JL audio 450/4 troubleshooting questions.

[rhempz01]

I just obtained a jl audio 450/4 for repair and it's very strange, it works and plays until it warms up and then it starts cutting out. Does anyone have any known problem areas on these amplifiers. I thested through the driver cards transistors and they test ok in curcuit. I didn't remove them so I don't know if some a leaking. Are the four blue and yellow potentiometers for the bias? Can they be turned down some and possible fix the issue?

 

[Perry Babin]

Does the amp start to draw more current as it warms up?

 

[rhempz01]

yes

 

[Perry Babin]

Is it the output section of the heatsink that starts to heat up?

 

[rhempz01]

yes

 

[Perry Babin]

Unless the current draw is very high, I doubt that it's the bias but you can reduce it to see if it makes a difference.

 

[rhempz01]

ok, ill check th eexact amout of current but it was pretty high. The amplifier itself works for a awhile i thought it was ok. After playing for about 5 min, it warmed up and sound was dim, and one of the channels stopped working. I pulled the outputs and dont find them at fault. Do the irf540N work in these amplifiers?
I tested through the opamps for voltage, All seem to have power. Someone has replaced the one of the power supply's
Rail, regulated, voltage is present. Ill go through the steps for the other amp with this one and see what i can find.

 

[Perry Babin]

I don't know if the 540N works in these amps.

If one channel quits, it could be that the large 47k resistors on the driver boards have bad solder connections.

 

[rhempz01]

ok i did touch them up and the outputs for the card seemed to stop heating up. I will rework the solder on the remaining cards a see how it effects the outputs.

 

[rhempz01]

I touched them up but if I run a 40hrz signal into the amplifier. And even touch the speaker output with my scope or load the channel down it starts drawing around 12amps on zero gain.

 

[Perry Babin]

What is 'zero gain'?

Does every channel do this or only one? If you are not sure, set the amp so that only one channel gets driven with signal at a time and recheck.

 

[rhempz01]

So while testing the amplifier, R608 burnt up, and on the channel 1 input there is a bad hum emmitting. The drivers on both u212,u412 get to 130°F. The two other driver card remain around 80-91°F at nominal level.

 

[rhempz01]

I felt and used the thermostat and didn't detect any hot opanos or anything on the input, the pnp driver transistor is shorted for the secondary power supply. Do you have part sources for these transistors. Or can I use the 2sa1797/2sc4672 I have?

 

[Perry Babin]

Those drivers should be OK.

The power supply FETs likely failed if the driver transistors failed.

 

[rhempz01]

This powesomeone changed this power supply previously, looks like eBay 65n15's and I don't know if the drivers were reused but they failed, the npn&pnp uy/wy sot89 transistors. I don't have many sot89 transistors in stick. Only the transistors mentioned above which are only 60v and bcx51

 

[rhempz01]

Thise drivers were shorted. Whatever the problem is that caused them to over heat and short is from channel 1 it has a very bad noise on that channel I'll look at the drivers and outputs for that channel. What were the original sencondary power supply mosfets used in these amplifiers?

 

[rhempz01]

Ok sothe amplifier is back together. And well when I apply a 1khz test tone at say 2v of input and even touch the speaker terminal with the scope it draws current, more than the 16amps my power supply can supply. No shorts I changed the drivers and power supply. Still just seems like there is an issue. I'll try a different source for input.

 

[rhempz01]

ok so the rear driver cards A1070/C3207 Driver cards transitors reach a high tempature of around 130F and the front cards are only reaching tempatures of 89F, one rear left channel has a low pitched hum as well. All three lights are lit up on the cards.
Would this be the drivers themselves or am i looking at it incorrectly?

 

[Perry Babin]

Are you sure that you don't have the front/rear confused? The rail voltage is different but I'd expect the front to run hotter with higher rail voltage.

The hum could be from the preamp section. Did you find out why it was drawing excessive current earlier?

 

[rhempz01]

Its very well possible. The cards are not to one channel they are just positioned in the rear closest to the mosfets one for each front and rear, either both left right channels.
I can verify the amount of rail, but since they seem to be from both front and rear left channels.
Theres also the low pitch hum on the left front
And the switch for the crossover makes a very loud audible pop when swich.
I will go get the board print locations to be more concise.