Home |
Search |
Today's Posts |
#1
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
I am a home hobbyist who enjoys restoring vintage stereo equipment. I
have a good working knowledge of electronic components and what they do. I have decent test equipment and good soldering skills, and most of the time I am able to track down and repair a particular problem. However, I do feel that I need to know more about the theory behind electronic circuits in order to improve my troubleshooting skills. Sometimes I find myself looking at a schematic and thinking, "Boy, I wish I knew more about what is supposed to be happening in this circuit!" Can anyone point to any good books or online sources where I can learn more about how to analyze schematics and recognize what is supposed to be happening? CASE IN POINT: I am attempting to repair a tape deck. One channel (the right channel) of the headphone amplifier doesn't work. The tape deck has perfect output through the line out jacks. Here is the schematic for one of the channels of the tape deck's headphone amp: http://i240.photobucket.com/albums/f.../amplifier.jpg I have reproduced the schematic in my own handwriting because the downloaded schematic was too fuzzy to scan. I *think* I have copied everything correctly. I cannot figure out why this particular circuit isn't working correctly. I have checked each component--and even replaced a few transistors even though all transistors tested fine--but nothing is improving. I have tested all of the electrolytics with my ESR meter and they all check out OK. I have checked all resistors and none of them have opened up or "gone high". Notice that there are some "typical" expected voltages indicated at certain points. These "typical" voltages have come right out of the service manual. All of the voltages marked "OK" are...well...OKAY. But the three voltages marked with a star (*) are NOT okay...they are all just a few millivolts each--around 20 to 30mV. All capacitors are electrolytic with the exception of the 4.7pF capacitor, which is ceramic. All resistors are 1/4 watt. If there is no "K" after the number, then that is the amount of actual ohms it has. For the record, I replaced Q204, Q206, and Q208 but still got exactly the same results. What am I overlooking? |
#2
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
"EADGBE" wrote in message
I am a home hobbyist who enjoys restoring vintage stereo equipment. I have a good working knowledge of electronic components and what they do. I have decent test equipment and good soldering skills, and most of the time I am able to track down and repair a particular problem. However, I do feel that I need to know more about the theory behind electronic circuits in order to improve my troubleshooting skills. Sometimes I find myself looking at a schematic and thinking, "Boy, I wish I knew more about what is supposed to be happening in this circuit!" Can anyone point to any good books or online sources where I can learn more about how to analyze schematics and recognize what is supposed to be happening? http://jacquesricher.com/NEETS/ 24 PDFs that make up a pretty fair basic electronics course, fairly current as of 2003. US Navy Electricity and Electronics Training Series Module 1 - Introduction to Matter, Energy, and Direct Current Module 2 - Introduction to Alternating Current and Transformers Module 3 - Introduction to Circuit Protection, Control, and Measurement Module 4 - Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading Module 5 - Introduction to Generators and Motors Module 6 - Introduction to Electronic Emission, Tubes, and Power Supplies Module 7 - Introduction to Solid-State Devices and Power Supplies Module 8 - Introduction to Amplifiers Module 9 - Introduction to Wave-Generation and Wave-Shaping Circuits Module 10 - Introduction to Wave Propagation, Transmission Lines, and Antennas Module 11 - Microwave Principles Module 12 - Modulation Principles Module 13 - Introduction to Number Systems and Logic Circuits Module 14 - Introduction to Microelectronics Module 15 - Principles of Synchros, Servos, and Gyros Module 16 - Introduction to Test Equipment Module 17 - Radio-Frequency Communications Principles Module 18 - Radar Principles Module 19 - The Technician's Handbook Module 20 - Master Glossary Module 21 - Test Methods and Practices Module 22 - Introduction to Digital Computers Module 23 - Magnetic Recording Module 24 - Introduction to Fiber Optics |
#3
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
On Apr 19, 2:40*pm, EADGBE wrote:
