This article will discuss repair and calibration of the Jackson 561 tube tester. High voltages are present, repairs should only be attempted by a qualified technician. Copyrighted, all rights reserved.
The Jackson 561 appears to be a rare (uncommon) model. Very little documentation exists, and I have seen only a small number of them for sale.
The model 561 is a combination of the Jackson 634 and the 648. Photo below shows Jackson 561 (right); Jackson 648A (left).
If the 634 and 648 mated, the 561 would be its offspring. It shares the following characteristics from the 648: (a) the same meter, (b) variable leakage control, (c) color scheme, and (d) case design. From the 634, it shares (a) the same test method (which Jackson calls “Dynamic Output”), (b) Shorts Test control, (c) lack of a Noise test, and (d) function control layout. In fact, the 634 tube setup chart can be used (with slight modification) with the 561, as explained later.