JL Audio 500/1 v1 Rev. 10 Boards #2

[RiLoWa]

I am working on another 500/1 seperate from the one I posted about a few weeks earlier.

I replaced the IRFZ44's and their gate resistors and the IRF540's.

When I powered it up, it cycles on and off. Green LED will also shut off during the cycling. I am using a 5A fuse and 3 Ohm resistor in line with the B+. I know sometimes these amps are sensitive to having the resistor in line, so I removed it and it pops the 5A fuse right away. I checked for solder bridges and there are none. While checking resistances on the outputs, I find that the two inner pair of 540's seemed a little low compared to another amp (K Ohms instead of M Ohms) so I back tracked to U500 which looks like it has over heated and two resistors that don't measure right. R669 should be 10 Ohms but measures inconsistantly ~50K and the resistor next to it should be 100 Ohms and measures ~2.5K.

If I replace those parts, are they the only ones that are suspect considering the symptoms, or are there more areas I need to check?

[Perry Babin]

Remove the outputs and short the pins indicated in the attached photo. That should give you a clean square wave on the input to the MIC4427. Yo can then check the output signals.

If you have to replace it, removing the two adjacent capacitors makes it much easier.

[RiLoWa]

With the scope set to 5V div and 5mS div, I get two solid traces at 0 and +5VDC on both inputs and one solid trace at +5VDC on the outputs of the 4427.

The amp continues to cycle on and off, but after several cycles it quits and apears to remain on. I haven't measured the current draw at this point, but the power supply's analog panel meter indicates ~2A. The inline resistor is hot to touch.

[Perry Babin]

Are you saying you get two lines using a single input on the scope?

If so, set the time base to ~10us and adjust the trigger level to see if you can get a lock.

[RiLoWa]

OK, scope set to 5us div. AND AC coupling, I have a nice square wave on both inputs.
On Output A (Pin7) I have a sawtooth waveform.
On Output B (Pin5) I have +5VDC

[Perry Babin]

It appears to be defective , assuming that you have supply voltage on the IC.

Use this to continue troubleshooting until you get a clean signal all of the way to the outputs. For the high-side FETs, you'll have to connect the source legs to ground to see the square wave. This is ONLY to be done with no outputs in the circuit.

[RiLoWa]

I do have +15V on Pin6 of the 4427.

So, replace the 4427 and the two resistors for sure.

When I look up the data sheet for this MIC4427BM I see it shows a delay time of 40ns. There are several varieties of this chip with different delay timings, does it matter with how it used here?

I have to order the 4427, are there other things that I need to verify now so I can put in one order?

[Perry Babin]

You can check the high-side drivers by confirming that you have a good signal to the gates of the high-side FETs.

Order the 4427 that has the same part number.

[RiLoWa]

Here is what I have on the high side gates:
5V Div. Probe x10, 5us Div.

[Perry Babin]

Did you ground the 3rd leg of the high-side FETs?