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.
Nigel
Does that mean the humming is to be expected? Or does the hum mean something is wrong?
Does that mean the humming is to be expected? Or does the hum mean something is wrong?
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.
Nigel
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
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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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.
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.
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.
That is not fair, badraven. I appreciate you may be frustrated, but several of us here have followed along for 190 posts here, not 4 or 5, 190. I don't think that merits a bitter response like that. We are trying to help you learn how things work and with greater understanding, you will find it less difficult to find the solution. Try keeping your neurons gleeful then...
Tools are tools, but they only do what they are designed to do. SHorting the driver bases together does one thing, it zero biases the amp to its coldest state. You problem has been that the amp wants to work but overheats soon. SHorting the bases forces the amp to its coolest condition, and that MAY allow you more time with it powered up to search for what is wrong. What it does NOT do it point out some specific failure. The fact the amp hums more is secondary to the fact it SHOULD be running cooler that way. The fact it now hums does not tell us some particular thing to look for, it allows us to do the looking without burning up the amp.
Zero biasing the amp makes it run cooler, but it is an unatural condition for the amp, so if it impairs the hum cancelling nature of the circuit, consider it a side effect.
DC coupled amps are not simple to work on, they are one big loop, which means that everything tends to affect everything else. I may be wrong, but throughout this, I think you tend to be looking for that one-bad-part that will fix it all up. You seem to focus on there being some part causing it all, and your fall-back appraoch is to replace things. I write that off to a lack of troubleshooting experience. There is no shame in that, we all start somewhere, I certainly recall many times I learned things the hard way, with smoke in my nostrils.
The difficulty here is for someone like myself, I'm not speaking for the others, to communicate to you what you need to know to troubleshoot this amp, but also what you need to know to undertsand the advice we are giving and the CONTEXT it belongs in. I fix these for a living, you certainly CAN test these circuits. What you generally cannot do is take some reading and say AHA, THAT part right ther is what is wrong. If it were that simple, you;d have been done in five posts.
Tools are tools, but they only do what they are designed to do. SHorting the driver bases together does one thing, it zero biases the amp to its coldest state. You problem has been that the amp wants to work but overheats soon. SHorting the bases forces the amp to its coolest condition, and that MAY allow you more time with it powered up to search for what is wrong. What it does NOT do it point out some specific failure. The fact the amp hums more is secondary to the fact it SHOULD be running cooler that way. The fact it now hums does not tell us some particular thing to look for, it allows us to do the looking without burning up the amp.
Zero biasing the amp makes it run cooler, but it is an unatural condition for the amp, so if it impairs the hum cancelling nature of the circuit, consider it a side effect.
DC coupled amps are not simple to work on, they are one big loop, which means that everything tends to affect everything else. I may be wrong, but throughout this, I think you tend to be looking for that one-bad-part that will fix it all up. You seem to focus on there being some part causing it all, and your fall-back appraoch is to replace things. I write that off to a lack of troubleshooting experience. There is no shame in that, we all start somewhere, I certainly recall many times I learned things the hard way, with smoke in my nostrils.
The difficulty here is for someone like myself, I'm not speaking for the others, to communicate to you what you need to know to troubleshoot this amp, but also what you need to know to undertsand the advice we are giving and the CONTEXT it belongs in. I fix these for a living, you certainly CAN test these circuits. What you generally cannot do is take some reading and say AHA, THAT part right ther is what is wrong. If it were that simple, you;d have been done in five posts.
I put forward a proposal a long time back to remove all the semiconductors one by one and check them on a multimeter. Then while they are out check all the passives. Also check the pcb for broken/blown tracks.
This is my last resort method but it has never let me down.
I tracked one fault down to the last transistor I took out which had an hfe of 1 !
This is my last resort method but it has never let me down.
I tracked one fault down to the last transistor I took out which had an hfe of 1 !
Enzo
Enzo
Forgive me if I sounded bitter, I assure you I am not. I am frustrated through. Not with the amp, with answers I get at times, more often than not it seems. It doesn't do me or anyone else any good to say something like: If the amp hums when you short the base terminals of Q18 and 19 together it means there is something wrong. Surely you see the ridiculousness of that statement. We all already know there is a problem with the amp, that's why the suggest of shorting the base terminals was offered. That's all I am saying. I apologize if I seemed insensitive.
