The other day I was testing my Merlin, with a Canon Eos 400D and a Canon 75-300mm IS lens.
The problem is, when the lens is extended to 300mm the weight of the camera plus lens is slightly too much for the lens... at least when used rougly horizonally it's fine, but the moment you point the lens down to any real degree it has a tendancy just to rotate down under gravity.
I was thinking therefore if I got a metal plate with a whole drilled in it that sandwiches between the camera and the mount sticking out of the back of the camera, with a couple of holes at the other end of the plate allowing a couple of long bolts (with number of nuts added to reflect the weight of the lens). Do you think this arrangement would counterbalance the weight of the lens, and relieve the problem?
1) tighten the counter-screw of the vertical arm movement (you have to dismantle the Merlin´s vertical grey cap) 2) if possible avoid to move the vertical arm by hand - always use the motor. Otherwise you wear off the glitch (? don´t remenber the name). 3) simply hang a little weight onto the camera to balance the lens. I made a little strap onto which i can fix different weights if needed. This way you can use really heavy lenses. Regard vibrations after movements due to any free hanging weights!
If you want something you´ve never had, then you´ve got to do something you´ve never done.