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#1
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
Good News! This headphone amplifier has been REPAIRED and is now working perfectly. Here's what happened: I have been taking parallel measurements between the "good" and "bad" channels since the beginning. I took voltage measurements, resistance measurements, tested ALL capacitors, and tested ALL transistors. But someone in my original thread (I can't remember who offhand -- sorry) did mention checking the circuit board traces. "No need," I thought. "The circuit board and all solder joints look fine to me." But in a fit of desperation, I nonetheless decided to whip out a tester I hadn't used yet: my magnifying glass. That's when I found it -- a *microscopic* crack in the trace between the emitter of Q208 and its adjoining 3.3 ohm resistor. The crack was so incredibly tiny that I easily overlooked it--repeatedly. I took one of my jumper leads and clipped one end of it to Q208's emitter and the other end of it to the 3.3 ohm resistor. I then took voltage measurements. EVERYTHING WAS NOW ON SPEC: Q205's collector now measures +7.09V, and Q206 now has the following measurements: B: +6.51V C: +7.08V E: +5.84V I was able to bridge the crack with a well-placed solder trail, and the headphone amp now sounds great. A BIG "THANK YOU" TO EVERYONE WHO HELPED ME OUT! I OWE YOU BIG TIME. |
#2
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
"EADGBE" wrote ...
That's when I found it -- a *microscopic* crack in the trace between the emitter of Q208 and its adjoining 3.3 ohm resistor. The crack was so incredibly tiny that I easily overlooked it--repeatedly. I took one of my jumper leads and clipped one end of it to Q208's emitter and the other end of it to the 3.3 ohm resistor. I then took voltage measurements. EVERYTHING WAS NOW ON SPEC: Its hard to imagine how the output node (where the two 3.3 ohm resistors join) could be correct (6.4V) without the path through the lower 3.3 ohm resistor and Q208? But at least you got it working. Good for you. Trouble- shooting ("fault-finding" as they call it in the UK) discrete circuits is rapidly becoming a lost art. |
#3
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
On Tue, 22 Apr 2008 12:38:19 -0700, "Richard Crowley"
wrote: "EADGBE" wrote ... That's when I found it -- a *microscopic* crack in the trace between the emitter of Q208 and its adjoining 3.3 ohm resistor. The crack was so incredibly tiny that I easily overlooked it--repeatedly. I took one of my jumper leads and clipped one end of it to Q208's emitter and the other end of it to the 3.3 ohm resistor. I then took voltage measurements. EVERYTHING WAS NOW ON SPEC: Its hard to imagine how the output node (where the two 3.3 ohm resistors join) could be correct (6.4V) without the path through the lower 3.3 ohm resistor and Q208? Pure coincidence - it was the breakdown voltage of the reverse biased base-emitter of Q207. But at least you got it working. Good for you. Trouble- shooting ("fault-finding" as they call it in the UK) discrete circuits is rapidly becoming a lost art. Quite so - when all the components are right, but there is still a problem, go for the board. d -- Pearce Consulting http://www.pearce.uk.com |
#4
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
On 2008-04-22, Don Pearce wrote:
On Tue, 22 Apr 2008 12:38:19 -0700, "Richard Crowley" wrote: "EADGBE" wrote ... That's when I found it -- a *microscopic* crack in the trace between the emitter of Q208 and its adjoining 3.3 ohm resistor. The crack was so incredibly tiny that I easily overlooked it--repeatedly. I took one of my jumper leads and clipped one end of it to Q208's emitter and the other end of it to the 3.3 ohm resistor. I then took voltage measurements. EVERYTHING WAS NOW ON SPEC: Its hard to imagine how the output node (where the two 3.3 ohm resistors join) could be correct (6.4V) without the path through the lower 3.3 ohm resistor and Q208? Pure coincidence - it was the breakdown voltage of the reverse biased base-emitter of Q207. Rubbish. ( Well, possibly :-) ) Q207 was indeed hard off with its base at 0V. Q208 was not connected to the output because of the fault. The output junction was therefore biassed only by the emitter voltage or Q204 (8V nominal) via the 15 k resistor. If the measurement was made with a 20 kohm/V multimeter on the 10 V range (200k) then I would expect the potential divider effect to pad that 8 V down to about 7.4 V. EADGBE actually posted that the node was 7.3V (not the nominal 6.4 V of the schematic). OK, I admit it could have been reverse bias breakdown but I prefer my version as it doesn't require that coincidence. (Well OK I do make an assumption about the measurement kit instead ...) -- John Phillips |
#5
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
On 22 Apr 2008 20:09:23 GMT, John Phillips
wrote: On 2008-04-22, Don Pearce wrote: On Tue, 22 Apr 2008 12:38:19 -0700, "Richard Crowley" wrote: "EADGBE" wrote ... That's when I found it -- a *microscopic* crack in the trace between the emitter of Q208 and its adjoining 3.3 ohm resistor. The crack was so incredibly tiny that I easily overlooked it--repeatedly. I took one of my jumper leads and clipped one end of it to Q208's emitter and the other end of it to the 3.3 ohm resistor. I then took voltage measurements. EVERYTHING WAS NOW ON SPEC: Its hard to imagine how the output node (where the two 3.3 ohm resistors join) could be correct (6.4V) without the path through the lower 3.3 ohm resistor and Q208? Pure coincidence - it was the breakdown voltage of the reverse biased base-emitter of Q207. Rubbish. ( Well, possibly :-) ) Q207 was indeed hard off with its base at 0V. Q208 was not connected to the output because of the fault. The output junction was therefore biassed only by the emitter voltage or Q204 (8V nominal) via the 15 k resistor. If the measurement was made with a 20 kohm/V multimeter on the 10 V range (200k) then I would expect the potential divider effect to pad that 8 V down to about 7.4 V. EADGBE actually posted that the node was 7.3V (not the nominal 6.4 V of the schematic). OK, I admit it could have been reverse bias breakdown but I prefer my version as it doesn't require that coincidence. (Well OK I do make an assumption about the measurement kit instead ...) Mmmmm...... 