Hi,

my friend had a car audio amplifier and when he was installing it he by mistaken interchanged the power polarites. Now the amplifier is in protection mode and when i took a look at it the output transistors seem working fine. What could be the fault?

 

Oh, lots of things could be wrong now, starting with it's internal power supply.

Doctor, it hurts when I do this.

Well then don't do that.

 

is there a way how can i check it? but if the internal power supply goes wrong no supply thus no leds will be on?

 

Open it and post high resolution pictures of the PCB from different angles (top and bottom).

If you see any burnt component take an extra picture of that area (also both sides of the PCB).

Post the amplifier model number and manufacturer.

Troubleshooting an unknown circuit usually involves drawing the schematic of this circuit or at least part of it.

 

no burnt components seem to be on the pcb.


on the board i have written AP-2300TB, the model of the amp is boschmann PCH-2600SX

 

note i took 2 power mosfets out of the board to check them....which seem ok when i checked them.

 

Second picture, near the bottom, next to the red jumper, probably a protection diode- I would start with checking that.

 

Looks like the resistors have been fried, check if they are open circuit, they look like 1 ohm( judging by the colour codes brown, black silver)

 

i checked that diode, it gives 0.547v in one side and OL in the other side. That means it is ok? Dodgydave to which resisotrs are you refering?

 

i checked that diode, it gives 0.547v in one side and OL in the other side. That means it is ok? Dodgydave to which resisotrs are you refering?

The big green resistors. If they are burnt you need to check also all the components in the TO-220 packages. What is printed on them? Are they all the same model? can you take a better picture of them?

How did you check the MOSFETs you took out?

 

i checked that diode, it gives 0.547v in one side and OL in the other side. That means it is ok? Dodgydave to which resisotrs are you refering?[/QUOTE]

The large green-ish cylinders with coloured rings on them, there are six on each side of your pcb, they look like they have been heated up!!

Also what are the transistors in the T0220 packages next to them, there are six per side? (looks like you have taken one out on the right side)

Just as a suggestion take a look at this diagram IT WONT BE FOR YOUR AMP, http://3.bp.blogspot.com/-zNZi3Xw8j...AAAdg/gimcj9Y_n8g/s1600/Actrk600schematic.jpg


your circuit will probably be the same output stage, class B or "push-pull" ( IGNORE VOLTAGES) beacuse its a stereo amp, you will have 6 transistors per side, 3 P-channel and 3 N-channel Fets
, a total of 12 transistors, which is what you have on your pcb, the resistors would be the ones marked up as R32 to R41.

 

when i check with the meter through diode check the big green resistors i beeping. the colors are red-black-silver-brown-orange. The TO-220 pakcages are BD911/BD912 for the power mosfet of the audio side and IRFZ24N for the power supply side.

I checked the mosfets by diode check and checking if they give me a voltage drop between the pins. between legs 1-2 and 1-3 it is 0.481v

 

the large resistors perhaps your seeing them heated up because they have a sort of glue over them....they are giving me 0.5ohm when i test them with meter..

 

when i check with the meter through diode check the big green resistors i beeping. the colors are red-black-silver-brown-orange. The TO-220 pakcages are BD911/BD912 for the power mosfet of the audio side

I checked the mosfets by diode check and checking if they give me a voltage drop between the pins. between legs 1-2 and 1-3 it is 0.481v

the BD's are bipolar transistors. if you cannot detect any faulty components draw the circuit from the protection LED (I assume there is a LED?) backwards and find out how it is driven. If it's a uC, bad luck, if not you may be able to find the problem.

btw, measuring in-circuit can be very misleading if you don't have a schematic.

 

i talked again to my friend. what he did exactly is that when he was wiring the supply he touched by mistake the positive terminal to the enclosure of the amplifier.