Hello, i have an Amp with an LM1876T for driving the tweeters. One channel is burning the output-resistor and the Chip LM1876T becomes untouchably hot. What could be the reason for this?
If you mean the resistor that is in series with a small cap (the zobel network), then the cause is oscillation and instability, which in turn is often caused by layout and grounding problems.
That sounds a large value cap for a zobel network... 0.1uF would be more typical.
The cause is 100% caused by oscillation of the chip but I'm afraid I've no quick fix because the issue is normally caused by wiring issues (returning grounds to the wrong point and so on). You should use a star ground and make sure to keep output currents away from the sensitive inputs of the chip.
The cause is 100% caused by oscillation of the chip but I'm afraid I've no quick fix because the issue is normally caused by wiring issues (returning grounds to the wrong point and so on). You should use a star ground and make sure to keep output currents away from the sensitive inputs of the chip.
You are right! It is a 0.1uF cap.
The amp is not self made. It is a commercial one. Could it be the supply part? There are a lot of 220uF and 22uF caps? I have checked the supply voltage and there are 25V. The voltage is regulated by an 7915 and an 7815.
The amp is not self made. It is a commercial one. Could it be the supply part? There are a lot of 220uF and 22uF caps? I have checked the supply voltage and there are 25V. The voltage is regulated by an 7915 and an 7815.
If its commercial then it sounds like a fault somewhere. Ideally an oscilloscope would be used to see what was actually going on.
Eliminate the easy stuff first. Make sure the problem isn't caused by some external connections... in other words power just the amp on its own with nothing else connected.
Its unlikely to be a faulty part in the zobel network but you should confirm the resistor is OK by measuring it and that the cap is OK by dabbing another cap of similar value across it. Also check the cap isn't short circuit.
Beyond that and it could be 'anything', even a crack in the print somewhere that is causing an open ground condition.
Eliminate the easy stuff first. Make sure the problem isn't caused by some external connections... in other words power just the amp on its own with nothing else connected.
Its unlikely to be a faulty part in the zobel network but you should confirm the resistor is OK by measuring it and that the cap is OK by dabbing another cap of similar value across it. Also check the cap isn't short circuit.
Beyond that and it could be 'anything', even a crack in the print somewhere that is causing an open ground condition.
Still measured! Over the cap is no short circuit. The problem is, if i switch the amp on it lasts 5-10 seconds to begin smocking.
The resistor burns because the amplifier is generating lots of high frequency energy at its output, in other words it is acting like an oscillator.
If you put an oscilloscope on the output you might see for example a 300 kHz sine or squarewave. Because the frequency is so high the cap presents no 'resistance' and so the full output of this oscillation appears across the 4.7 ohm causing it to burn.
Its remotely possible the cause could be a faulty chip. If its a commercial and it has worked OK in the past then its worth trying a new one. Beyond that and its either physical damage (cracked print) or a failed component around the chip.
On a test bench armed with a scope and its an easy repair... diagnosing at a distance is more difficult.
If you put an oscilloscope on the output you might see for example a 300 kHz sine or squarewave. Because the frequency is so high the cap presents no 'resistance' and so the full output of this oscillation appears across the 4.7 ohm causing it to burn.
Its remotely possible the cause could be a faulty chip. If its a commercial and it has worked OK in the past then its worth trying a new one. Beyond that and its either physical damage (cracked print) or a failed component around the chip.
On a test bench armed with a scope and its an easy repair... diagnosing at a distance is more difficult.
That looks a decent enough circuit board and it being a commercial product pretty much rules out a design or layout problem.
As the chip is in a socket I think you have to just try a replacement. I see the chip is unscrewed from the heatsink. Normally the chip would have an insulating but thermally conducting washer to isolate it electrically from the heatsink.
Has this amplifier worked OK in the past or has it some unknown history (like an ebay purchase) ?
Also replace the 4.7 ohm that has been burning.
As the chip is in a socket I think you have to just try a replacement. I see the chip is unscrewed from the heatsink. Normally the chip would have an insulating but thermally conducting washer to isolate it electrically from the heatsink.
Has this amplifier worked OK in the past or has it some unknown history (like an ebay purchase) ?
Also replace the 4.7 ohm that has been burning.
This is a new chip i ordered on ebay. I didn't mounted the washer and the screw to test it. It has worked flawless over years. Then the left channel tweeter stoppt working and i put the amp with its speakers back in theyr box over the last years. Now i would like to use the system again but i have still the problem with the tweeter.
Its very possible that the oscillation problem is damaging the tweeter because there is just so much high frequency energy being produced.
I've no magic fix I'm afraid... if it was on a bench with an oscilloscope available then it would be a different story. That said it should still be fixable. You need to look at the data sheet for the IC which shows typical applications. Try bridging any power supply decoupling caps, perhaps one is open circuit, and make sure the 4.7 ohm is still OK. If that goes open circuit then the amp will oscillate.
I've no magic fix I'm afraid... if it was on a bench with an oscilloscope available then it would be a different story. That said it should still be fixable. You need to look at the data sheet for the IC which shows typical applications. Try bridging any power supply decoupling caps, perhaps one is open circuit, and make sure the 4.7 ohm is still OK. If that goes open circuit then the amp will oscillate.
There are three individual answers to that 🙂
1/ Measure the physical size of what is fitted and select something similar.
2/ Normally a 2 watt carbon or metal film would be a good choice.
3/ If you are testing an amplifier at very high frequencies and maximum output then you need something larger still.
(in normal use i.e. music at normal volume, a 0.5 or 1 watt would be fine. It should never get even warm with typical music)
1/ Measure the physical size of what is fitted and select something similar.
2/ Normally a 2 watt carbon or metal film would be a good choice.
3/ If you are testing an amplifier at very high frequencies and maximum output then you need something larger still.
(in normal use i.e. music at normal volume, a 0.5 or 1 watt would be fine. It should never get even warm with typical music)
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