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Banderson's remark CR2 is for protection got me thinking about the circuit again. The design for for the balanced amplifiers required the negative supply. Because the +20v and -6.3v supplies are derived from the same source without a ground reference within the supply itself, the circuit externally determines it.
I have attached a drawing to outline my reasoning. The power supply is actually stacked 20 plus 6.3. The voltages with respect to ground is determined by the ratio of the +20v load and -6.3v load. Because the same amplifiers are across the +20volts and -6.3 volts, the ratio should remain consistently the same. Therefore the 7.2v zener is for protection. If the =20 rail develops a short, it would push the -6.3v very negative unnecessarily damaging components. The 7.2 volt zener therefore prevent the lower rail dipping below -7.2 volts. It also explains wht the zener appears to be a chassis mount hight powered zener external to the circuit board.
I do not think the zener is faulty. I do however think the negave supply is drawing slightly more current than it should hence the dip to ablout 3 volts.
I am going to reason this through over the next day and if you have any questions on my reasoning, let me know.
I would advise not touching the zener as I think it is okay and removing it may jeopardize other circuitry it was intended to protect.
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