JMFahey If Q20 and 21 are shorted, wouldn't that cause the fuse to blow? Now that I can beginning to see how this amp works I am thinking these have been what was causing the intermittent humming and cutting out in the past.
About your comments on this amp. Some people say these amps sound like crap but I have loved the sound of this thing since I got it. It has a very nice clean channel and the overdrive is quite versatile, in my opinion of course. Thanks!
About your comments on this amp. Some people say these amps sound like crap but I have loved the sound of this thing since I got it. It has a very nice clean channel and the overdrive is quite versatile, in my opinion of course. Thanks!
So is there a way ti test the transformer and the smoothing caps to verify their integrity? I don't think I will spend the money for the transformer since it is so expensive but the caps are within reason.
BR
JMFahey Tests
Checking resistance on Q20 18 19 21 are giving me all sorts of crazy readings.
Q20 was 1.1 and going down. 18 was .89 and going down 19 was .67 and going up and 21 was .65 and going down. I am using the heat sink as ground since its ground when the PCB is mounted to the chassis.
Diode test:
Q20 BE .036 BC .036
Q18 BE .623 BC .620
Q19 BE 1.911 BC 1.911
Q21 BE 0 BC 0
Just for Reference I am using a MAC Tools EM710 DMM
Mac Tools Online Store - Digital Multi-Meter with Backlit Display
2) now with amp off, even better unplugged, you will take some resistance readings.
To be more precise, we will use the "Diode test" setting, which applies a couple mA to the probes and shows the voltage reading between them.
We will measure the big suspects, the output and driver transistors.
We *expect* to find around 0.65V (650mV) across BE and BC junctions, one way; open or much higher (over 2V) the other way, and also open CE, both ways.
If any reading in a transistor is close to "0", it's an immediate suspect.
Carefully remove it and remeasure "outside".
Your multimeter will probably show a higher value, say 1/2 or 1 ohm, but that's fine, regular multimeters also add their own internal resistance (wiring/cpnnectors/switches/probes) which never is "0", what we don't expect there is "open" or a *large* value , sy 10 ohms or more.
Post results.
PS: just as an extra test, measure resistance to ground from Q20/21 collectors.
We don't want to find them shorted to ground.[/QUOTE]
Checking resistance on Q20 18 19 21 are giving me all sorts of crazy readings.
Q20 was 1.1 and going down. 18 was .89 and going down 19 was .67 and going up and 21 was .65 and going down. I am using the heat sink as ground since its ground when the PCB is mounted to the chassis.
Diode test:
Q20 BE .036 BC .036
Q18 BE .623 BC .620
Q19 BE 1.911 BC 1.911
Q21 BE 0 BC 0
Just for Reference I am using a MAC Tools EM710 DMM
Mac Tools Online Store - Digital Multi-Meter with Backlit Display
2) now with amp off, even better unplugged, you will take some resistance readings.
To be more precise, we will use the "Diode test" setting, which applies a couple mA to the probes and shows the voltage reading between them.
We will measure the big suspects, the output and driver transistors.
We *expect* to find around 0.65V (650mV) across BE and BC junctions, one way; open or much higher (over 2V) the other way, and also open CE, both ways.
If any reading in a transistor is close to "0", it's an immediate suspect.
Carefully remove it and remeasure "outside".
Your multimeter will probably show a higher value, say 1/2 or 1 ohm, but that's fine, regular multimeters also add their own internal resistance (wiring/cpnnectors/switches/probes) which never is "0", what we don't expect there is "open" or a *large* value , sy 10 ohms or more.
Post results.
PS: just as an extra test, measure resistance to ground from Q20/21 collectors.
We don't want to find them shorted to ground.[/QUOTE]
Dear badraven, in a nutshell, yes, your output transistors are shorted , replace both oputputs and drivers, meaning Q18/19/20/21 .
Be careful, pamper the PCB, the *one* part that you can't buy anywhere , use a good solder sucker, maybe even a little solder wick to better clean the remaining solder.
You want to be able to pull parts without wiggling, a torn pad is a PITA.
When ordering the transistors, also get some insulating micas for them and a little silicone/thermal grease.
Google and print those transistors datasheets, mainly to know how that case is called , so if it's , say, TO247, you'll need TO247 sized micas, etc.
Also which leg is called E, B or C so we all speak the same.
Calling them "ABC" is confusing
A "rail" is oldspeak to refer to all points in a circuit connected to the same voltage, so "+42V rail", which by the way is written so in theschematic, means all points connected to +42V, meaning Q18/20 collectors, the top side of R88/91/101, C57, D38 cathode, etc.
