JL 450/4 channel 2

[Shane73]

I have a JL Audio 450/4 that has issues with channel 2. I bought the amp with the low ohm light on and no sound on channel 2. I pulled the output fets and checked them with a dmm and they checked out okay. When I powered the amp up without the channel 2 output fets the low ohm light went off and the amp played normal on 1,3 and 4. I also cheched the power fets and c3198 driver. All checked out okay. I measured the rail voltage and got 43 volts. Now, the issue seems to be that I am getting 32.6 volts to pin 3 of q203 and q203 but also 30.2 volts to pin 1 of same transistors. This only occurs on channel 2. Channel 1 gets 32.6 volts to pin 3 but no voltage to pin 1 and it works fine. Any ideas. Thanks.

 

[Perry Babin]

You cannot use voltage readings to compare channels when the feedback/servo loop is open (which it is with the outputs out of the circuit).

Are you sure that the outputs that you removed are not leaky?

Did someone possibly set the bias too high in the defective channel?

 

[Shane73]

I ordered a new set of irf540 to replace the output transistors. Channel 1 now causes low ohm light to come on and the power fets run warm and I end up pulling nearly 50 amps from my power supply. Channel 1 operates at low power fine but distaorts at high power. I pulled out output fets for channel 1 and it runs fine on 3 and 4. Still get good rail voltage on supply line for 1 and 2. Should I replace the power fets and drivers anyways?

 

[Perry Babin]

Are you testing with all semiconductors tightly clamped to the heatsink? It's possibly for them to quickly fail when not clamped.

 

[Shane73]

Yes they were all clamped when the amp was powered up.

 

[Shane73]

I placed a known good set of irf540 in channel 2 and still received nothing. What is the small circuit board with the three green leds? Is it some type of driver circuit. It is what is supplying voltage to pin 1 of the fets on channel 2. No other channel is receiving voltage on pin 1 at idle.

 

[Perry Babin]

The vertical boards with the LEDs are the audio driver boards. All LEDs should be lit.

You have Q203 twice in the initial post. Did you mean Q208 and 203?

 

[Shane73]

Sorry. It is 203 and 208.

 

[Shane73]

All 4 of the audio driver boards have 3 leds that are lit. When I traced back the lead from pin 1 of the output fets, it lead to the audio driver board.

 

[Perry Babin]

It's normal for the low side FETs (Qx03 and Qx08) to have negative rail voltage on the source leg and very nearly negative rail voltage on the gate leg.

 

[Shane73]

I am reading 30+ volts positive voltage on gate leg.

 

[Shane73]

I ordered new ifr540 for the output fets and 50n06 for the power fets but I don't eant to put the output fets in channel 2 until I figure out what is the issue. I used the output fets from channel 1 on channel 2 just to see if I got any sound at all. It is completely dead, no sound or any output at all.

 

[Perry Babin]

What is the DC voltage measured directly across the speaker terminals for channel 2?

Have you operated all switches and potentiometers through their entire range while listening for audio from channel 2?

 

[Shane73]

There is 0 votage from the speaker terminals on channel 2. I tried all switches and pontentiometers with the only change occurring when I switched from 1,2,3 and 4 channels to just 1 and 2, I got some type of feedback on channels 3 and 4. They just stated buzzing with no audiable music.

 

[Perry Babin]

What are you using for a signal source?

 

[Shane73]

I have used a tone generator app from my tablet pushing through a car audio cd player (aux input). I have also used just the cd player.

 

[Perry Babin]

Is the CD player being powered from the same power supply as the amp?

Are the RCA shields intact on the head unit?

 

[Shane73]

Cd player is from a seperate power source. Rca cables are shielded. It is the the same setup I have used to fix 4 other amps in the last week without an issue. I am just not familiar with JL amps.

 

[Perry Babin]

Connect the grounds from the two power supplies together. JL amps can do weird things when the shields are not grounded. And it's not only JL amps.

When I asked if the shields were intact, this is what I wanted you to check:
http://bcae1.com/images/rca/temporaryrcashieldrepair.html

 

[Shane73]

I will try that. Right now I have the power fets and output fets out.