Please note that this discussion is not limited to Panasonic's SV-3700.
Unlike Sony, Panasonic's DAT machines have never had problems making accurate recordings to tape. It is important, however, to regularly replace all wear-item parts especially brakes and clutches because they are inexpensive AND their "good" condition ensures the stability and performance of the machine. (Part numbers listed below.)
Apparently, after Panasonic relocated production of DAT machines to a different factory, various problems arose. You will notice, for example, that SV-3700s manufactured after late '94 (serial number AA4...) had their case paint changed from "textured" to shiny. Inside, the upper head drum changed from having relief rings to not. The position of the record current adjustment also changed from mid-rotation to full counter clockwise.
Since the change, the heads do not:
But, the number one reason why the error rate / distortion goes up after locating a START ID is due to the little soft brake UNDER the take-up reel. You should always change ALL three brakes about every year if the machine receives heavy use.
One way to tell if there is a tape path / tension problem is to look at the reflection of the mechanism in the tape between the take-up side precision guide and the capstan. Go forward and back in PLAY using the shuttle wheel. The reflection will indicate whether the tape is skewing. Only 1/8th of the "soft" brake touches the under side of the reel table, so when it wears out, there is no braking action.
When in reverse play the most difficult mode poor back tension and/or and an extremely out-of-whack exit guide ( the last one before the tape re-enters the shell ) will aggravate the tape path in reverse. In many cases, the guide will not correctly adjust because the plastic that secures the machined brass becomes damaged. Take up tension should be between 10gm/cm and 15gm/cm. Tolerance: not less than 8gm/cm or more than 18 gm/cm. Nominal Supply tension is 6 gm/cm. Tolerance: -1/+2 gm/cm. Back tension when the take-up reel becomes the supply reel in reverse play should be 10 gm/cm.
All of the aforementioned parts are not user-serviceable items unless you already have extensive transport experience.
Mode and Load switches: