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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
In the interest of clarification, I have added some information to the
schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. |
#2
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
"EADGBE" wrote ...
In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. Can't see what parts external to the circuit shown could cause any of your symptoms. The input and output are decoupled with capacitors. Furthermore, you confirm that the internal, DC side of the input and output are at the prescribed voltage. You have also confirmed that the supply voltage is OK. There is nothing else externally that could affect the circuit. I am assuming that the node at the lower left corner finds its way back to ground somewhere. The question remains why the collector of Q205 is at 131mV rather than the predicted 7V. Q205 is simply not conducting and that is causing all the rest of the symptoms. One of the marvelous things about stereo is that you've got two identical channels. So you have a good one to compare it to. What is the base voltage of Q205 on the working side? And what is it on your broken side? |
#3
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
Richard Crowley wrote: "EADGBE" wrote ... In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. Can't see what parts external to the circuit shown could cause any of your symptoms. The input and output are decoupled with capacitors. Furthermore, you confirm that the internal, DC side of the input and output are at the prescribed voltage. You have also confirmed that the supply voltage is OK. There is nothing else externally that could affect the circuit. I am assuming that the node at the lower left corner finds its way back to ground somewhere. The question remains why the collector of Q205 is at 131mV rather than the predicted 7V. Q205 is simply not conducting and that is causing all the rest of the symptoms. One of the marvelous things about stereo is that you've got two identical channels. So you have a good one to compare it to. What is the base voltage of Q205 on the working side? And what is it on your broken side? All points to Q205 collector being open apart from one thing. On the revised schematic, the output voltage at the junction of the two 3R3 resistors is measured as being correct. That doesn't add up at all. I could do with seeing this voltage checked. I've put the circuit into Pspice and played around with various fault conditions to see what the volts do. Q205 is the only thing that seems to reproduce the symptoms. d |
#5
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Amplifier Problem - Revised Schematic
bz wrote in
98.139: Since the OTHER voltages around Q204 are correct, the collector of Q204 should be LOWER than the 11.9 rail by half a volt or so. It should thus be 11.4 on Q204 collector and Q205 base. If it isn't, then Q204 is probably bad, open on the base/collector junction. OR, the Q204 collector is not connected to to the 12 k resistor and the base of Q205. You said you replaced Q204. You could have a bad transistor(new parts can be bad), a part soldered poorly, or a transistor soldered in wrong. Check for broken traces and make sure that the orientation is correct on Q204's replacement. -- bz 73 de N5BZ k please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set. remove ch100-5 to avoid spam trap |
#6
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
"EADGBE" wrote in message ... In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. Make sure your transistors are oriented the right way round, i.e. B,C & E really are where you think they are. I remember once being stuck with a repair where the lead out of a replacement transistor was not the same as the original. Couldn't work out what the hell was going on for quite a while, but C and E were reversed. (don't just use a diode check for this) Gareth. |
#7
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
"Gareth Magennis" wrote in message ... "EADGBE" wrote in message ... In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. Make sure your transistors are oriented the right way round, i.e. B,C & E really are where you think they are. I remember once being stuck with a repair where the lead out of a replacement transistor was not the same as the original. Couldn't work out what the hell was going on for quite a while, but C and E were reversed. (don't just use a diode check for this) Gareth. As an example: BC184 and BC184L have different pinouts. Gareth. |
#8
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Amplifier Problem - Revised Schematic
Gareth Magennis wrote: "Gareth Magennis" wrote "EADGBE" wrote In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. Make sure your transistors are oriented the right way round, i.e. B,C & E really are where you think they are. I remember once being stuck with a repair where the lead out of a replacement transistor was not the same as the original. Couldn't work out what the hell was going on for quite a while, but C and E were reversed. (don't just use a diode check for this) As an example: BC184 Centre base ('TO-18 style'). and BC184L Base at one end. have different pinouts. Gareth. |
#9
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Amplifier Problem - Revised Schematic
I have revised the schematic further by adding the voltages at the base of Q205 and collector of Q204.... http://i240.photobucket.com/albums/f...mplifier-2.jpg I guess I don't have enough equipment to fix this problem. I can't afford a 'scope right now. This will have to go on a shelf until I can deal with it. |
#10
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
You might want to look at pc scopes. Or ebay. If all you are doing is
audio and possibly lower rf frequencies there are lots of decent used scopes on ebay. "EADGBE" wrote in message ... I have revised the schematic further by adding the voltages at the base of Q205 and collector of Q204.... http://i240.photobucket.com/albums/f...mplifier-2.jpg I guess I don't have enough equipment to fix this problem. I can't afford a 'scope right now. This will have to go on a shelf until I can deal with it. |
#11
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
In article e79927cb-9552-41f4-81e9-0c380ce099b0
