Jl audio 300/2 protection mode

[IAN160981]

Just wondering If anyone has had this problem with their JL audio amp and has any ideas on what the problem may be. The problem is that the amps "low ohm light" comes on after the amp starts up. The power light comes on when remote voltage has been applied but after 2-3 seconds the low ohm light comes on regardless if any speakers are connected and rca cables connected or not. When the amp goes into protection it still amplifies any input but only 25% of the possible output is heard. It also makes a high pitched noise from roughly were the power connections go in to the amp when the protection light comes on. When no input audio is connected the amp draws approx 15amps. Is there any reference voltages/resistance i can check to try and identify the problem.

[ppia600]

Measure the resistances between the Drain and Source legs on the output transistors.(second and third legs) If they are very low (.1 to 100 ohms for example) that will be a good place to start. If the outputs short/go bad the protection light will come on with no speaker connected.

[IAN160981]

Hi ppia600, thanks for your your quick reply. I measured the output transistors (am i right in saying the output transistors are the ones attached to the heatsink) The resistances between drain and source are the following: -

1. 3.2r
2. 3.0r
3. 90.0k
4. 71k
5. 88k
6. 81.6k
7. 103.6k
8. 125.7k
9. 53.6k
10. 3.4k
11. 2.1k
12. 2.0k
13. 2.1k
14. 2.1k
15. 2.1k
16. 2.1k

would i be right in saying transistor 1 & 2 are bad considering they both measure less than 5 ohm. Does this indicate a shorted transistor.
Thanks

[Perry Babin]

#2 looks like the one that's defective. Pull 1 and 2 and check them out of the board.

There are 8 outputs (4/channel). 11-16 are power supply FETs. 9 and 10 are likely the rectifiers.

[IAN160981]

Thanks for your reply Perry Babin. I have pulled them from the board and now i think you are right in what you were saying about trans 2 being defective. measurements are as follows -
between pin 2 & 3 = 3.0r
between pin 1 & 2 = 28.9r

I cant seem to get a proper measurement from transistor 1 though out of the board. Unless my meter is at fault but i keep getting totally different readings depending on what i have my meter set to (eg - meter set to 200 , 2000, 20k, 200k, 2000k)
Is this normal.

The transistors are both the same (IRF540)

Thanks for all your help.

[Perry Babin]

It's likely OK. FETs have to be checked either in a a specific order or by shorting the gate/source as is shown in the FET section of the following page:
Basic Amplifier Repair - Transistor Test Applet Link

If #1 is OK, reinstall it and clamp the transistors to the sink. Center the clamp on #1 so the clamp will stay in place. Power it up through a 10 amp fuse. Does it produce clean audio (low to moderate power)?

[IAN160981]

I have just done the diode check from the link. This seems to confirm our findings because with transistor 1 the diode check produces a reading of 580 one way and open circuit the other way. transistor 2 however reads 003 both ways round. I will reinstall trans 1 and post back with the results. Thanks again Perry Babin you've been a great help.

[IAN160981]

That must be the one. now its back together there is no protection light, it doesnt overheat after 5 minutes and although i am 1 output transistor short the sound is awesome. now just have to get a replacement transistor/s. How many should i replace , just the defective one or multiple. Thanks very much for all the help, it is much appreciated.

[Perry Babin]

At the very least, you have to replace 1 and 2 but I'd suggest that you also replace 3 and 4. Don't apply power without having all of the transistors clamped to the sink. The new ones may require re-setting of the bias current and could cause excessive current draw until it's properly set.

When you reinstall the new transistors, bend the center leg similar to the one in the attached photo. If it's left straight, the transistor won't lay flat against the heatsink and will fail prematurely.

You'll also need to apply heatsink compound between the transistors and the insulator. The heatsink compound from Radio Shack is good enough. Apply a thin layer to the transistor before you reassemble the amp. Remove all of the old heatsink compound (without removing the insulator - unless there is a lot of dust/debris behind the insulator) before applying new compound. Apply about as much as you see in the second photo.

[IAN160981]

Thanks for the instructions, all sounds good apart from the fact i dont seem to have an insulator between the transistors and sink. i will order the transistors today. i was going to use the same thermal paste that is used for pc cpu heatsinks as i have a load of that. this should do the same job? I will post back to let you and everybody else know how i get on. thanks again