The AT4 and AT6 appeared somewhere in the late teens to early twenties according to some of our old catalogs of the day about the same time as the famous DU4/DU6, ZR4/ZR6 and were promoted as the high end heavy duty line. They are smaller in size but pack a respectable wallop in spite of themselves. They had a higher price point and were not as popular as the more common ZR and DU series. Many survive today in working condition. They shared a wound armature design like the DU and ZR series of the day which produces a fatter longer duration spark than stationary coil designs. Some early Eisemann literature suggests this design optimal for larger engines. The wound armature suffered from the usual woes of insulation seepage and sticky goo gluing the armature in place, lack of lubrication,and ample supply of dirt, grit, and neglect. The condensers live in the base of the armatures and suffer from the same. They soak up moisture from too much Oregon Rain in the wrong places and develop electrical leakage, pitting at the points, wimpy spark and discouraging words. Despite all the woes,even when all was not well electrically and/or mechanically, they worked well.
When there is a failure to fire, the first place to look is the contact surfaces. Polish them smooth and shiny with a Cratex bar and/or Crocus cloth. Files are too coarse, remove too much material and leave a rough surface that will tarnish easily. The easiest way to service the contacts is to carefully remove the point spring screw and remove the breaker lever and remove the screw holding the stationary contacts for best access to them. Always inspect the breaker lever pivot pin for rust, polish smooth and relube before reassembly. Be carefull not to cross thread or let the point spring screw fly across the shop. Set the contacts to .015". The lock nut and adjusting screw are metric sizes as are most of the fasteners on this model despite being American made. When reassembling the mag, wash and repack the thrust ball bearings with a good black molybdenum grease. Here is some factory service info to keep yours alive and well.
If you can find imitation parts for any Bosch products, please let us know!
These strange looking wrenches often appear at flea markets and swap meets. They are a good investment as the points adjusting screw and lock nut are metric and very hard to reach to adjust.