50W Stereo Kit amp runs for 1 min, clips then turns off.

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Hey guys.
I've recently bought and assembled an Altronics K5130 50W Stereo Amp (http://www.altronics.com.au/index.asp?area=item&id=K5130) powered by a 25+25 300VA Toroidal Transformer (http://www.altronics.com.au/index.asp?area=item&id=M5525).
After around 1 minute of run time the amp starts to clip and the sound becomes tinny for around 2 seconds. After this the speakers go quiet. The only way to get sound back is to turn off the amp for around 10-20secs and then the whole process repeats again.
Does anyone have any ideas as to why this is happening?
 

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The chips will go into protection mode with under or over voltage. Have you measured your power supply? Is it split rail or single supply?

EDIT: I have just looked at the Power Supply schematic you have photographed. This shows a supply of + / - 15v using voltage regs. The LM3886 needs at least +/-28v.
 
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Correct me if I am wrong here, but I see the green wire going to the wrong voltage. one is V+ (from cap) and the other side goes to V-:eek:
one wire should be V- and the other V+...
ok I guess the voltage is good ...But if you see the smallest cap on the right...IT IS WRONG!!!!
 
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I've measured the transformer output, the rails and the source voltage. Even after the amp stops producing sound, these values are all withing the values stated in the manual. It seems that some componant may be overheating but the power transistors are luke warm at most. 15V is only used if a pre-amp is to be connected. The rails are running at around 37V. Hope this helps.
 
The wrongly polarized electrolytic is probably not reason for the failure - althoug it should still be fixed.
From the description it sound more like the amp going into oscillations and then goes into protect mode.
To prevent that, you might want to parallel the 22k feedback resistor with a 50pF capacitor maybe in series with another 22k resistor.

Check out testcircuit 2 on page 5:
http://www.ti.com/lit/ds/symlink/lm3886.pdf

You can solder this to the underside of the PCB on the two leads on the 22kohm resistor.
edit: I would just use the 50pF or 47pF capacitor directly parallel over the 22k feedback resistor
 
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Al6000 could you measure the dc resistance of the speakers you are driving? with 35V rails you should really not be running less than 8 ohm speakers.

for a nominally 8 ohm speaker you may get a reading of perhaps 6 ohms across the speaker terminals.

Tony.
 
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