JL Audio 300/2 test/set bias

[vchhat]

Hi All,

Am hoping someone can help, i have a jl 300/2 amp and had a bad transistors so replaced all 4 on that channel (Q300,301,304,305). However, I'm aware that i need to set the bias on this channel again...can someone help me on how to do this?

From other posts i have got "turn the blue potentiometer 'in the repaired channel' fully counter-clockwise and power the amp up (via a 10 amp fuse <-- very important). Turn the potentiometer clockwise (slowly) until you just see a slight increase in idle current. Do this with no load and no signal"

However i'm not sure where to test i.e literally where do i put each probe? And what range do i need the meter to be able to read? I'll upload a pboto if that helps..

Many thanks in advance
Many thanks in advance

[Perry Babin]

Those instructions are for setting the bias by watching the amp meter on the 12v power supply.

If you want to set the bias by the voltage across the test points, measure the voltage across the test points for the channel that has not been repaired. Set the other channel the same. I've found that the bias is generally about 4mv across the test points.

The transistors must be clamped tightly against the heatsink when doing this. You need to re-check it after allowing the amp to idle for a minute or so. It's often best to measure the voltage across the test points from the bottom of the board to prevent shorting to the busbars on the top of the board (may not be an issue on the 300/2).

[vchhat]

Hi Perry, thanks for your help really appreciate it -sounds straightforward enough...

However, one school-boy error that I've made is that I seem to have mis-placed the insulator for the transistor...any suggestions on what I could use instead or what it's called in the market place?

Thanks again

[Perry Babin]

They use mica insulators. You can get the smaller, individual mica insulators from virtually any electronics distributor. You'll need to apply new heatsink compound as well.

[vchhat]

Thanks i got some mica, installed amp and all was working really well...until i turned the volume up and it's gone again!

However this time i had the input to the amp ran from the speaker outputs rather than the pre-outs (this was because while the amp was out for repair i had temporarily ran the speakers directly from the stereo by connecting the speaker outputs of the stereo to the rca cables which go to the amp then the amp end of the rcas connected to the wires which run to the speakers) would/could this high input have damaged the amp? If so any tips on where to start checking would be a great help...

Thanks again
Vishak

[Perry Babin]

Which LEDs are lit on top of the amp?

[vchhat]

Just the power LED.

Strange that I'm not getting the low ohm lights as when i checked the transistors in situ, the ones I replaced seem to have shorted again

Confused..

[Perry Babin]

Are you sure that they were laying flat against the heatsink?

Was there sufficient heatsink compound between the transistor/insulator and between the insulator/heatsink?

Did you recheck the bias after the amp was allowed to idle for a few minutes?

Did you recheck it after the amp warmed up?

[vchhat]

Hi Perry, yes to all of the above..however I can go through the steps again as I replace the FETs again..

Do you think it could be related to the high level of input when it should've been receiving low? Would that cause other damage apart from the FETs?

Thanks for your help, really appreciated!