I bought a rebuilt a833, a '70 23 spline. I have no experience with these transmissions. I'm converting from a torqueflite to manual. I'm going to contact the builder with the issue, but it's after hours and I though I'd check here first in case its something simple that I am just not understanding.
I have all the shift levers in the neutral position. It seems to be in a gear that is close to 1:1 If I put any of the levers into gear, the input shaft just locks up completely. The only exception is when you go into what should be third gear, it seems to be OVER driving the output shaft (this is NOT supposed to be an overdrive.) The interlock system seems to be working correctly...when you put any lever into gear, the other levers won't move. All the levers seem to move and detent correctly.

Summary:
all levers in neutral = 1:1 ratio
Reverse engaged = locked up
1st gear engaged = locked up
2nd gear engaged = locked up
3rd gear engaged = some overdrive ratio
4th gear engaged = locked up.
All levers move as expected and lockout system works as expected.

Anybody know a simple fix? Am I overlooking something simple?

1:1 is the input shaft driving straight through the main shaft. So ... the issue is the front shift fork is keeping 4th engaged. I suspect the fork got cocked on the install and that's the problem. Pull the side cover for a look. Whether you have trans experience or not, it should be obvious.

Why does a '70 have overdrive - or did you order that ??

NP didn't make O.D. 833 trannies in 1970
I think they where offered in the A body six cylinder cars in either 1974, 1975 or 1976 for the first time and then offered in the pickups and vans a little later

I did NOT order an overdrive, but I did request the 3.09 first gear. That makes me worried that they put in the overdrive gearset, which is not what I wanted.
After googling how the transmission works, I’m guessing some misalignment during assembly is causing direct drive to be engaged when the 3/4 lever is in neutral.... just as you said. I’ll find out when I get home after work.
Would have been nice if they’d done a function check before sending it out.

I ran a 833 in an alum overdrive case for years so I'd assume that you could reverse it and put OD gearset in a cast iron 833 case. Hopefully that's not what's going on here. Been so long but the OD flips 3 & 4 so if you are truly seeing an overdrive condition when you are in what's supposed to be 3rd that may be it. Hopefully all this is not the case for you and just a fork issue.

It is possible to "miss" the fork when installing the side cover and push it forward.

Got home and took off the cover and it is not as simple as I hoped. The forks were in the right place.
Once I was in there and moving the sliders by hand, the problem did NOT go away. But now that I can see what is going on, I can better report what is happening.

It is stuck in third gear, even when the sliders are in the neutral position,. I was wrong about it being 1:1. Now that I can see the gears turning, I can see that it is slightly less. When I engage third gear, nothing changes. Any other gear and it is locked up. Third gear is not disengaging, even when the 3/4 slider is moved to the neutral position.

It looks like an issue with the synchro. For all the other synchros, it looks like there is about an 1/8" gap between the brass synchro teeth and the gear engagement teeth when the slider is in neutral. For third gear, the brass synchro is slam up against the synchro teeth and I think slightly engaged with them.

At least now I know what to tell the builder when I give him a call

Knowledge is power, now you understand the insides a little better
Try using a flat bladed screw driver or another lever to see if you can possibly disengage the 3/4 slider from third gear
Don't get real aggressive on prying on it though
I have made them stick on a early Ford tranny when speed shifting and had to remove the cover to get it to disengage
Good luck, maybe you should fix it yourself
There not really hard to work on
Good luck

I would agree that you should try to pry the brass syncro loose. But here's how it works ... when you want to engage a gear, the brass gear is pushed against the mating cone of the gear - the purpose is to syncronize the speeds of both the output shaft and the input gear so that when the steel teeth of the syncro mesh with the teeth on the gear the impact is lessened and wear is minimized. The brass syncro ring is kept in relative position by three steel dogs. Its actually the dogs that in your case are keeping 3rd engaged. Pry that ring loose and things should move properly.

BUT there are two styles of dogs, thinner old style and wider new style. the thin old ones will fit newer syncro assemblies but the new won't fit the old . But regardless, the wrong dogs should never be used in the wrong assembly. Unfortunately you won't be able to see those . But there is the possibility that's the cause of your grief.

On the other hand it could be as simple as the brass ring mistakenly got slammed on during assembly and prying loose will be the solution. In fact, if there was no lube applied during assembly, I can understand a dry ring hanging up on the cone. There is nothing that pulls those rings loose, they just sort of float till called upon to do their job.

My bet is the end of one of the 3 struts on the 3/4 synchro has cracked . The end the strut has broken off.
This allows the strut to move to far outward on the hub . Then the slider cant move back to the neutral posistion because the strut is jammed. It happened to mine. Cheap fix but disassembly is required.
Doug

All's well that ends well.

I contacted the builder and he said the synchro ring was jammed and that is something that happens sometimes during shipping. I'm thinking a transmission is going to need a pretty good jolt for that to happen, but anyway....
So I just needed to pry the ring loose (as suggested above), which I did and now everything works.

Thanks everybody!

This is a typical "that's my story and I'm sticking to it" - Did someone drop it on its end from a rooftop ?!?!?

If the trans was assembled with everything dry, I can see a brass synchro ring "sticking" to the gear. After all, the synchro ring is a brake intended to grip its gear.