temperature of jl 300/4 after output transistor replacement? - diyAudio
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Old 14th August 2012, 08:35 PM   #1
DcMaN is offline DcMaN  United States
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Join Date: Aug 2010
Default temperature of jl 300/4 after output transistor replacement?

Just replaced some output transistors on a jl 300/4, but i noticed the amp runs hot, i let it play for about 45 minutes.The amp gets hot enough so you cant keep your hand on it for more than a few seconds(only on the output transistor side of the heatsink). Mind you in channels 1/2 im driving a sub bridged @ 4ohm, and in channels 3/4 im driving a speaker bridged @ 4ohms. Just trying to see if the temperature of this amp is normal, or perhaps i need to do some more adjusting of the BIAS on all the channels. As of now I have the bias adjusted to 1 o'clock on all 4 channels. Is this a normal position for the potentiometer to be at ? I replaced channels 1-4 output transistors with IRF540A
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Old 14th August 2012, 10:42 PM   #2
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There's no 'normal' position for the bias. When components are replaced in any channel, the bias must be readjusted.

The output transistors are going to operate at higher temperatures than the power supply transistors. The outputs are operating in 'linear' mode. The power supply transistors are operating in switch mode. Linear operation dissipates more power (in the form of heat) than switch mode.
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Old 17th August 2012, 04:00 PM   #3
DcMaN is offline DcMaN  United States
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Perry, i keep losing output on channel 3, but only on channel 3. At this point, im almost certain something on the driver board for that channel is broken. Basically makes a staticy noise and a small pop comes from the speaker, then the low ohm light comes on, and also the amp that starts drawing 20-25 amps at idle. I ensured that all output fets were clamped tightly to the heatsink. Its just weird because sometimes ill let it play for 45 minutes to an hour and it would be fine, then all of a sudden i go to turn it on the next day to check it out again and it would fail sometimes with in seconds of being on.
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