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Hi, I have worked on quite a few of these and the dropping resistors are always overheating. These are R111 and R112 in the schematic. I was wondering if anyone could enlighten me as to why they are deliberately unmatched(270ohm and 330ohm both 7 watt). The 270 is on the positive leg and the 330 is on the negative. Are these values flexible? My local supplier has many higher wattage resistors but none exactly that ohm. Any suggestions?
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Are they failing or just getting darn hot?They are dropping a lot of voltage, what is across each one, 30-35v? So they are dissipating 3-4 watts? That is a lot of heat, and though they are 7w resistors, they still get hot. If you want, you can mount a couple of those aluminum Dales on the chassis wall and run wires. Mouser or someone like them is only a couple days away by UPS.
Those are three watt zeners, they need a certain level of current through them to zene properly, plus the current for the circuits they serve. Why uneven? Looking at the schematic, I'd have to guess the additional load on the +15 by the channel switching circuits.
SO I wouldn;t drift too far off value. COuld you move up to 330 and 390 or 330 and 470? probably, but you'd want to monitor the results with a scope.
Of course if you felt like re-engineering the thing, yo could replace the zeners with Vreg ICs like 7815.Education is what you're left with after you have forgotten what you have learned.
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