# My first attempt at fixing an amp



## sonikaccord (Jun 15, 2008)

My JBL PX300.4 goes into protect with nothing hooked up but the power wires.
The pinout voltages of the tl494 are
1. 1.2mV
2 4.98V
3 56.8mV
4 4.97V
5 1.465V
6 3.66V
7 1.3mV
8 12.31V
9 .6mV
10 .6mV
11 12.3V
12 12.3V
13 4.98V
14 4.98V
15 4.98V
16 3.88V

The amps power light flickers on sporadically. I applied ground to pin 4 and the amp wanted to switch on but it always went back into protect.

This pin has thermal, dc offset, and i believe short protection on it.
I can upload the schematics if needed


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## Morganpartee (Jul 2, 2009)

Looks like something is burnt out preceding that pin then. Check diodes.


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## sonikaccord (Jun 15, 2008)

I've found something that corresponds to the amp coming on for a split second and going back into protect. The rear channel has a small offset voltage and the left channel has zero. When the amp comes on the offest voltage is at about 0V and then when it goes back into protect it jumps up to about 180-190mV then slowly decreases back down and the cycle repeats itself.

Could this be a bad cap somewhere?


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## sqshoestring (Jun 19, 2007)

Usually pin 3 is high if it is protecting, depending on how they are using the 494. If 3 and 4 are low and it still does not run, then I'd wonder about the IC being bad. Assuming 9 and 10 run the power supply if you ground 4 the power supply should light up. On the other hand I'd find out why pin 4 is high, if you can lift something before it try to see if the voltage is from the IC or elsewhere, and if elsewhere then what. I'm not that familiar with JBLs.


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## sonikaccord (Jun 15, 2008)

Every time I ground pin 4, the power supply will come on and then go right back into protect. I'm assuming something is turning on a transistor that's connected to the protection circuit but I'm not sure where to find it.


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## DanMan (Jul 18, 2008)

Check out the Diy audio forum. Perry Babin, the dude who made the BCAE site, is extremely helpful with amp issues.


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