It really clarifies things, because measuring *anywhere* on a rail must yield the same value.
Note: if you don't know what any word means, google it; I will gladly help you to repair your amp but this is not Electronics 101, this said with the utmost respect and trying to get to the point, which is repairing this amp.
I trust you will
Be careful, pamper the PCB, the *one* part that you can't buy anywhere , use a good solder sucker, maybe even a little solder wick to better clean the remaining solder.
You want to be able to pull parts without wiggling, a torn pad is a PITA.
When ordering the transistors, also get some insulating micas for them and a little silicone/thermal grease.
Google and print those transistors datasheets, mainly to know how that case is called , so if it's , say, TO247, you'll need TO247 sized micas, etc.
Also which leg is called E, B or C so we all speak the same.
Calling them "ABC" is confusing
A "rail" is oldspeak to refer to all points in a circuit connected to the same voltage, so "+42V rail", which by the way is written so in theschematic, means all points connected to +42V, meaning Q18/20 collectors, the top side of R88/91/101, C57, D38 cathode, etc.
It really clarifies things, because measuring *anywhere* on a rail must yield the same value.
Note: if you don't know what any word means, google it; I will gladly help you to repair your amp but this is not Electronics 101, this said with the utmost respect and trying to get to the point, which is repairing this amp.
I trust you will
No, the output transistors wouldn't cause a fault like that - Q20/21 will cause the fuse to blow (and are doing so) - but replacing just those will most likely result in them failing again, you need to replace EVERYTHING that's damaged or faulty in the DC stage of the power amp.
This damage has almost certainly been caused by something you did, as it wasn't like it before you took it apart - any slight short circuit you might cause, even just for milliseconds, can easily blow the amp.
If a transformer was damaged, it would probably be visible. Blown outputs also mean more damage downstream. Your driver transistors (little ones also on the heatsink) and the emitter resistors are also most likely toast. In the wise words of Enzo, "throwing parts at a failure is like throwing sponges at a rainstorm."
Last edited:
JMFahey
JMFahey - No offense taken. You are correct, this is not necessarily a training forum for electrical terms. Even though in a way it actually is. Leaning what terms mean and how things work i.e. conventions etc. is also a matter of learning where to find the info I need. i.e. definitions etc.. I'm learning to do all these things. I appreciate your patience as my learning curve accelerates in this area.
Thanks for clarifying what a rail is. I have been seeing that and now I understand what it mean.s
With that said, I understand about being gentle when desoldering. I have soldering wick and I have a premium sucker if I need it.
No one has answered my questions about where to get these transistors. I am thinking of checking with Radio Shack here in town in hopes they have these items in stock. But they are normally kinda high on their prices and I have never thought of them as carrying quality items. So maybe that is a bad choice.
As for the pin conventions of the transistors, you said B C E (L to R) to alleviate confusion so that is how I am listing. If you look at my earlier post you will see that I acknowledged your request and agreed to follow it.
Again Thank you!
BR
JMFahey - No offense taken. You are correct, this is not necessarily a training forum for electrical terms. Even though in a way it actually is. Leaning what terms mean and how things work i.e. conventions etc. is also a matter of learning where to find the info I need. i.e. definitions etc.. I'm learning to do all these things. I appreciate your patience as my learning curve accelerates in this area.
Thanks for clarifying what a rail is. I have been seeing that and now I understand what it mean.s
With that said, I understand about being gentle when desoldering. I have soldering wick and I have a premium sucker if I need it.
No one has answered my questions about where to get these transistors. I am thinking of checking with Radio Shack here in town in hopes they have these items in stock. But they are normally kinda high on their prices and I have never thought of them as carrying quality items. So maybe that is a bad choice.
As for the pin conventions of the transistors, you said B C E (L to R) to alleviate confusion so that is how I am listing. If you look at my earlier post you will see that I acknowledged your request and agreed to follow it.
Again Thank you!
BR
Dear badraven, in a nutshell, yes, your output transistors are shorted , replace both oputputs and drivers, meaning Q18/19/20/21 .
Be careful, pamper the PCB, the *one* part that you can't buy anywhere , use a good solder sucker, maybe even a little solder wick to better clean the remaining solder.
You want to be able to pull parts without wiggling, a torn pad is a PITA.
When ordering the transistors, also get some insulating micas for them and a little silicone/thermal grease.
Google and print those transistors datasheets, mainly to know how that case is called , so if it's , say, TO247, you'll need TO247 sized micas, etc.
Also which leg is called E, B or C so we all speak the same.