I am a home hobbyist who enjoys restoring vintage stereo equipment. *I have a good working knowledge of electronic components and what they do. *I have decent test equipment and good soldering skills, and most of the time I am able to track down and repair a particular problem. However, I do feel that I need to know more about the theory behind electronic circuits in order to improve my troubleshooting skills. Sometimes I find myself looking at a schematic and thinking, "Boy, I wish I knew more about what is supposed to be happening in this circuit!" Can anyone point to any good books or online sources where I can learn more about how to analyze schematics and recognize what is supposed to be happening? CASE IN POINT: I am attempting to repair a tape deck. *One channel (the right channel) of the headphone amplifier doesn't work. *The tape deck has perfect output through the line out jacks. Here is the schematic for one of the channels of the tape deck's headphone amp: http://i240.photobucket.com/albums/f.../amplifier.jpg I have reproduced the schematic in my own handwriting because the downloaded schematic was too fuzzy to scan. *I *think* I have copied everything correctly. I cannot figure out why this particular circuit isn't working correctly. *I have checked each component--and even replaced a few transistors even though all transistors tested fine--but nothing is improving. I have tested all of the electrolytics with my ESR meter and they all check out OK. I have checked all resistors and none of them have opened up or "gone high". Notice that there are some "typical" expected voltages indicated at certain points. *These "typical" voltages have come right out of the service manual. *All of the voltages marked "OK" are...well...OKAY. But the three voltages marked with a star (*) are NOT okay...they are all just a few millivolts each--around 20 to 30mV. All capacitors are electrolytic with the exception of the 4.7pF capacitor, which is ceramic. All resistors are 1/4 watt. *If there is no "K" after the number, then that is the amount of actual ohms it has. For the record, I replaced Q204, Q206, and Q208 but still got exactly the same results. What am I overlooking? Do you have an oscilloscope?? If you do, it is a simple matter of tracking the signal on the good side and comparing it to the "lack of signal" in the bad side, stage by stage. The training source given by the 2nd poster looks to be great!!!. Bob Hofmann |
#4
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
Arny gave you a list of some teaching materials for electronics theory, however,
most of them seem to focus on more advanced systems theory. I don't know to what level these texts assume the reader has progressed. It's been so long since I looked at a basic theory text that I can't recommend any without a bit of searching. Perhaps another reader can be of more assistance. If your focus is on solid state equipment, then you'll need to focus your studies on theories of transistor circuits. The most basic component that you'll need to study hard and really understand is, of course, the transistor. In reference to your tape deck problem, If you could think about how the various voltages in the circuit are derived, then it might be a little clearer. Remember that transistors are current operated devices, that is, a small current through the base-emitter of the transistor modifies the current between the collector and emitter. In normal linear circuits, a transistor's B-E junction is forward biased, while the B-C junction is reverse biased. Forward-biased: current flows; reverse-biased: no current flows. Assuming silicon transistors, the B-E junction is forward-biased with a voltage of approximately 0.5 to 0.7 volts between the base and emitter. Always use the emitter as a reference point for voltage measurements around a transistor. For an NPN device, the base is positive with reference to the emitter. For a PNP, the base is negative to the emitter. In your tape circuit, the big clue to the problem is lack of any voltage at the collector of Q205. Q205 must be turned on, at least partially, in order for any voltage to appear at its collector (which is showing no voltage). It is the only path that can supply the current to Q203 is through the E-C circuit of Q205. If Q205's B-E is properly forward biased, current should flow through the C-E circuit. Measure the B-E voltage of Q205. If it's in the vicinity of 0.5 - 0.7 volts, then the transistor should be conducting current from its emitter to its collector. If the voltage there is less than that, then the transistor can't conduct, thereby shutting off the C-E circuit. If the transistor's B-E junction is properly forward biased, then it should be conducting. If it isn't conducting, then the transistor is either open or there is another fault, and further troubleshooting is needed. My bet on your tape deck's fault is on Q205. Let me know if that fixes it. Hope this helps you get started, -- Dave M MasonDG44 at comcast dot net (Just substitute the appropriate characters in the address) Some people are like Slinkies. Not really good for anything, but they bring a smile to your face when pushed down the stairs. "EADGBE" wrote in message ... I am a home hobbyist who enjoys restoring vintage stereo equipment. I have a good working knowledge of electronic components and what they do. I have decent test equipment and good soldering skills, and most