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got the amp free when i bought a guitar at a garage sale.found a bad cap also noticed board discolored and solder joints on the 2 10a 120ohm resistor (r-37 r-38) looked bad.replaced the 4 50v 4700uf caps and the 2 resistors.amp works but resistors got up to 300 degrees befor i shut it down.any ideas?
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Isn't this your original post: http://music-electronics-forum.com/t34294/ ?
It is normal for them to get very hot. If there is a problem you will not have your plus and minus 15volts at those resistors. Do you have proper +/-15V ?Originally posted by Enzo
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40v in16.5v outComment
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Ok, so your voltages are good (where I said 15V I meant 16V.) So the resistors are doing what they are supposed to and the heat is normal.
If you are still in doubt, consider this: those resistors are expected to get so hot that the designer had to use something bigger than 5watt (Tom showed the math in the other thread). They expected that a 5watt resistor would burn out under normal conditions so they used 10watt resistors there. That is a lot of heat.Originally posted by Enzo
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I had that same amp and R37/R38 were badly discolored too. Lots of heat on those parts and over time it will cause trouble. The problem I had with the amp was that it would power up for a few minutes then it would turn off and back on by itself. Replaced R37/R38 and that fixed the problem. Replacing those wire wound resistors is a great idea as per preventative maintenance.When the going gets weird... The weird turn pro!Comment
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If you take a look at the datasheet for a 120 ohm 10 watt cement resistor , you will see that the temperature rise, at 1/2 load, is 150 C.
That is 302 degrees fahrenheit.
Right on the money.
Link: http://www.yageo.com/documents/recen..._SQP%20NSP.pdf
A 15 watt resistor will be less.
It will just boil water.
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