20k/V multimeters are pretty rare beasts these days. I have an old AVO 8 but I don't think I'd have the confidence to estimate two decimal places in its voltage readings ;-) My money is still on reverse breakdown. Trannies are almost as good as zeners in this configuration. d -- Pearce Consulting http://www.pearce.uk.com |
#6
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
EADGBE wrote:
That's when I found it -- a *microscopic* crack in the trace between the emitter of Q208 and its adjoining 3.3 ohm resistor. The crack was so incredibly tiny that I easily overlooked it--repeatedly. Jeepers. You would normally find that around any PCB-mount jack or DC socket. Unless the whole thing has had a severe jarring. May have been a heat/cool fatigue thang. geoff |
#7
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
In article
, EADGBE wrote: Good News! This headphone amplifier has been REPAIRED and is now working perfectly. Here's what happened: I have been taking parallel measurements between the "good" and "bad" channels since the beginning. I took voltage measurements, resistance measurements, tested ALL capacitors, and tested ALL transistors. But someone in my original thread (I can't remember who offhand -- sorry) did mention checking the circuit board traces. "No need," I thought. "The circuit board and all solder joints look fine to me." But in a fit of desperation, I nonetheless decided to whip out a tester I hadn't used yet: my magnifying glass. That's when I found it -- a *microscopic* crack in the trace between the emitter of Q208 and its adjoining 3.3 ohm resistor. The crack was so incredibly tiny that I easily overlooked it--repeatedly. I took one of my jumper leads and clipped one end of it to Q208's emitter and the other end of it to the 3.3 ohm resistor. I then took voltage measurements. EVERYTHING WAS NOW ON SPEC: Q205's collector now measures +7.09V, and Q206 now has the following measurements: B: +6.51V C: +7.08V E: +5.84V I was able to bridge the crack with a well-placed solder trail, and the headphone amp now sounds great. You should *always* bridge the crack with a strand or two of fine wire; that'll keep it from cracking again, Real Soon Now. Isaac |
#8
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
"isw" wrote in message ... I was able to bridge the crack with a well-placed solder trail, and the headphone amp now sounds great. You should *always* bridge the crack with a strand or two of fine wire; that'll keep it from cracking again, Real Soon Now. Why use fine wire? I just use an off-cut from a resistor leg for normal size traces. Much easier to place and they're pre-tinned as well. MrT. |
#9
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
"Mr.T" wrote ...
"isw"wrote ... I was able to bridge the crack with a well-placed solder trail, and the headphone amp now sounds great. You should *always* bridge the crack with a strand or two of fine wire; that'll keep it from cracking again, Real Soon Now. Why use fine wire? I just use an off-cut from a resistor leg for normal size traces. Much easier to place and they're pre-tinned as well. "Fine" is a relative term. And some PC boards are too dense and use traces that are too narrow to use resistor lead wire. I typically use 30Ga wire-wrap wire for that purpose. But I agree that just bridging the gap with a blob of solder is not an optimal solution. |
#10
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
"Richard Crowley" wrote in message ... You should *always* bridge the crack with a strand or two of fine wire; that'll keep it from cracking again, Real Soon Now. Why use fine wire? I just use an off-cut from a resistor leg for normal size traces. Much easier to place and they're pre-tinned as well. "Fine" is a relative term. But not so much when you say "a strand or two". And some PC boards are too dense and use traces that are too narrow to use resistor lead wire. Which is why I already said "for normal size traces". That covers a lot of audio gear. MrT. |
#11
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
In article ,
"Mr.T" MrT@home wrote: "isw" wrote in message ... I was able to bridge the crack with a well-placed solder trail, and the headphone amp now sounds great. You should *always* bridge the crack with a strand or two of fine wire; that'll keep it from cracking again, Real Soon Now. Why use fine wire? I just use an off-cut from a resistor leg for normal size traces. Much easier to place and they're pre-tinned as well. Depends on the width of the trace... Isaac |
#12
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
"isw" wrote in message ... You should *always* bridge the crack with a strand or two of fine wire; that'll keep it from cracking again, Real Soon Now. Why use fine wire? I just use an off-cut from a resistor leg for normal size traces. Much easier to place and they're pre-tinned as well. Depends on the width of the trace... Which is why I said "for normal size traces". The post I replied to OTOH said "*always*". That is what I objected to. (Doesn't anybody read before replying?) MrT. |
#13
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
"Mr.T" wrote ...