Now if Nigel had said do this and expect the amp to sound like its going to blow up when you turn it on, that would be different. My amp isn't humming like a tube amp plugged into a two prong socket. Its humming like its going to shoot a flame out or something. You have got to understand something, I am the newbie here. I don't have your experience. I don't know what normal heat is on an output transistor is. Nor do I know what is considered a normal hum when I cross those terminals. If it was buzzing like I had a cell phone sitting next to the amp, I wouldn't worry about it.
Do you guys honestly think I don't appreciate you sticking with me through this, trust me I do. I honestly wonder why anyone is still here after 190 posts.
Believe me, I am not expecting to find "The" part that is causing the problem. I'm looking for any evidence I can find that will tell me the area to look. Aside from from a couple resistors being out of spec, everything I check seems to be within spec. I took Nigel W's advice and started pulling transistors and checking them, but they all have checked out so far. I placed an order with Mouser on Friday for enough transistors to replace everyone one. I got resistors, zeners and diodes. No matter what I find, I think I will have everything I need.
I think we have determined that the fuzz is from the preamp area but the output transistors are getting hot so either there is two problems or I am missing the boat here.
I am sorry if I have offended, and I am sure I have. All I am asking is: if you tell me to do something and a spring snake is going to jump out of the can when I pop the lid, let me know please. That way I'm not going OMG!!!!
Believe me guys I am reading. I am doing what one of the Nigels said to do. Learn about Class A biasing, AB biasing etc. That is what I have been reading the last couple of days.
BR
Enzo
Forgive me if I sounded bitter, I assure you I am not. I am frustrated through. Not with the amp, with answers I get at times, more often than not it seems. It doesn't do me or anyone else any good to say something like: If the amp hums when you short the base terminals of Q18 and 19 together it means there is something wrong. Surely you see the ridiculousness of that statement. We all already know there is a problem with the amp, that's why the suggest of shorting the base terminals was offered. That's all I am saying. I apologize if I seemed insensitive.
Now if Nigel had said do this and expect the amp to sound like its going to blow up when you turn it on, that would be different. My amp isn't humming like a tube amp plugged into a two prong socket. Its humming like its going to shoot a flame out or something. You have got to understand something, I am the newbie here. I don't have your experience. I don't know what normal heat is on an output transistor is. Nor do I know what is considered a normal hum when I cross those terminals. If it was buzzing like I had a cell phone sitting next to the amp, I wouldn't worry about it.
Do you guys honestly think I don't appreciate you sticking with me through this, trust me I do. I honestly wonder why anyone is still here after 190 posts.
Believe me, I am not expecting to find "The" part that is causing the problem. I'm looking for any evidence I can find that will tell me the area to look. Aside from from a couple resistors being out of spec, everything I check seems to be within spec. I took Nigel W's advice and started pulling transistors and checking them, but they all have checked out so far. I placed an order with Mouser on Friday for enough transistors to replace everyone one. I got resistors, zeners and diodes. No matter what I find, I think I will have everything I need.
I think we have determined that the fuzz is from the preamp area but the output transistors are getting hot so either there is two problems or I am missing the boat here.
I am sorry if I have offended, and I am sure I have. All I am asking is: if you tell me to do something and a spring snake is going to jump out of the can when I pop the lid, let me know please. That way I'm not going OMG!!!!
Believe me guys I am reading. I am doing what one of the Nigels said to do. Learn about Class A biasing, AB biasing etc. That is what I have been reading the last couple of days.
BR
Enzo
Enzo I have a question specifically directed to you.
I have been thinking about your post from yesterday.
First you said DC coupled amps are hard to work on because everything effects everything else in the amp. Fair enough, I am coming to realize that. Which of course is making it impossible to find the offending parts.
Then you said that I seem to have the idea that I am looking for a single part that is causing my problem. The correct answer on my part is obviously yes. I am looking for a single or multiple offending part(s). What else would I be doing?
Then you say that I keep reverting to the strategy of just changing parts. Which shows my lack of troubleshooting experience.
