amp repair

I've just got a phoenix gold XS2300 off ebay and it doesn't work.

I wired it up to some 6x9's i had lying around to test it, the power light comes on ok but the sound coming out of the speakers (one more than the other) was fierce crackling. The speaker cone was going crazy and after about 10 secs i see the dreaded smoke from the voice coil.

I've never done any real electrical repairs before so i need all the help i can get. I've been reading up on various websites and it seems these symptoms are most likely due to blown output transistors.

What else is likely to have gone with the output transistors? can i check the power transistors while they are on the pcb or do they need removing? How can i test the input side of the amp is ok?

Pcb:

Also there is a burnt resistor (circled) but i cant read the stripes on it to replace it.

https://bcae1.com/repairbasicsforbcae1/repairbasics.htm

https://bcae1.com/repairtutorialcdinfo.htm

https://bcae1.com/tranfet.htm

Some links...Doesnt it look like this amp has been repaired at least once already...?

Your not the first person to direct me to that site, and what a fantastic site it is!

I cant see any evidence of previous repairs.

The latest development is a clicking sound a few seconds after its powered up, sort of like a relay opening and closing realy fast

Thanks.

The "goop" under the transistors is pretty sloppy...That is what lead me to think they have been replaced...?

Yeah i guess it looks worse in the pictures that it is.

To me it looks all origonal, with no eveidence of repair work, but i'm not an expert.

the solder joins on all the transistors look spot on to me:

[IMG]https://i10.photobucket.com/albums/a120/uel_f/CIMG3875.jpg[/IMG]

I get 0.713 one way, reverse the leads and i get 1.465.

Both are about the same.

I know normally that would mean the diode has failed, but i dont know with zeners

The reading starts lower, and climbs up to the said values where it settles.

They are not shorted.  Did you use a real camera or was it a phone?  If it was a camera, set it to the Macro Focus mode and take a closer picture of the resistors that burnt.  The Macro Focus mode will have a green flower symbol.  This allows your camera to focus much better when taking close up pictures.

Also follow the traces from the burnt resistor to the next components each direction.  What component is next in line?

yes it was a digital camera used, its just that when the pictures are hosted and put up on this board they are resized and loose all there quality. Can i send them to you via e-mail?

I'll check the adjacent components and report back.

Thank you.

The only burnt resistor I see is the one near the zener diodes.  Have you checked the output transistors to see if any are shorted.  You DO NOT HAVE TO REMOVE THEM to check them.  If you turned the blue and white adjustments in the amp, turn them both fully counterclockwise.  If you have not adjusted them, DO NOT ADJUST them.  What is the number off of the actual transistor that has the burnt resistors are near?

i am an idiot wrote:

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The only burnt resistor I see is the one near the zener diodes.  Have you checked the output transistors to see if any are shorted.  You DO NOT HAVE TO REMOVE THEM to check them.  If you turned the blue and white adjustments in the amp, turn them both fully counterclockwise.  If you have not adjusted them, DO NOT ADJUST them. 

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That burnt resistor isn't actually blown though, its supposed to be a 100 ohm resistor and its reading 97.3 ohms. none of the output transistors read shorted across any 2 of their 3 legs and I havent turned the trim pots.

i am an idiot wrote:

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what is the number off of the actual transistor that has the burnt resistors are near?

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You mean the output transistors? they are IRF540's

Should all 3 of the op-amps (at the rca plug end)  read the same resistance across thier respective legs? its just the 1 near the gain and bass boost pots is giving different readings compaired to the other 2, which give the same readings. They are all the same part number!

Do you know what the one near the gain pot does? or how to test it?

Thanks for your help, its much appreciated.

None of the output transistors read shorted.

When i connect  my 12v source to the amp, after a few seconds, when this clicking sound starts, it drops down to 10v, like its sucking far too much current. Any thoughts on this? 

Cheers.

i am an idiot wrote:

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Check the outputs again.  It may not read near a dead short.  Black meter lead on the tab of the transistor, red lead on the right leg it should read no lower than .400 with the meter set to the diode test function. 

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for the 4 transistors i get:

0.555   0.552   0.557   0.553

i am an idiot wrote:

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Black lead to tab and red lead to the left leg should read considerably higher than the previous readings.

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0.843   0.810   0.843   0.811

Turn both of the blue/white variable resistors fully Counter Clock Wise.  Place a single 10 amp fuse in the amp.  If it is unmounted from the heat sink, clamp all transistors back down and turn it on again.  Let me know what happens.