Adjusting antenna retraction time

[77450slowner]

The antenna on my 77 450sl will not retract fully. Initially I thought it was a problem with antenna mast but have just installed a new mast and am seeing the same problem.

From what I can determine, the problem seems to relate to the difference in the amount of time the antenna motor is running when extending vs retracting. The motor runs for a few seconds longer during the extend period vs the retraction so there is always a few inches that are left when the power shuts down.

I have an after market Kenwood stereo system. Anyone had a similar problem and any advise on how to correct this?

Thanks

77450slowner