of the time I am able to track down and repair a particular problem. However, I do feel that I need to know more about the theory behind electronic circuits in order to improve my troubleshooting skills. Sometimes I find myself looking at a schematic and thinking, "Boy, I wish I knew more about what is supposed to be happening in this circuit!" Can anyone point to any good books or online sources where I can learn more about how to analyze schematics and recognize what is supposed to be happening? CASE IN POINT: I am attempting to repair a tape deck. One channel (the right channel) of the headphone amplifier doesn't work. The tape deck has perfect output through the line out jacks. Here is the schematic for one of the channels of the tape deck's headphone amp: http://i240.photobucket.com/albums/f.../amplifier.jpg I have reproduced the schematic in my own handwriting because the downloaded schematic was too fuzzy to scan. I *think* I have copied everything correctly. I cannot figure out why this particular circuit isn't working correctly. I have checked each component--and even replaced a few transistors even though all transistors tested fine--but nothing is improving. I have tested all of the electrolytics with my ESR meter and they all check out OK. I have checked all resistors and none of them have opened up or "gone high". Notice that there are some "typical" expected voltages indicated at certain points. These "typical" voltages have come right out of the service manual. All of the voltages marked "OK" are...well...OKAY. But the three voltages marked with a star (*) are NOT okay...they are all just a few millivolts each--around 20 to 30mV. All capacitors are electrolytic with the exception of the 4.7pF capacitor, which is ceramic. All resistors are 1/4 watt. If there is no "K" after the number, then that is the amount of actual ohms it has. For the record, I replaced Q204, Q206, and Q208 but still got exactly the same results. What am I overlooking? |
#5
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
EADGBE wrote: Notice that there are some "typical" expected voltages indicated at certain points. These "typical" voltages have come right out of the service manual. All of the voltages marked "OK" are...well...OKAY. But the three voltages marked with a star (*) are NOT okay...they are all just a few millivolts each--around 20 to 30mV. There's something amiss with the biasing of the output emitter-follower stage. The voltage on the base of Q208 should be about 1.2V less (not 0.2V less I think that 6.8V should be 5.8V) than the voltage on the base of Q207. That suggests possibly zero or very low current in Q205's collector circuit. Suggest you measure the voltage across the 12k connected to Q205's base-emitter. Graham |
#6
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
On Apr 19, 5:04 pm, "DaveM" wrote:
In your tape circuit, the big clue to the problem is lack of any voltage at the collector of Q205. Q205 must be turned on, at least partially, in order for any voltage to appear at its collector (which is showing no voltage). It is the only path that can supply the current to Q203 is through the E-C circuit of Q205. If Q205's B-E is properly forward biased, current should flow through the C-E circuit. Could you repeat this part, please? There is no Q203 in this circuit. Thanks for your help! |
#7
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
On Apr 19, 5:17 pm, Eeyore
wrote: There's something amiss with the biasing of the output emitter-follower stage. The voltage on the base of Q208 should be about 1.2V less (not 0.2V less I think that 6.8V should be 5.8V) than the voltage on the base of Q207. That suggests possibly zero or very low current in Q205's collector circuit. Suggest you measure the voltage across the 12k connected to Q205's base-emitter. You are ABSOLUTELY CORRECT. I took another good, hard look at the service manual schematic (very fuzzy), and the base voltage of Q208 is indeed +5.8V. How did you deduce this? I'm not sure about the 12K resistor across Q205's base-emitter. I will take a voltage measurement across that resistor and post it here. |
#8
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
|
#9
Posted to rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
On Sat, 19 Apr 2008 12:40:50 -0700 (PDT), EADGBE
wrote: http://i240.photobucket.com/albums/f.../amplifier.jpg As others have said, replace Q205. Or, first lift one end of the 15K feedback resistor for a surprise about the output DC voltage. Then replace Q205. All good fortune, Chris Hornbeck |
#10
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
"EADGBE" wrote in message
