transistor amp not working after shorting output

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#241
If all is checked you could power up the amp without the two resistors and without the two big transistors.
Connect the power supply and look for parts that might become hot. I mean hot, not warm 🙂
If that's the case or the lightbulb keeps glowing, something is wrong.
If all is well, connect an audio source with a volume control.
Connect a small speaker to the output.
Play some music very softly and listen if you hear something.
The amp will have no power because the big transistors are not installed but you should hear something.
 
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#243
Yes, it will glow at startup for just a second or two.
After that it should stay dim.

If something is wrong and the amp would draw too much current, the bulb will save it from doing that.
The bulb will draw the current, not the amp.
 
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#247
Are you sure you fixed this loose resister leg?

LooseLeg.jpg
 
#248
oscroft said:
After reading the thread, here are a few thoughts for a newcomer to amp electronics...

I have a number of multimeters, including the exact same one you have here. And, within its range limits, it's fine. It might not be as accurate as a better one, but it should be easily accurate enough - but not for low-value components like emitter resistors.

When using a light bulb as current limiter, you need to know what to expect from it. If an amp is good, the bulb should light up fairly brightly just briefly (as the power supply capacitors are charged), and then dim. But if the bulb stays bright, switch off! It means you have a dead short in there somewhere.

There's one piece of test kit I'd strongly recommend to any newcomer, a cheap Chinese-made component tester - I'll add a photo of one below. It should cost around €20. I use mine all the time, probably even more than my multimeter. The real beauty is that you don't need to know what a component is, which way round the pins go, whether a transistor is NPN or PNP - the tester will tell you all of that. For accurate measurements, no - but for a quick check of a component, it's a real time saver.

Good luck.

View attachment 1254433
Click to expand...
 
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#256
sg97 said:
should bulb shine without anything connected to outlet ?
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What you're doing is putting two things in series, the bulb and the amp. If there's no bulb, there's no complete circuit, so no current can flow. Similarly, if the amp is not in the loop then current can't flow either. But you should test it without the amp first, and if the blub lights then you know you've wired it wrong.

If it's wired correctly, current can only flow when both the bulb and the amp are connected in series. And then the bulb does two things.

First, it gives you an idea of how much current is flowing. With a good amp, the bulb should light fairly brightly for probably less than a second (as the power supply capacitors charge, which takes current), and then go very dim (as an idle amp with the volume down and not playing anything should draw very little current).

Second, the bulb limits the current that can flow. The maximum current that can flow is limited by the bulb and the amp added together. So if the amp has a short somewhere, the current is still limited by the bulb - it's then just like an ordinary bulb in the home, which does not have an amplifier in its circuit.

That gives you time to see if there's a short (because the bulb stays bright), and switch off before there's any damage. Connecting up a shorted amp without some sort of current limiter might simply result in an immediate bang and smoke (and you might be able to guess how I know that 😉).
 
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#258
oscroft said:
What you're doing is putting two things in series, the bulb and the amp. If there's no bulb, there's no complete circuit, so no current can flow. Similarly, if the amp is not in the loop then current can't flow either. But you should test it without the amp first, and if the blub lights then you know you've wired it wrong.

If it's wired correctly, current can only flow when both the bulb and the amp are connected in series. And then the bulb does two things.

First, it gives you an idea of how much current is flowing. With a good amp, the bulb should light fairly brightly for probably less than a second (as the power supply capacitors charge, which takes current), and then go very dim (as an idle amp with the volume down and not playing anything should draw very little current).

Second, the bulb limits the current that can flow. The maximum current that can flow is limited by the bulb and the amp added together. So if the amp has a short somewhere, the current is still limited by the bulb - it's then just like an ordinary bulb in the home, which does not have an amplifier in its circuit.

That gives you time to see if there's a short (because the bulb stays bright), and switch off before there's any damage. Connecting up a shorted amp without some sort of current limiter might simply result in an immediate bang and smoke (and you might be able to guess how I know that 😉).
Click to expand...
i tested it and bulb is not working on its own, i will test amp shortly, but if i understand this normal operation is dim bulb and if it shines brightly than we have a problem xd right
 
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