you want to know.

I just had to take my Nakamichi CR-7 to Tom van der Huff in the Netherlands for a full service, he is the man to go to for Nakamichi. He is very generous with his time, I sat with him while he service the deck. He talked me through everything he was doing and some time he would ask me if I wanted more in-depth explanation to some parts that was not so easy to follow.

Even though he showed me everything, would I attempt it myself next time, not a chance in hell would I do that. Somethings are best left to the pros.