We often see 'Z' as representing the impedance of a 'network' and somehow these hybrid resistor 'networks' are indicated with a Z. As in Z1, Z2 etc. on the artwork and in the schematic. Many years ago they used to use customized modules with resistors, capacitors, diodes, transistors and even specialized I.C.'s to save space. They were ceramic hybrids often with printed laser trimmed resistors and generally coated with green epoxy and also often a brownish yellow encapsulation. In the television and radio circles they were often called 'Plates'. I have seen schematics marked as "HY' or 'HM' (Hybrid Module) and often seen them called 'I.C.' as in I.C. 10 etc. Among Zenith and many others they would not release any information on what was contained in the 'plate' and when these went bad and were no longer available that was an end-of-life for your device. So reverse engineering them became very common. Case in point, the 'greenie' used in Tektronix TDS420 power supplies. An 'unobtainium' part even since the day those scopes were released. They have been reverse engineered and can be duplicated today. Besides being space saving these little wonders often protected a product from being easily duplicated. Cheers!!
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