JL 300/2 Plays But No Power Light

Shane73

Member
2016-02-18 1:57 am
I have a JL Audio 300/2. When I bought it, the power light would come on but it had no sound nor did the thermal or low ohm lighylt come on. I opened it up to find all 6 power fets were blown. Replaced them with 50n06 fets. The amp no plays fine, but shorthly after the power light quit working. Any ideas as to what it may be. This is the first JL amp I have worked on. Thanks
 

Shane73

Member
2016-02-18 1:57 am
I replaced the gate resistors when I replaced the power fets. The output fets checked out fine. I checked the solder joints and continuity of the wires to the leds and they are ok. I don't seem to have any ground for the leds. There is a transistor for the grounding wire. I replaced it but it didn't correct the issue.
 

Shane73

Member
2016-02-18 1:57 am
I replaced the gate resistors when replacing the power fets. Tha amp powers on and plays music fine with no issues. The leds check out fine. I resoldered the led wires to be sure. It appears I have no grounding for the led lights. There is a transistor in the ground loop for the leds and I replaced it, but still no luck.
 

Shane73

Member
2016-02-18 1:57 am
Do you have 0v across the LED and also read greater than 1v when checking on diode check?

Do you read approximately 0v from collector to ground for the LED grounding transistor?



There is 11.53 volts on the positive side of the power indicator led. There is a 2200ohm sm resistor prior to the led and it shows 11.53 volts on each side of the sm. There is 11.5 negative volts on 2 legs of the transistor on the negative side of the led but no voltage on the 3rd leg. I am also showing 11.53 volts on the positive side of the thermal indicator led but the amp does not get hot at all. Tge voltage shows as soon as the amp shows up but neither led is lit.
 

Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
The grounding transistor has its emitter grounded (0v). The collector is connected to the LED via a resistor. The 12v is passing through the LED and resistor. The base leg cannot be more than about 0.6v above the emitter. Driving it to 12v would destroy the base-emitter junction.

In the photos I have. The transistor is marked Q3. There appears to be a 2.2k pulldown resistor and a 10k driving resistor. Those would form a voltage divider and prevent the base from seeing anything near 12v so there may be another problem.