earthquake ph120.3 in protection mode

[Rakshark]

hello everybody, i have been reading here for a while, this is my first post here. i have an earthquake ph-120.3 amplifier which is stuck in the protection mode even with all speakers removed, rca cables unpluged. i metered the speaker outputs and there is a reading of +- 4v on there. any help would be great!

 

[Perry Babin]

If no one can help, post a photo of the inside of the amp.

 

[Rakshark]

hi Perry , sorry about the late reply,here is a picture of the board. many thanks

 

[Perry Babin]

Is it still going into protect mode with the output transistors out of the circuit?

 

[Rakshark]

hi there, yes it is, a day before it shut down there was a humming sound coming from the amp.

 

[Perry Babin]

Check Q510 and the corresponding transistors in the other channels.

What is the IC that's been replaced in the power supply?

 

[Rakshark]

hi there, i checked the transistors and these are the readings as follows:
1.638.1 ohms 1. 0.634 diode test
2.513.6 2. 0.518
3.ol 3. ol
4.ol 4. ol
5.512.7 5. 0.517
6.601.9 6. 0.607
these readings were very similiar for all six of them, used your method for testing smd transistors. the ic is a 8 pin one,has a number on it: 358A, i tried to remove it thats why it looks like that. the power supply fet's only have +13v on the centre leg. output transistors have +3.8v on all legs. i checked power supply fet's and found one faulty one. many thanks

 

[Rakshark]

help from anyone would be appreciated .
thanks in advance.
Rakshark
http://www.google.co.za/url?sa=t&rc...sg=AFQjCNH1r13iJixYUdxFGJno57Zyb2XsQA&cad=rja

 

[Perry Babin]

Post the DC voltage on all pins of the LM358 and the TL494. Copy and paste the following into your reply. Place the black probe on the amplifier's ground terminal.

LM358
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

TL494
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:

 

[Rakshark]

LM358
Pin 1: 8.70
Pin 2: 8.93
Pin 3: 8.40
Pin 4: 0.002
Pin 5: 5.17
Pin 6: 9.42
Pin 7: 0.008
Pin 8: 10.36

TL494
Pin 1: 0.240
Pin 2: 7.92
Pin 3: 0.242
Pin 4: 0.024
Pin 5: 3.106
Pin 6: 6.69
Pin 7: 0.002
Pin 8: 12.71
Pin 9: 0.001
Pin 10: 0.001
Pin 11: 12.70
Pin 12: 10.36
Pin 13: 8.43
Pin 14: 8.43
Pin 15: 8.43
Pin 16: 8.00

kind regards.

 

[Perry Babin]

It appears that both ICs may be defective. Does pin 3 of the LM358 connect directly (0 ohms) to pin 14 of the 494?

 

[Rakshark]

hi Perry, sorry about late reply, tried changing 494 and no difference, dont have smd 358 to try, pin 3 of 358 connects to a cap marked fc10 which one leg connects to pin 2 of 358 which connects to pins 13.14,15 of 494. i removed both ics to see if any difference , no change.

many thanks

 

[Perry Babin]

Follow pin 1 and 7 from the LM358 and see if they connect to any pins on the 494. Theere may be diodes or resistors in-between the two IC. Capacitors aren't a concern because they can't pass DC from one IC to the other.