Combines 1660 slips out of second gear

Your thinking the right way. Check the detents_cables first but if those don't fix problem then the transmission will have to come out and second gear set replaced. Have seen a bungee cord on the shifter in the cab work in an emergency to finish a day or two but it will wear the shift fork in the transmission.

Check to make sure you don't have chaff and dbris piled up in your shift linkage at the transmission. This can act as a counter spring and pop the rascal out of gear without there being problems in your transmission.

Sorry to tell you but you will have to have 2nd gear replaced. The gear gets worn and it won't stay in. We had the same thing in a 2166. labour wasn't bad but the parts were a killer for price.