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Old 11th February 2013, 11:19 PM   #191
Enzo is offline Enzo  United States
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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.
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Old 11th February 2013, 11:25 PM   #192
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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 !
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Old 12th February 2013, 01:01 AM   #193
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Default 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

Quote:
Originally Posted by Enzo View Post
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.
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Old 12th February 2013, 01:02 AM   #194
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Default Nigel

I am doing this, one item at a time. Thanks!

BR


Quote:
Originally Posted by nigelwright7557 View Post
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 !
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Old 12th February 2013, 09:39 PM   #195
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Default 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

Quote:
DC coupled amps are not simple to work on, they are one big loop, which means that everything tends to affect everything else.

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.
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Last edited by badraven; 12th February 2013 at 09:43 PM.
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Old 12th February 2013, 09:47 PM   #196
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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.
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Old 12th February 2013, 10:12 PM   #197
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Quote:
Originally Posted by nigelwright7557 View Post
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.
To check hfe I have to build a circuit for testing don't I? If so do you have a schematic for one?

Thanks!
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Old 12th February 2013, 10:15 PM   #198
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Quote:
Originally Posted by badraven View Post
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.
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Old 12th February 2013, 10:19 PM   #199
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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
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Old 12th February 2013, 10:21 PM   #200
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The meter will have 2 setting on the rotary dial NPN and PNP, select which ever your transistor is. Then put the transistor leads into the BCE socket in the correct order.
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