Amplifier Repair Question (Large Image WARNING!)

[scott_jcp]

A friend of mine passed on to me an amplifier that needs to be repaired. It is a EV TAPCO PA-700. He plugged it into the wall, and flicked the switch, and flames shot out of the fan hole (there were no inputs attached and no outputs attached).

Upon opening the amp, I discovered that a fair number of resistors on one channel were fried, but everything else appears generally okay. Do I just need to replace the destroyed components? I don't have a schematic, but the other channel is in perfect condition, so I should be able to work off of that.

Here are some pictures of the destoryed channel, and its corresponding good channel:
Section 1:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

Section 2:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

Section 3:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

I'd appreciate any suggestions I can get on this, as I've never repaired an amp before.

 

[sek]

Hi,

if You have never repaired an amp before, this one will become difficult.

I think I see burnt transistors, too!

My recommendation would be to clean the board without desoldering or cutting any components, first.
This way you can check out how much the board material is affected and also distinct most of the components that look damaged.

Then draw the schematic according to the other channel and mark every component that has been damaged. By posting a schematic with all the damage described, I'm sure someone on the forum can help (I don't know this particular amp).

Sebastian.

PS: The transistors we can see are the drivers for TO-3 power transistors underneath the board below the screws, right? Are those still okay?

 

[scott_jcp]

from my cursory examination, the big output transistors all look in mint condition.

 

[Stocker]

one of the big high-voltage-rated red capacitors is blasted as well.

 

[astouffer]

Resistors are passive devices and fail because something else caused them to. If a resistor is burned up then replacing the resistor won't solve the problem. The most likely culprit is a transistor going bad.


Adam

 

[richwalters]

http://www.diyaudio.com/forums/showthread.php?goto=newpost&threadid=55479

Complaint...........this has virus attached.......plus absurdly long and blocks cookies...

richj

 

[Enzo]

You cannot tell if the large power transistors are OK by looking. You will have to check them with a meter to see if they are shorted.

I don't recall what the power transistors are in that one, but if they are the J49 and K134 pair, you are screwed. They wil cost more than a new amp.

 

[sss]

u can test the transistors with a multimeter if u got one , put it on diode test , the transistor is like 2 diodes ..........blah blah , hope u know that allready