For those of you interested in modifying an Otari MX 5050 for direct head wiring for using with an external playback amp such as the Bottle Head here is how I did it on a friend's Mark III version which is the same as a MX 5050 BII but with a console case and the electronics on a bridge above the deck. When hooking up this machine for use with the BottleHead I found that I needed to isolate the RCA jacks from the chassis and connect the shield of the head cables separately to the chassis. In hindsight I placed the RCA jacks too close to the blue connector. I should have placed them in a little more open location.
On the console machines the Otari head amp can easly be disconnected from the headstack by not connecting the cable to the bridge. You may find that you have ground loop hum problems on the BII and BIII machines without disconnecting the Otari amp which would require some extra and experimentation.