MF 1552 Engine stalls when PTO turned on

[Jackstand]

2006 Massey 1552. PTO was fine, suddenly locked up and stalls engine when engaged.
PTO has an electrical engagement switch. I assume the PTO clutch is not disengaging.
How can I determine whether the problem is internal or external to the transmission? I have no reference material at all on this tractor.
I don't have a "theory of operation" for how the PTO clutch and clutch brake work on this transmission. Haven't found anything helpful on YouTube. Does anyone have a link to a reasonable description/ explanation? For reference, I am a 30-year auto mechanic with minimal tractor experience.

Thanks in advance

 

[ptsg]

Do you have anything attached to the PTO? Like an implement or something that could be locked up.

The way the PTO works is the switch only turns the PTO valve solenoid ON and OFF. The PTO control valve controls the clutch pack for the PTO, it will engage/disengage and lubricate the clutch. The PTO brake is spring actuated. Whenever the PTO is OFF, the spring just actuates the brake. Once the PTO turns On, the pressure will release the brake.

You sure is not the seat switch acting up? Thinking you're off the tractor and turning Off the engine?

 

[CobyRupert]

Shooting from hip:
Two theories:
1) Tractor engine literally stalls because it's trying to turn a locked/braked PTO. PTO solenoid engages, clutch engages, but brake is still on.
2) Safety interlock. Likely seat switch. Would tractor "normally / previously", turn itself off if you got out of seat with PTO on?

Edit: Basically in agreement with what PTSG said ^.

 

[Jackstand]

Thank you PTSG and CobyRupert for your ideas. The problem is inside the transmission. If the owner fixes it rather than trades it in, I'll add the diagnosis and repair to this thread.
I eliminated the seat switch circuit as a possibility, I believe, by confirming that power to the fuel shut-off relay is never lost when the engine stalls.
I had not realized this tractor also has a mechanical PTO lever on the fender. That lever won't move at all and the linkage is free.

 

[ptsg]

If the lever won't move, then something bad happened inside to the point of jamming the gears.

I would suggest try to put the lever in Neutral and trying to rotate the shaft by hand but since the lever won't move it's kinda pointless.