Dude what else is there? You guys have all but flat out told me that you can't troubleshoot a DC coupled amp. Yet several of you tell me you repair amps for a living.
Nigel W tells me to pull each transistor and check them. I have pulled several checking them with my meter, all the while keeping in mind what you said about how checking them with the meter in this fashion does not put real world stress on them like the circuit does.
I asked a few weeks ago for a suggestion on what meter to get for checking caps. Nigel told me not to worry about them because that is seldom ever the problem.
I've replaced the several resistors that were out of spec and replaced the output transistors again just in case R106 being so far out of spec might've damaged them.
The one thing I can not find in all the reading I have been doing is a procedure for troubleshooting amps.
If there isn't a standard procedure for troubleshooting a DC coupled amp, and changing parts is the way amateurs do it. and since you say that like its an indictment. My question to you is: How do you find the problem? You must have some logical path you follow. I have posted the measurements each of you have asked for. I have coupled and shorted and looped etc. Someone here must have some idea of what we are looking for???
It sounds like you're telling me the amp can't be diagnosed and if I change parts I'm a greenhorn. Which of course I am!
BR
Enzo I have a question specifically directed to you.
I have been thinking about your post from yesterday.
First you said DC coupled amps are hard to work on because everything effects everything else in the amp. Fair enough, I am coming to realize that. Which of course is making it impossible to find the offending parts.
Then you said that I seem to have the idea that I am looking for a single part that is causing my problem. The correct answer on my part is obviously yes. I am looking for a single or multiple offending part(s). What else would I be doing?
Then you say that I keep reverting to the strategy of just changing parts. Which shows my lack of troubleshooting experience.
Dude what else is there? You guys have all but flat out told me that you can't troubleshoot a DC coupled amp. Yet several of you tell me you repair amps for a living.
Nigel W tells me to pull each transistor and check them. I have pulled several checking them with my meter, all the while keeping in mind what you said about how checking them with the meter in this fashion does not put real world stress on them like the circuit does.
I asked a few weeks ago for a suggestion on what meter to get for checking caps. Nigel told me not to worry about them because that is seldom ever the problem.
I've replaced the several resistors that were out of spec and replaced the output transistors again just in case R106 being so far out of spec might've damaged them.
The one thing I can not find in all the reading I have been doing is a procedure for troubleshooting amps.
If there isn't a standard procedure for troubleshooting a DC coupled amp, and changing parts is the way amateurs do it. and since you say that like its an indictment. My question to you is: How do you find the problem? You must have some logical path you follow. I have posted the measurements each of you have asked for. I have coupled and shorted and looped etc. Someone here must have some idea of what we are looking for???
It sounds like you're telling me the amp can't be diagnosed and if I change parts I'm a greenhorn. Which of course I am!
BR
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You can approach things from a different angle but you would need some circuit knowledge.
Things like CCS can be looked at to see if the voltages on them are OK.
But the rest is down to testing in or out of circuit the rest of the components.
Obviously a DMM wont put working voltages on the transistors/diodes but it will give a good reliable answer to if it is completely dead or not.
You should definitely do a Hfe check on any transistors you take out. I have seen me find them with gains of 1 which is clearly wrong.
Things like CCS can be looked at to see if the voltages on them are OK.
But the rest is down to testing in or out of circuit the rest of the components.
Obviously a DMM wont put working voltages on the transistors/diodes but it will give a good reliable answer to if it is completely dead or not.
You should definitely do a Hfe check on any transistors you take out. I have seen me find them with gains of 1 which is clearly wrong.
Nigel
To check hfe I have to build a circuit for testing don't I? If so do you have a schematic for one?
Thanks!
To check hfe I have to build a circuit for testing don't I? If so do you have a schematic for one?
Thanks!
Most multimeters have one built in.
If yours has it will have a socket with BCE on it.
My Mac meter does not have that feature but I have a cheaper unit that does. The instructions are in Japanese. I don't supposed you have something you could scan and post on how to use that feature do you? Or maybe a site that covers the subject? Thanks!
The meter is an Excel XL830L I have searched for English instructions but had no luck,
BR
The meter is an Excel XL830L I have searched for English instructions but had no luck,
BR