Bruce,
The square pad is TEKs way of designating the positive side of any polarized device. So Yes, square pad is + side of the Capacitor. This also applies to diodes as well. Sometimes Tek will use the square pad to designate the number one pin of an IC, however, this is not universally true. The number one pin can also be designated by a small round dot on the PC board, adjacent to the pad. The number one pad is sometimes not identified in any manner. My experience is working many of these 4xx Series as well as other contemporary Tektronix instruments. This holds true on almost all the instruments that I have personally repaired. I do not always depend on the color codes on these caps, I tend to rely on the values stated in the schematics and in the parts list. One thing that I try to do is take a picture of the area of the board where I am working, BEFORE I make any changes and I try to document exactly these sort of markings for later reference.
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Michael Lynch
Dardanelle, AR
Mlynch001