... On Apr 19, 5:04 pm, "DaveM" wrote: In your tape circuit, the big clue to the problem is lack of any voltage at the collector of Q205. Q205 must be turned on, at least partially, in order for any voltage to appear at its collector (which is showing no voltage). It is the only path that can supply the current to Q203 is through the E-C circuit of Q205. If Q205's B-E is properly forward biased, current should flow through the C-E circuit. Could you repeat this part, please? There is no Q203 in this circuit. Thanks for your help! Yes, you're right... I should have said The only path that can supply the current to Q206 is through the E-C circuit of Q205. The voltages around Q205 are the clues to your problem. Find out why Q205 isn't conducting and you'll find the problem. -- Dave M MasonDG44 at comcast dot net (Just substitute the appropriate characters in the address) Some people are like Slinkies. Not really good for anything, but they bring a smile to your face when pushed down the stairs. |
#11
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
FWIW, the base voltage of Q205 is +11.09V. This was measured at the junction of the base terminal and the 12K resistor. |
#12
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
On Sat, 19 Apr 2008 14:36:36 -0700 (PDT), EADGBE
wrote: On Apr 19, 5:04 pm, "DaveM" wrote: In your tape circuit, the big clue to the problem is lack of any voltage at the collector of Q205. Q205 must be turned on, at least partially, in order for any voltage to appear at its collector (which is showing no voltage). It is the only path that can supply the current to Q203 is through the E-C circuit of Q205. If Q205's B-E is properly forward biased, current should flow through the C-E circuit. Could you repeat this part, please? There is no Q203 in this circuit. Thanks for your help! Q205 is biased just fine. It has nearly 5V emitter to collector, There is a problem around Q206, though. This transistor provides thermal regulation for the quiescent current in the output stage. If you look at the base and emitter voltages, you will see that it has become reverse biased by 0.4V and there is possibly no voltage difference between collector and emitter. It is not easy to see why this might be. What is the voltage on the collector of Q208? It is misbiased right now, possible by a bad Q206. Just looking at the basic bias conditions, I would say Q204 ok Q205 ok Q206 problem - maybe C-E short Q207 ok Q208 reverse biased B/E So whatever is wrong, I think it is happening down around that bottom right hand corner. Perhaps more to the point, how do all these points measure on the good channel? -- Pearce Consulting http://www.pearce.uk.com |
#13
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
Don: I'm not sure, but you might be reading my schematic incorrectly. There SHOULD be +7V at Q205's collector...but right now, there is only 20 - 30mV. The +7V figure is what the service manual indicates is the "proper" voltage for Q205's collector. Regarding Q206, the actual voltages for the base and emitter are both currently reading just a few millivolts, just like the collector of Q205. The indicated voltages for Q206 are, once again, merely from the service manual, and the figure for the emitter was written incorrectly. The proper voltage for the emitter of Q206 is +5.8V. |
#14
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
alt.binaries.e-book.technical
|
#15
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
EADGBE wrote: Eeyore wrote: There's something amiss with the biasing of the output emitter-follower stage. The voltage on the base of Q208 should be about 1.2V less (not 0.2V less I think that 6.8V should be 5.8V) than the voltage on the base of Q207. That suggests possibly zero or very low current in Q205's collector circuit. Suggest you measure the voltage across the 12k connected to Q205's base-emitter. You are ABSOLUTELY CORRECT. I took another good, hard look at the service manual schematic (very fuzzy), and the base voltage of Q208 is indeed +5.8V. How did you deduce this? Because there should be 2 Vbe's difference betwen those 2 points (to turn on the complementary emitter follower stage). I'm not sure about the 12K resistor across Q205's base-emitter. I will take a voltage measurement across that resistor and post it here. That will be helpful. Graham |
#16
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
EADGBE wrote: FWIW, the base voltage of Q205 is +11.09V. This was measured at the junction of the base terminal and the 12K resistor. That suggests a Vbe of 0.81V in which case Q205 *ought* to be well turned on and conducting strongly but it isn't. Replace Q205. Graham |
#17
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
Don Pearce wrote: On Sat, 19 Apr 2008 14:36:36 -0700 (PDT), EADGBE wrote: On Apr 19, 5:04 pm, "DaveM" wrote: In your tape circuit, the big clue to the problem is lack of any voltage at the collector of Q205. Q205 must be turned on, at least partially, in order for any voltage to appear at its collector (which is showing no voltage). It is the only path that can supply the current to Q203 is through the E-C circuit of Q205. If Q205's B-E is properly forward biased, current should flow through the C-E circuit. Could you repeat this part, please? There is no Q203 in this circuit. Thanks for your help! Q205 is biased just fine. It has nearly 5V emitter to collector, NO. Those are the service manual values. Q205's collector is actually almost at ground potential, i.e. not conducting. Graham |
