--- In TekScopes@..., "Goran" <goran.krusell@...> wrote:
I don't have the specs in front of me to compare, but with some effort I could find an old catalog with the non"A" model to see what changed.
There is one thing I do know. At the time the 7904 went to the "A" model, Tek had a policy that the model suffix would not increment unless there was a significant specification or feature change for the customer. Improved reliability, manufacturing cost savings, fixing the annoying power supply start up, etc, would not merit the "A" model for the 7904.
Consider the 7A13 upgrade to the electronic DMM for displaying the comparison voltage. The old "odometer" version was prone to mechanical alignment problems that caused it to bind up and ultimately strip its plastic gears. I don't think I have ever seen an old one that was actually well used that did not suffer this problem. The DMM totally eliminated this reliability problem, and changed a significant portion of the circuitry and mechanical parts list. Yet the model remained a 7A13.
This was not always the case. The 310A is essentially the same circuitry as the 310, with no change in performances or features. It returned to ceramic strips. The original 310 tried out this new technology of "etched circuit boards" but before plated through holes were invented. Using rivets for feed throughs proved intermittent, resulting in a reliability nightmare. Sometime in either the late 1960's or early 1970s Tek adopted the policy to denote major internal changes with no performance enhancement using large serial number breaks rather than model number increments.