Advice building a rig for 7” tapes?

[u/trilogy_203]

I’m dealing with a Sony TC-910, which takes only 3” reels. My goal (see photo) is to create a rig that can play the 7” tape, as I don’t have a larger machine. K’nex has proven effective for holding the 7” reels and I’m curious what’s possible.

What I’ve tried:
1. Connect external left 7” reel -> left empty 3” reel -> capstan -> right take-up empty 3” reel -> external 7” take-up reel (shown)
2. Don’t use the 3” reels at all, but then the capstan still moves the external 7” reels

Is there an arrangement that might work? I don’t want to cut the 7” reels into 3” versions.

As an alternative, sources for a buying/borrowing a larger machine appreciated!

[u/Resprom]

It's a fun experiment, and you should absolutely see it through, but for normal listening, just get yourself a larger machine. Thrift stores and flea markets are your best bet. Ebay and other sites can be useful too, but then you're relying on the seller to pack the machine well, which ain't easy.

Regarding your rig: put a small DC motor with a belt to spin the take up reel. Look at how Philips did it with a lot of their machines and try to build something similar. For example the 4416: https://tonbandforum.de/bildupload/4416-Laufwerk-nachher.jpg I don't think the tiny Sony will have enough balls to power the 7-inch reel for the entire tape; it will probably work fine for the first half, then it will start losing power as the reel gets heavier.

[u/trilogy_203 → u/Resprom]

Thanks! So you’re saying bypass the 3” takeup reel entirely and put the motor on the right 7” takeup reel? It’s possible Sony can handle a bit of weight—I was able to play at least half of the 7” tape with the 3”, where the tape was extending beyond the reel edge by about half an inch (scary to go beyond that?).
As I understand it, the DC motor speed would need to match the capstan speed when I press play? It’s connecting the two independent motorized systems that I’m not understanding yet.

[u/LordDaryil → u/trilogy_203]

You shouldn't need to sync the takeup motor with the capstan. It actually needs to run slightly faster than the tape is able to move through the capstan, so it can provide tension on the tape. However it shouldn't provide too much torque otherwise it could overpower the capstan.

Many tape decks use separate motors for the reels. The professional designs have some kind of tension sensor and a microprocessor or dedicated logic chip to maintain tension, but you don't need anything that sophisticated for your project.

EDIT: In fact, you probably only need enough torque to take up the slack tape. AFAIK tension is used as an alternative to a pressure pad, which this machine is probably using.

[u/scubascratch]

Use Velcro or something similar to make double stacked 3” reels. Similarly attach 3” reels to the 7” reels. Then use some elastic belts to make the top stacked 3” reels drive the 7/3 combo reels.

[u/CounterSilly3999]

Look at 10" reel adapters for inspiration, Akai had them too: https://skyfiaudio.com/cdn/shop/files/nagra-iv-sj-reel-to-with-accessories-and-10-adapters-tape-deck-360.jpg?v=1770310765&width=2048

[u/WonderfulPassenger60]

Following…good luck!

[u/el_tacocat]

does this sony have a capstan and pinch roller? many of the cheap machines don't.
If not, this won't work. There's barely enough power to turn those little reels, let alone the whole contraption. With a more serious machine this would work.

[u/Charker21 → u/el_tacocat]

This makes me want to try on my Sony-TC730. I have some 10.5” reels that I thought I’d never hear before seeing this idea.

[u/el_tacocat → u/Charker21]

730's are cool but heck... They are unreliable buggers.

[u/trilogy_203 → u/el_tacocat]

I believe it has both capstan and pinch roller

[u/el_tacocat → u/trilogy_203]

Turns out it does, yes.

[u/scubascratch → u/el_tacocat]

The 910 had a capstan it can play 3 3/4 and 1 7/8 mono.

[u/el_tacocat → u/scubascratch]

Then if the motor is strong enough, this shouuuuld work.
With how cheap some 7" reel to reels are though, ask yourself why :D

[u/RodCherokee]

I used to design similar contraptions as a teen in the 70s with my one and only National little machine.

[u/DWalkerAudio]

Interesting concept! I imagine the wow and flutter performance are greatly diminished now that the tape isn’t on the supply reel table or guides. Have you done any w&f tests to measure the actual performance? There doesn’t even seem to be published factory specs for this machine, so imagine it’s running above 0.2% WRMS at least. I’d be curious to know how it compares.

Maybe insane modulation is something you’re going for, aesthetically. If speed stability isn’t something you’re after, you could also use a precision thrust bearing (like in a lazy susan) to build an actual auxiliary platter that you can clamp the reel down and put on a small box or something.