One of my little chip amps has failed! - diyAudio
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Old 27th March 2013, 08:52 AM   #1
fap is offline fap  Australia
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Default One of my little chip amps has failed!

G'day all, this evening one of my little DIY chip amps based on an LM1875T stereo amplifier chip suddenly failed without warning whilst listening to a radio program.

The .5 amp mains fuse blew and a new one resulted in 'sizzles' from the power supply area. I have a spare DIY amplifier of the same type that I'd previously built which I've put into service.

I've had a quick look at the faulty unit and all wiring looks normal and intact but one thing I found which is odd is that one of the solder connections on the legs of one of the power supply filter capacitors seems to have lost all solder! I'm sure that it had been properly soldered. Did it vaporise or melt? Regards, Felix.
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Old 27th March 2013, 09:12 AM   #2
Mooly is offline Mooly  United Kingdom
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I doubt it. A (good) soldered joint would take all the current a chip amp PSU could throw at it.

Probably the chip has failed either as a result of some problem or on its own. As long as the bridge is OK and the chip is good there's little else left really.

Use a bulb tester (and no speakers) for powering up after repair.
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Old 27th March 2013, 09:24 AM   #3
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G'day mate, yes I suspect one of the chips has probably failed and stressed the power supply. That particular capacitor was leaking badly and has been replaced but still no go. Oh well, I'll look at it later. It gave good service for quite a few years. Regards, Felix.
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Old 27th March 2013, 09:26 AM   #4
Mooly is offline Mooly  United Kingdom
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Take the chip out and check the PSU is OK on its own.

Then fit a new chip (and use a bulb tester... it saves excess current eing drawn if there is a fault)
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Old 27th March 2013, 09:44 AM   #5
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it can happen that one of the big caps in the psu went bye bye.
replace anything bigger than 1000 uF cap along with the chip and most probably all will be fine.
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Old 27th March 2013, 09:51 AM   #6
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Cap blown? Solder joint melted?
Check the rectifier diodes...
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Old 27th March 2013, 09:52 AM   #7
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G'day all, one question, as the power supply is common to both channels how do I (quickly) identify the faulty chip? Regards, Felix.
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Old 27th March 2013, 10:34 AM   #8
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G'day all, on the bridge rectifier, that's interesting. How common are rectifier failures? Regards, Felix.
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Old 27th March 2013, 02:30 PM   #9
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if there was a short in the buffer cap and the fuse ratings where not selected properly then the chanses are around 100% or more.
but its unlikely in your case.
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Old 27th March 2013, 04:29 PM   #10
Mihkus is offline Mihkus  Estonia
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Yep bridge rectifier failure is common.

Once i said to a guy who had blown amp and strong mains noise in the output
"Your bridge is shorted"
And noone belived me, after few days he came back after very long testing and measuring "You were right"

But only one side was blown half bridge per rail...
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