"isw" wrote ... You should *always* bridge the crack with a strand or two of fine wire; that'll keep it from cracking again, Real Soon Now. Why use fine wire? I just use an off-cut from a resistor leg for normal size traces. Much easier to place and they're pre-tinned as well. Depends on the width of the trace... Which is why I said "for normal size traces". The post I replied to OTOH said "*always*". That is what I objected to. (Doesn't anybody read before replying?) I suspect that the standard "normal" trace width may have shrunk while you weren't looking. Or maybe you have the luxury of working on older, more human-scale equipment. |
#14
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
Richard Crowley wrote:
"Mr.T" wrote ... "isw" wrote ... You should *always* bridge the crack with a strand or two of fine wire; that'll keep it from cracking again, Real Soon Now. Why use fine wire? I just use an off-cut from a resistor leg for normal size traces. Much easier to place and they're pre-tinned as well. Depends on the width of the trace... Which is why I said "for normal size traces". The post I replied to OTOH said "*always*". That is what I objected to. (Doesn't anybody read before replying?) I suspect that the standard "normal" trace width may have shrunk while you weren't looking. Or maybe you have the luxury of working on older, more human-scale equipment. In any case, a small wire (like one strand of some zip cord--30 gauge?) is an improvement over nothing at all. jak |
#15
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
"Richard Crowley" wrote in message ... I suspect that the standard "normal" trace width may have shrunk while you weren't looking. Nope, but what's normal for an audio amp is not necessarily "normal" for a computer motherboard. Since we weren't discussing computers, they are not included in my comment. Or maybe you have the luxury of working on older, more human-scale equipment. Yes, since most micro-miniature equipment is not really designed to be worked on, and we were discussing an audio amp, not digital electronics. As usual, people can argue endlessly about linguistics, frankly I'm not interested. MrT. |
#16
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
On Apr 24, 9:47 pm, "Mr.T" MrT@home wrote:
Yes, since most micro-miniature equipment is not really designed to be worked on, and we were discussing an audio amp, not digital electronics. MrT: I must add that most micro-miniature equipment is also NOT WORTH being worked on! Analog rules! :-) P.S.: I am taking everyone's advice and adding a bit of fine wire to the solder trail I used to fix the circuit board. Good advice, it is! |
#17
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
In article , EADGBE wrote:
On Apr 24, 9:47 pm, "Mr.T" MrT@home wrote: Yes, since most micro-miniature equipment is not really designed to be worked on, and we were discussing an audio amp, not digital electronics. MrT: I must add that most micro-miniature equipment is also NOT WORTH being worked on! Analog rules! :-) P.S.: I am taking everyone's advice and adding a bit of fine wire to the solder trail I used to fix the circuit board. Good advice, it is! Sometimes its better to add a complete wire from end to end and forget the trace. greg |
#18
Posted to rec.audio.tech
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Amplifier Problem - ***SOLVED***
"GregS" wrote in message
In article , EADGBE wrote: On Apr 24, 9:47 pm, "Mr.T" MrT@home wrote: Yes, since most micro-miniature equipment is not really designed to be worked on, and we were discussing an audio amp, not digital electronics. MrT: I must add that most micro-miniature equipment is also NOT WORTH being worked on! I dunno. If you have a few $100s invested in something, or say the better part of a $grand, even a few $100 spent on getting it back into service can be justified. Analog rules! In the larger view, there is no dichotomy between analog and digital. P.S.: I am taking everyone's advice and adding a bit of fine wire to the solder trail I used to fix the circuit board. Good advice, it is! Sometimes its better to add a complete wire from end to end and forget the trace. Good advice. The worst case of circuit board repair I ever encountered were complete longitudinal fractures of 2 major boards in a Heath AR-15 receiver. I seem to recall that both the multiplex and input boards were severed. When I picked up the package at the APO, parts literally streamed out of a ripped corner of the box. I used the wire-replacement/reinforcement technique, and the receiver ran flawlessly for years afterward. |
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