#18
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
EADGBE wrote: I am a home hobbyist who enjoys restoring vintage stereo equipment. I have a good working knowledge of electronic components and what they do. I have decent test equipment and good soldering skills, and most of the time I am able to track down and repair a particular problem. However, I do feel that I need to know more about the theory behind electronic circuits in order to improve my troubleshooting skills. Sometimes I find myself looking at a schematic and thinking, "Boy, I wish I knew more about what is supposed to be happening in this circuit!" Can anyone point to any good books or online sources where I can learn more about how to analyze schematics and recognize what is supposed to be happening? In order to troubleshoot *discrete* transistor circuits that may be a tall order. They don't even teach this stuff much at college or Uni any more ! Graham |
#19
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
In article
, hr(bob) wrote: Do you have an oscilloscope?? If you do, it is a simple matter of tracking the signal on the good side and comparing it to the "lack of signal" in the bad side, stage by stage. That's an expensive way - although of course a good one. For many years I managed with a small battery amp and speaker. -- *White with a hint of M42* Dave Plowman London SW To e-mail, change noise into sound. |
#20
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
On 19 Apr, 22:40, EADGBE wrote:
I am a home hobbyist who enjoys restoring vintage stereo equipment. I have a good working knowledge of electronic components and what they do. I have decent test equipment and good soldering skills, and most of the time I am able to track down and repair a particular problem. However, I do feel that I need to know more about the theory behind electronic circuits in order to improve my troubleshooting skills. Sometimes I find myself looking at a schematic and thinking, "Boy, I wish I knew more about what is supposed to be happening in this circuit!" Can anyone point to any good books or online sources where I can learn more about how to analyze schematics and recognize what is supposed to be happening? Visit http://www.e-micro.ro/index.php?topic=57.0 |
#21
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
EADGBE writes:
[...] Can anyone point to any good books or online sources where I can learn more about how to analyze schematics and recognize what is supposed to be happening? I had this text in one of my engineering courses at the University of South Florida (Tampa) and in my opinion it would be an excellent book for the purpose you have in mind: @BOOK{sedra, title = "{Microelectronic Circuits}", author = "Adel~S.~Sedra and Kenneth~C.~Smith", publisher = "Saunders College Publishing", edition = "third", year = "1991"} -- % Randy Yates % "My Shangri-la has gone away, fading like %% Fuquay-Varina, NC % the Beatles on 'Hey Jude'" %%% 919-577-9882 % %%%% % 'Shangri-La', *A New World Record*, ELO http://www.digitalsignallabs.com |
#22
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
OK, the plot thickens somewhat... Some of you have told me that, in my non-functioning amplifier circuit, the most likely culprit was Q205. I have replaced Q205 and the circuit STILL doesn't work. It is exactly the same as before. There is only 131.1mV on the collector of Q205, despite +11.09V on its base and +11.59V on its emitter. Aaargh! |
#23
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
EADGBE wrote: OK, the plot thickens somewhat... Some of you have told me that, in my non-functioning amplifier circuit, the most likely culprit was Q205. I have replaced Q205 and the circuit STILL doesn't work. It is exactly the same as before. There is only 131.1mV on the collector of Q205, despite +11.09V on its base and +11.59V on its emitter. And yet you continue to refuse to *directly* measure it's Vbe which is what's important. One other thing. Do you know how to test a transistor ? Graham |
#24
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
On Apr 21, 6:41 pm, Eeyore
wrote: And yet you continue to refuse to *directly* measure it's Vbe which is what's important. According to the datasheet for Q205, the Vbe is: MINIMUM: -0.58 TYPICAL: -0.62 MAXIMUM: -0.68 These figures were arrived at using the following test conditions: Vce = -6.0V Ic = -1.0mA As I have said, I am a hobbyist. Not sure how to do "real world" testing for Vbe or why it's important. I do know how to test NPN and PNP transistors using the diode function of a multimeter, that's it. |
#25
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
EADGBE wrote:
