Old Car (240z) -- 'Fried' Two Amps...

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#41
miniman82 said:
Zeners can't be tested for resistance, only breakdown voltage and reverse bias. If you were able to get a reading, I'd say they are suspect (leaking/shorted). Perry is right, zeners have been used in electronics like this for a long time as a method of regulating voltages. They can be fickle though, in the old days it wasn't out of the ordinary for them to cause problems by oscillating. They usually will have a small value bypass cap nearby to help avoid this (something like .01uf is typical), and I also agree that 100 ohms series resistance may be a bit low.

If the rails are sitting around 35v for example, that would mean the resistor needs to drop 20v to arrive at the regulated voltage of 15. When you plug those values into ohms law, you wind up needing a resistor of 4 watts to withstand the heat generated. That's a pretty large amount of heat for modern electronics, I would expect something closer to 400 ohms, placing the resistor value right at 400. I'd go with 470 ohm 2-watt metal film type, which is a more common value that's easy to find.

In order for the math to work correctly though you need to know what your rail voltage actually is, so don't go with what I wrote above it's just an example. Also it's possibly the zeners were simply a casualty of another problem, it's obvious something caused a lot of current to go through those resistors so you need to trace back the problem and correct it.
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Thanks for the clarification on the Zeners. I did use the Diode mode. I tried a couple other diodes on the board and they looked good, but the new ones I just put in look like they failed again...I got 10 in the pack, so I guess it's a good thing I got some reserves 🙂 But, will have to figure out what else is wrong now. I'll look around some more....

I'll have to learn some more to cover/check for the suggestions you have provided above, but I will work toward that.

I got the resistors already and put them in. They are 2-watt 560ohm.
 
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#45
Pretty common for voltage to rise with an old regulator when the amps fall short.

Hope it didn’t take its toll on the instrument panel or anything like that.

If you haven’t already, it would be good to check out or just replace the battery cables, engine to chassis cable. Believe it or not, Walmart actually had some decent gauge ones for very reasonable.

Hope you get it sorted out soon.
 
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