JBL Px300.4


2007-04-11 9:42 pm
I am starting to work on a jbl Px300.4 amp that is in protect mode. none of the power supply fets seem to be shorted and it is not drawing high current. Checked the outputs for shorts and they all appear to be fine. i also checked the power supply drivers and the audio drivers and i could not find any shorts. there were many bad/overheated solder joints with the typical oxidation rings on them that I re-soldered mostly in the audio output drive stages. could it be bad opamps in the input section? has anyone seen common problems with these amps?

here are the TL494 voltages.

1: .001
2: 4.98
3. .063
4: 4.97
5: 1.48
6: 3.68
7: .001
8: 13.98
9: .001
10: .000
11. 14.00
12: 14.00
13: 4.98
14: 4.98
15: 4.98
16: 4.42

also, does anyone have schematics for this amp or one like it?
Some external circuit is shutting down the TL494 by pulling pin 4 (duty cycle control/soft start) high to 5V. Pin 4 should be near ground to allow normal operation and maximum duty cycle. The rest of the voltages seem correct.

Consider drawing an schematic of whatever is connected to pin 4 (usually additional protection circuits) and studying the voltages. Something is probably triggering the protection circuit.


2007-04-11 9:42 pm
OK. I found the problem. I isolated pin 4 of the tl494 and grounded it. I started the amp up through a 2 ohm current limmiting resistor and the amp tried to turn on but was drawing high current. i found that a small diode was shorted in the channel 1 audio drive section of the amp. once the diode was replaced the amp worked fine.

do you think this was a random failure or was the amp overdriven to cause such a failure?

Thanks for the help.