Calling them "ABC" is confusing
A "rail" is oldspeak to refer to all points in a circuit connected to the same voltage, so "+42V rail", which by the way is written so in theschematic, means all points connected to +42V, meaning Q18/20 collectors, the top side of R88/91/101, C57, D38 cathode, etc.
It really clarifies things, because measuring *anywhere* on a rail must yield the same value.
Note: if you don't know what any word means, google it; I will gladly help you to repair your amp but this is not Electronics 101, this said with the utmost respect and trying to get to the point, which is repairing this amp.
I trust you will
Nigel
Nigel I'm not trying to say I didn't cause a problem. I'm just saying I can't find a single joint that is shorted. The couple of joints I suspected and desoldered had a track between them and were resoldered. Every time I sit down to do something with this amp I am looking over all the joints to see if I am missing something obvious. But nothing so far. I am using a lighted 5x magnifying glass and 2x reader glasses. The truth is I only reflowed the joint on the board from the end of the PCB with the power supply connections to the right side (in the picture I uploaded) of Q21. In the pic follow the right side edge of the chip across the board (down in the pic) I got interrupted and didn't reflow any others.
I am still learning how to read the schematic so maybe you could tell me the items down wind of Q18, 19, 20 and 21 that need to be replaced. I might as well get it all done.
BR
Nigel I'm not trying to say I didn't cause a problem. I'm just saying I can't find a single joint that is shorted. The couple of joints I suspected and desoldered had a track between them and were resoldered. Every time I sit down to do something with this amp I am looking over all the joints to see if I am missing something obvious. But nothing so far. I am using a lighted 5x magnifying glass and 2x reader glasses. The truth is I only reflowed the joint on the board from the end of the PCB with the power supply connections to the right side (in the picture I uploaded) of Q21. In the pic follow the right side edge of the chip across the board (down in the pic) I got interrupted and didn't reflow any others.
I am still learning how to read the schematic so maybe you could tell me the items down wind of Q18, 19, 20 and 21 that need to be replaced. I might as well get it all done.
BR
As Nigel says replacing output transistors doesn't always fix the original fault.
If the amp output was shorted then sometimes jut replacing output transistors will fix it.
If some part of the driver or bias circuit has gone wrong then you will keep blowing output transistors.
Not for a novice but what I do is remove all output transistors ( or disconnect them) and route the driver output back into the LTP and then I can check bias and DC offset without blowing fuses.
Otherwise you need to check all resistors/transistors in the driver circuit too.
If the amp output was shorted then sometimes jut replacing output transistors will fix it.
If some part of the driver or bias circuit has gone wrong then you will keep blowing output transistors.
Not for a novice but what I do is remove all output transistors ( or disconnect them) and route the driver output back into the LTP and then I can check bias and DC offset without blowing fuses.
Otherwise you need to check all resistors/transistors in the driver circuit too.
nigelwright7557
I'll do what I need to do. The amp is broke so it just experience for me at this point. Can you identify the "Driver Circuit" for me or are you saying to pull and check every piece on the board? Thanks!
BR
I'll do what I need to do. The amp is broke so it just experience for me at this point. Can you identify the "Driver Circuit" for me or are you saying to pull and check every piece on the board? Thanks!
BR
I would first replace output transistors then fire up the amp with a lamp in series with the mains. A 40 watt lamp is fine.
If the lamp comes on then there is another fault to fix.
If not try it with some music.
Likewise - I often do the same - and quite often run it for a good while like that with a scope on the output and a suitable dummy load (a LOT higher than the speakers impedance).
Nigel
I don't have a scope, although I do plan to get one when the opportunity presents its self.
Can you decipher this for me please. "route the driver output back into the LTP and then I can check bias and DC offset without blowing fuses."
I did a search for LTP Electronics and came uip with all sorts of crazy results.
Please explain this connection your describe and how to test it. Or direct me to where I can find the info I need. That way I will be ready when my parts arrive.
Thanks!
BR
I don't have a scope, although I do plan to get one when the opportunity presents its self.
Can you decipher this for me please. "route the driver output back into the LTP and then I can check bias and DC offset without blowing fuses."
I did a search for LTP Electronics and came uip with all sorts of crazy results.
Please explain this connection your describe and how to test it. Or direct me to where I can find the info I need. That way I will be ready when my parts arrive.
Thanks!
BR
Dear badraven.
I'm trying to make this simple because I think *today* you want to have your amp working.
So I *try* to keep math/tech stuff as little as possible , although of course some is still necessary, and give you "paint by the numbers" instructions.
