Probably not the best time to reply, as the forum's going off-line tomorrow
I've just pulled a new AT1 off the shelf and wired it up to a motor on the bench.
The 5v terminal does give out 5v.
I then wired up a 5K pot and changed parameter 10 to 2, so enabling external speed control.
I get the full frequency range from 0-50Hz as expected.

The last time I heard of someone having this problem was when they mis-wired the pot, and had the wiper to the wrong terminal.
Most pots I've seen have the wiper as the centre terminal, but maybe not all.

Also, whilst the wiring schematic refers to 10v/5v my own actual AT1 has the wiring terminal labelled as 5v (only)
The basic schematic is used for more than one model, so it may be that the AT3 has a 10v terminal instead of a 5v one.
The manuals supplied are pretty basic, and show 15v & 24v as the other outputs, depending on whether they are AT1,2, or 3. My AT1 terminal block is labelled as 12v, so don't put too much store on the manual's labelling of voltage outputs.

Bill