ANOTHER another weird one!

[Bad Raven]

Why Meeeeeeeeeee! 

75mm brushed 8mm motor quad assembled by me.  Been flying perfectly for six months, many have commented on how fast and "on rails" it flies. Able to easily blast through tiny gaps with reliability.

It has started intermittant fast yaw spins. These can occur while in hover as much as when flying, so not yaw washout. Then it goes back to flying OK for maybe 15 to 30 seconds or more, then boom, more high speed spins. It stays flat and stable while spinning, I can go hands off sticks and it just hovers there perfectly but spinning furiously, then decides to behave with no input from me.

Def not the Tx as other quads and planes are OK. SO, motor(s)?  FC?

I suppose motor(s) are the obvious first thing to change, but they all have normal spin, all have normal end float, all have normal sound, and when it is working its just as on rails as normal. And I'd expect a motor to cause instability.

Oh Dear.................   

[trebor]

I would guess    motors 

[Bad Raven]

I would guess    motors 

You'd think with a model room filled with "stock"  that I'd have replacement motors to hand.     

Stupid Boy. 

Ordered on fastest method. 

[shawdreamer]

If its as you describe "spinning on the spot" but otherwise stable hover then I wouldn't be inclined toward a motor issue myself.

a screwy/glitchy motor would cause multiple instability issues across just about all axis not just Yaw (that is unless two motors on opposite corners just happen to suffer the same issues at exactly the same time when it happens)

If its yaw spinning like its being commanded to in a controlled manner then my guess would be a signal issue and seen as your not getting any similar issues from other models then Id further guess its on the models end (maybe the RX)

[Bad Raven]

Yes, I'd agree with all of that SD.

I have just in the last forty five minutes cured it.

When I'd tested several models, the one reported was the only one to exhibit the symptom, and I did go back and fore over several sessions and several models inc Heli, Plane and quad and back to the iffy quad to prove repetitive, and flew other models from the Tx on a couple of sessions with no issue. (over an hour on Monday!)

However, starting setting up a new model today, and it had minor yaw twitches visible on betaflight receiver screen, really tiny compared to other quads actions, but def there.

SO, I took the Tx apart, and with its channel monitor on, and wobbling wires and fittings with the gimbal locked unable to move found I could affect yaw occasionally by moving its hall effect gimbal wire at the sensor end, which proved after wire replacement and stripping down to be frayed inside its insulation.

SO, it was the Tx, at least I'm guessing specifically that when it was changed from PVC to Silicone I did too good a job of tinning the wires before soldering on, and the soldered wire passed through the clamp, creating a stress point hidden in the wire!   

(Yes, I have checked the others!!!  )