That amp *is* repairable , but I had a not-too-good experience in another Forum (MEF) where a bunch of us, including our friend Enzo , tried to help a Texan Tech (won't give names) to repair a FM212.
What could have been solved in, say, 10 questions and answers streched for over 100 posts, basically because he did not answer what he was asked , as in "measure Q18 base voltage" and he answered "suspect one cap, pulled it to measure ESR, will probably replace it" which in due course degenerated in a discussion about capacitor dielectric influence on sound, and so on and on and on.
You already measured outputs and drivers, found shorts, fine. Replace them.
Then we'll go on
PS: mind you, that Tech I referred to, is a regular Tech, has all kinds of fancy equipment, and knows how to use it, simply can't focus on what he's doing and his mind jumps all over the place, thinking about 10 different things at the same time, instead of concentrating on the specific problem he's solving *now*.
Of course it takes much longer that way
I'm trying to make this simple because I think *today* you want to have your amp working.
So I *try* to keep math/tech stuff as little as possible , although of course some is still necessary, and give you "paint by the numbers" instructions.
That amp *is* repairable , but I had a not-too-good experience in another Forum (MEF) where a bunch of us, including our friend Enzo , tried to help a Texan Tech (won't give names) to repair a FM212.
What could have been solved in, say, 10 questions and answers streched for over 100 posts, basically because he did not answer what he was asked , as in "measure Q18 base voltage" and he answered "suspect one cap, pulled it to measure ESR, will probably replace it" which in due course degenerated in a discussion about capacitor dielectric influence on sound, and so on and on and on.
You already measured outputs and drivers, found shorts, fine. Replace them.
Post results.
Then we'll go on
PS: mind you, that Tech I referred to, is a regular Tech, has all kinds of fancy equipment, and knows how to use it, simply can't focus on what he's doing and his mind jumps all over the place, thinking about 10 different things at the same time, instead of concentrating on the specific problem he's solving *now*.
Of course it takes much longer that way
Last edited:
JMFahey I'm not trying to drag anything out. I think I have provided everything you have asked except for removing the transistors, which you said are shorted and need to be replaced. I figure I will remove them and replace them when the parts arrive. Less chance of losing things if they stay attached.
If however I have missed something please forgive me, I am being tossed a lot of ideas here. You basically said to concentrate on what you are telling me so as to avoid confusion. That is what I am doing but when others say something that peaks my interest I ask them to clarify. I did ask for help here. It would be rude for me to ignore people that seem to be earnestly trying to help. I mean there have been several open ended comments by people tossing a guess into the mix, of which I paid little attention.
I am trying to locate the parts on the Internet right now, including the two site you mentioned. I'm not doing a very good job of finding the parts though.
I did answer your question about R109 and R112 in post 39.
R109 = 0 ohms
R112 = .1 (20 scale)
Again, I am doing what you have said to do.
Thanks!
BR
If however I have missed something please forgive me, I am being tossed a lot of ideas here. You basically said to concentrate on what you are telling me so as to avoid confusion. That is what I am doing but when others say something that peaks my interest I ask them to clarify. I did ask for help here. It would be rude for me to ignore people that seem to be earnestly trying to help. I mean there have been several open ended comments by people tossing a guess into the mix, of which I paid little attention.
I am trying to locate the parts on the Internet right now, including the two site you mentioned. I'm not doing a very good job of finding the parts though.
I did answer your question about R109 and R112 in post 39.
R109 = 0 ohms
R112 = .1 (20 scale)
Again, I am doing what you have said to do.
Thanks!
BR
Dear badraven.
I'm trying to make this simple because I think *today* you want to have your amp working.
So I *try* to keep math/tech stuff as little as possible , although of course some is still necessary, and give you "paint by the numbers" instructions.
That amp *is* repairable , but I had a not-too-good experience in another Forum (MEF) where a bunch of us, including our friend Enzo , tried to help a Texan Tech (won't give names) to repair a FM212.
What could have been solved in, say, 10 questions and answers streched for over 100 posts, basically because he did not answer what he was asked , as in "measure Q18 base voltage" and he answered "suspect one cap, pulled it to measure ESR, will probably replace it" which in due course degenerated in a discussion about capacitor dielectric influence on sound, and so on and on and on.
You already measured outputs and drivers, found shorts, fine. Replace them.
Post results.
Then we'll go on
PS: mind you, that Tech I referred to, is a regular Tech, has all kinds of fancy equipment, and knows how to use it, simply can't focus on what he's doing and his mind jumps all over the place, thinking about 10 different things at the same time, instead of concentrating on the specific problem he's solving *now*.
Of course it takes much longer that way