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Old 7th February 2013, 03:51 PM   #181
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Originally Posted by badraven View Post
When I short the base of 18 to 19 what am I looking to find?
It sets the bias to zero, and prevents thermal runaway.

It gives you more chance when working on a faulty amplifier as it doesn't 'self destroy' quite as quickly.
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Old 7th February 2013, 09:46 PM   #182
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Default Nigel

Does that mean the humming is to be expected? Or does the hum mean something is wrong?

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Originally Posted by Nigel Goodwin View Post
It sets the bias to zero, and prevents thermal runaway.

It gives you more chance when working on a faulty amplifier as it doesn't 'self destroy' quite as quickly.
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Old 8th February 2013, 01:36 PM   #183
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Does that mean the humming is to be expected? Or does the hum mean something is wrong?
If there's nothing wrong with the amp, it shouldn't hum.
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Old 8th February 2013, 05:32 PM   #184
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???

Does the fact that the amp hums loudly when I show the base terminals of Q18 and 19 give anyone here an idea of what might be the culprit?
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Old 9th February 2013, 08:53 PM   #185
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I'll take that as a no.
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Old 10th February 2013, 12:46 PM   #186
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I'll take that as a no.
It's a 'no' because it's a DC coupled amp, pretty well anything anywhere can cause such symptoms. Any part you replace, when you've not cured the actual fault, is likely to fail again the instant you turn it on.
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Old 10th February 2013, 07:10 PM   #187
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It's a 'no' because it's a DC coupled amp, pretty well anything anywhere can cause such symptoms. Any part you replace, when you've not cured the actual fault, is likely to fail again the instant you turn it on.
I understand that. So the fact that shorting the base terminals of Q18 and 19 together causes the amp the hum loudly doesn't cause you to think that looking in "X" area is the place to look?

I asked that earlier and you round about told me that if it hums after shorting those terminals it means there is a problem with the amp. I don't mean to come off as sarcastic and I certainly do not wish to alienate anyone here but I think everyone here already knows that. There is obviously a problem with my amp, its the reason for this thread.

Obviously no one is obligated to help me. It is entirely a goodwill endeavor to participate here. But please don't offer something that is meaningless like the above.

When someone tells me they have a problem with their car I don't respond with: Does it start? Well then that means something is wrong. It would be silly for me to say that because if something isn't wrong, its a safe assumption they're probably not going be asking my advice about how to repair it.

When someone says to me: My car won't crank (southern lingo). The proper response (if I am willing to offer help) is: When you say it won't crank, you mean it doesn't turn over or it turns over but it doesn't start? Well it cranks but it doesn't start. When did it stop running? Well I parked it yesterday and today it won't start. From this I can offer a direction to go in like, is there gas in the tank?. Did you work on the car since you turned it off yesterday? Did you drive through a puddle just before you parked it? Is there fluid on the ground under the car, like gas or oil? With these questions I can head the person in the right direction (hopefully), I can offer for instance, turn the key on to the run position and check the red wire at the coil to see if you have 12.6V. If you do, unplug the connector and check terminal 1 and 3 and tell me what the resistance is. These are helpful input. Saying something like: no power at the coil means something is broke, is not helpful.

Enzo gives good advice because when he offers something he normally gives a little info so that I can understand what he is talking about. If he doesn't and I ask him about it he will normally follow up with something to clarify. Which is all I am asking of you fellas.

I understand I am the newbie here and I probably ask to many questions. I am sorry about that. I'm not expecting anyone here to just give me the answers, though that would be nice. I am trying to learn. If you tell me to look something up or get a reading from something, I do. I also realize you do not have the luxury of physically inspecting my amp, that you have to rely on my responses when you give me something to work with. If at some point you think I am not responding correctly, let me know. Believe me, I am not a prideful man. I am open to your critiques and your constructive criticism. Maybe I didn't catch something or I simply wasn't clear enough in my response. Believe me, I go through the same thing when I help someone long distance.

In automotive there are plenty of mysteries to solve. But there is a path, even if you have to cut your own. If "this" isn't working, its controlled by "that" so I look in "this area" and see why there is no power are point A. I know its the same in electronics. Maybe not exactly the same but I am sure its similar.

BR
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Old 10th February 2013, 07:22 PM   #188
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I would like to add: I can play the amp for about 10 minutes before it overheats. That's plugged directly to the wall socket. Whatever is causing the problem can't be that big since the amp plays almost normal.
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Old 10th February 2013, 07:26 PM   #189
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I understand that. So the fact that shorting the base terminals of Q18 and 19 together causes the amp the hum loudly doesn't cause you to think that looking in "X" area is the place to look?

I asked that earlier and you round about told me that if it hums after shorting those terminals it means there is a problem with the amp. I don't mean to come off as sarcastic and I certainly do not wish to alienate anyone here but I think everyone here already knows that. There is obviously a problem with my amp, its the reason for this thread.

Obviously no one is obligated to help me. It is entirely a goodwill endeavor to participate here. But please don't offer something that is meaningless like the above.
Reducing the bias to zero gives you more chance of finding the fault, and is a VERY common professional technique.

As I've already explained you can't point to a particular part in a DC amp because it's ALL interconnected, your car analogies are utterly useless.
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Old 10th February 2013, 09:33 PM   #190
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Alright then...
DC circuits can't be tested, you just change parts and hope the bad part(s) are part of the change. And if it all burns out again you change more parts till it works or you run out of money/patience.

Well then I guess its a waste of everyone's time for me to present my problem here. Although I have learned a lot reading and trying different things on my amp. At least its not blowing fuses now.
Maybe the moderator could place a caption right below the forum title to let people know that this is the place to come as long as your problem doesn't involve a DC coupled circuit.
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