On Apr 21, 6:41 pm, Eeyore wrote: And yet you continue to refuse to *directly* measure it's Vbe which is what's important. According to the datasheet for Q205, the Vbe is: MINIMUM: -0.58 TYPICAL: -0.62 MAXIMUM: -0.68 These figures were arrived at using the following test conditions: Vce = -6.0V Ic = -1.0mA As I have said, I am a hobbyist. Not sure how to do "real world" testing for Vbe or why it's important. I do know how to test NPN and PNP transistors using the diode function of a multimeter, that's it. That's a good starting point, and will likely identify 95% of any problems in an amp. Transistors and diodes, that is. Other things to look for are blown emitter resistors, other burned resistors, and bulging capacitors. Problms relalting to drifting component values are pretty rare. geoff |
#26
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
"EADGBE" wrote in message ... On Apr 21, 6:41 pm, Eeyore wrote: There is only 131.1mV on the collector of Q205, despite +11.09V on its base and +11.59V on its emitter. And yet you continue to refuse to *directly* measure it's Vbe which is what's important. According to the datasheet for Q205, the Vbe is: MINIMUM: -0.58 TYPICAL: -0.62 MAXIMUM: -0.68 These figures were arrived at using the following test conditions: Vce = -6.0V Ic = -1.0mA And from your own measurements there is only 0.50V Base-emitter, so the transistor is not correctly biased. Possibly a fault in a preceding stage, or even an O/C resistor or possibly a dry joint. Keep looking and measuring other voltages. MrT. |
#27
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
"Mr.T" MrT@home wrote in message
u... "EADGBE" wrote in message ... On Apr 21, 6:41 pm, Eeyore wrote: There is only 131.1mV on the collector of Q205, despite +11.09V on its base and +11.59V on its emitter. And yet you continue to refuse to *directly* measure it's Vbe which is what's important. According to the datasheet for Q205, the Vbe is: MINIMUM: -0.58 TYPICAL: -0.62 MAXIMUM: -0.68 These figures were arrived at using the following test conditions: Vce = -6.0V Ic = -1.0mA And from your own measurements there is only 0.50V Base-emitter, so the transistor is not correctly biased. Possibly a fault in a preceding stage, or even an O/C resistor or possibly a dry joint. Keep looking and measuring other voltages. MrT. Silly thought, but you *have* connected 205 the right way round when you replaced it......... S. -- http://audiopages.googlepages.com |
#28
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
One of the basic principles of troubleshooting -- which I think other
posters have been trying to get across -- is to look at the DC voltages on a device to see what they tell you about the device's operation. For example, in "normal" operation, a tube's plate voltage should be at least 100V higher than the cathode *, while the grid should be several volts lower. If this isn't so, something is wrong and you need to find out why. I was recently troubleshooting an electronic crossover with about 10 volts DC output. The output stage used an op amp driving a current-boost device. I had no schematic. The parts were difficult to remove, as there was no easy way to remove and flip over the PC board. I didn't want to go through the hassle of removed a 14-pin op amp from the top of the board, if it wasn't defective. A friend (a well-known audio designer -- yes, I'm name dropping) said "Look, the current-boost thingy has a gain of 1 for both AC and DC. The input and output should be the same, regardless of circuit design. What are you getting?" "There's 1V on the input, -3V on the output." "Then it _must_ be defective. Right?" Right. I clipped the leads and sucked out the stubs. Then I shoved in the replacement and carefully applied solder. It worked. Voltage checks provide really good troubleshooting clues -- if you know how to interpret them. * Obviously, the exact nominal voltages vary with the tube, the circuit, etc. |
#29
Posted to sci.electronics.repair,rec.audio.tech
|
|||
|
|||
WANTED: Good Source of Electronics Theory
William: That was very good advice. In a way, I'm trying to do the same thing, but from a "functional" standpoint as well. In other words, I know what the expected voltages should be, but I can't help thinking that if I knew how this circuit actually amplified the signal, I would be in a better position to troubleshoot it, hence my original request for good sources for learning electronic theory. (Some of the sources that have been recommended so far seem to be excellent, by the way.) Where exactly does the signal go?, what should happen when it gets there?, etc. I believe that these are questions that I should be able to answer. So far, what I have is a collection of components that all seem to test perfectly OK, but don't work, for some as-yet unknown reason. |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
WANTED: Source For Micro-Miniature Lights | Pro Audio | |||
Good source for belts? | Tech | |||
WANTED: Source for NOS and Used Tubes | Vacuum Tubes | |||
WANTED: Source for headphone cable replacement | Pro Audio | |||
Wanted: source for STK0029 or ECG1280 | Tech |