I have updated my article entitled “Rebuild & Calibration of Jackson 648A” to discuss how to eliminate the 1S5 tube from models 648 and 648A and upgrade to solid-state circuitry.

Newer versions of the “648″ (648R, 648S, etc…) use a solid-state circuit instead of a 1S5 tube. The benefit of the solid-state circuit is that the tester calibration should remain accurate much longer. If you do not mind checking your calibration on a regular basis, there is no need to perform this upgrade. It is more appropriate for power users.

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