8 Sep
2015
8 Sep
'15
9:21 a.m.
On 09/08/2015 05:53 AM, Noel Chiappa wrote:
My current best guess, after sleeping on it, relates to the fact that the
'magic' pin was an output from a delay line. Delay lines, in that era, were
apparently potted confections of inductors and capacitors. So maybe the extra
current drain with the probe on somehow affected one (or more) of the
capacitors in the delay line? A total WAG, but it's very mysterious!
Most potted delay lines are made the same way. You take a
piece of aluminum foil-coated paper and roll it into a
tube. You then wind extremely fine magnet wire onto the
tube. The foil is a ground plane, and one of the
distributed capacitor plates. The wire is the distributed
inductor and the other capacitor "plate". This technology
was used in the first color TVs, and I think pretty much
everybody used the same technology for digital delays.
Well, it could be the delay line is going bad.
Jon