Car amp diagnosis (help)

[dBrama]

I have an old jbl sub amplifier that i bought as "not working" in order to repair it.

With no speakers hooked up, the protection light turns on immediately, but slowly fades down, the power light flickers, the protection lights turns on and etc.

Ive taken it apart and all the components look fine, no burn marks or hot spots that i can tell. The only thing i noticed was shiny solder on the positive of a large capacitor. However I've tried removing it to test it, but its glued to the board.

Any advice on troubleshooting would be helpful.

 

[ChrisMarantz]

The first step in always measuring voltages on the board to see what looks good and and what looks bad. With car Amplifiers - I always suspect bad output transistors, usually from abuse - Chris

 

[gadget73]

If the voltages come out wrong with no load, pull the outputs and see what you get. A shorted output will drag the voltage down. Removing them should give you normal supply voltages. Sometimes there are handily placed jumpers that can be removed in order to isolate the power supply from the rest of the unit. Expect something like +/- 50v relative to ground. If you don't have both rails, its not going to work. A manual or some marking that indicates the exact voltage is helpful but its not going to be higher than the value on those big caps. Probably won't be too far under it either though.

 

[dBrama]

So after playing around with an oscilloscope, I found that my input power flickers with the light when the amp in on. I have proper voltage to the unit, but every 5 seconds it will flicker.

I found the main transformer was outputting virtually no voltage and showed as a straight line on my scope, but its not shorted.

From what i can tell, all the chips or working properly enough to at least let the amp turn on.

Any suggestions where to go from here?

 

[dBrama]

Just found the manual online and it says if the protect light blinks every 4 secs, there's DC voltage on the amplifier output

 

[gadget73]

Usually that means one or more shorted output transistors.

 

[dBrama]

So I found that all of the transistors running the transformer are shorted on the output to ground, so im going to replace all of those as well as the mosfets which seem shorted as well.

However i think my multimeter might be glitching because the faults arent always present and seem to choose when they want to be there.

 

[ChrisMarantz]

So I found that all of the transistors running the transformer are shorted on the output to ground, so im going to replace all of those as well as the mosfets which seem shorted as well.

However i think my multimeter might be glitching because the faults arent always present and seem to choose when they want to be there.

Did you remove the Transistors and Mosfets when you were testing them? it is usually best to remove them for testing - Chris

 

[petehall347]

if mosfets turn out to be bad there are more things to check . just same as anything else really