@f36g2000hsa.googlegroups.com, says... I have revised the schematic further by adding the voltages at the base of Q205 and collector of Q204.... http://i240.photobucket.com/albums/f...mplifier-2.jpg I guess I don't have enough equipment to fix this problem. I can't afford a 'scope right now. This will have to go on a shelf until I can deal with it. You don't need a scope for this one, just DC meter and brain power. For the output transistors, Q207 base should be about one Volt higher than Q208 base. If not, either Q206 is shorted or Q205 is not sourcing current. If I can believe the voltages you show on Q205, it probably has an open collector. |
#12
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
On 2008-04-22, EADGBE wrote:
I have revised the schematic further by adding the voltages at the base of Q205 and collector of Q204.... http://i240.photobucket.com/albums/f...mplifier-2.jpg I guess I don't have enough equipment to fix this problem. I can't afford a 'scope right now. This will have to go on a shelf until I can deal with it. If I understand this correctly the nominally 5.8 V point on Q208 base is close to 0 V but the output node is OK at +6.4 V. If that's right, Q208's VBE plus the drop across the lower 3.3 ohm output resistor is about 6.4 volts rather than about 0.6 V. That can't be so unless the lower 3.3 ohm resistor is open circuit, or it has 6.4 V across it and is boiling hot, or Q208 is open circuit B-E. -- John Phillips |
#13
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Amplifier Problem - Revised Schematic
You need to get a scope or at least a signal tracer and see where the signal
stops on the bad side. It's a stereo amp so it should be a no brainer to compare signals as you move through both sides. If all you have is a meter then you just don't have enough equipment to do repair work. "EADGBE" wrote in message ... In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. |
#14
Posted to sci.electronics.repair,rec.audio.tech
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Amplifier Problem - Revised Schematic
"EADGBE" wrote in message
In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. Have you followed the recommended procedure for debugging direct-coupled circuits like this? Step 1 - remove all transistors and check them for shorts and opens out of the circuit. An ohmeter usually suffices. |
#15
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Amplifier Problem - Revised Schematic
In article , EADGBE wrote:
In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. Where did you get this diagram. Is it completely from scratch ? greg |
#16
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Amplifier Problem - Revised Schematic
On Apr 22, 10:34 am, (GregS) wrote:
Where did you get this diagram. Is it completely from scratch ? Greg: No, it was copied, carefully, from the original schematic. I would have scanned the original schematic, but it is very fuzzy. I did check my schematic against the other channel's schematic, which was a bit clearer but did not have the expected voltages indicated, and was cluttered with the actual component values. I have checked, re-checked, and RE-checked the accuracy of my hand- drawn schematic, comparing it to the original, and there are no mistakes. |
#17
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Amplifier Problem - Revised Schematic
I have just discovered that the voltage measured directly at the emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere from -300mV to -16mV. |
#18
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Amplifier Problem - Revised Schematic
In article , EADGBE wrote:
I have just discovered that the voltage measured directly at the emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere from -300mV to -16mV. Thats no good. It seemed most was OK, and I imagined you have no scope. I was going to suggest injecting a signal at the bases of the imput stage. Holding a metal probe with your hand should give equal sounds from left and right on corresponding points of each amp. You are going to have to check the Q208 more thoroughly. greg |
#19
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Amplifier Problem - Revised Schematic
On 2008-04-22, EADGBE wrote:
I have just discovered that the voltage measured directly at the emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere from -300mV to -16mV. By subtraction from the 7.3 V you said was present at the junction of the two 3.3 ohm resistors there's nominally 7.3 to 7.6 V across the lower one. So it looks like: - that lower 3.3 ohm resistor is open circuit; or - it's dissipating about 16 watts because of another fault and presumably getting rather hot which may lead to it going o/c soon (depending on its power rating which I would guess doesn't support a 16 watt dissipation); or - it isn't actually connected to the emitter of Q208 so check the soldering. -- John Phillips |
#20
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Amplifier Problem - Revised Schematic
"EADGBE" wrote ...
I have just discovered that the voltage measured directly at the emitter leg of Q208 is a NEGATIVE voltage that fluctuates anywhere from -300mV to -16mV. You show the collector of Q208 as being ground. Is that same ground not the reference for all your reported voltage measurements. How could it possibly be negative? There are no sources of anything below ground shown in your schematic. What is the voltage on the "to volume control pcb" node at the bottom left corner of the circuit? Is it ground/zero? Is that node *really* disconnected from the rest of the ground reference (collector of Q208, bottom of the 680 resistor, etc.)? It seems like it ought to be ground, but you show it as a separate node. It seems suspicious that the base of Q204 is measured to be the proper voltage, but the collector seems to be pretty much up at the supply rail. That indicated to me that Q204 isn't conducting properly and preventing proper bias on Q205. We have asked you several times about measuring the equivalent nodes in the other (working) side, but you seem to have ignored this valuable source of information about how a proper circuit behaves. |
#21
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Amplifier Problem - Revised Schematic
On Apr 22, 12:44 pm, "Richard Crowley" wrote:
We have asked you several times about measuring the equivalent nodes in the other (working) side, but you seem to have ignored this valuable source of information about how a proper circuit behaves. Richard: Sorry about not posting that information. I wasn't ignoring this request, I promise. Just hadn't gotten around to it yet. I will post the same schematic, but this time with the measurements taken from the "good" channel. And I will also take measurements at the node(s) you inquired about. |
#22
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Amplifier Problem - Revised Schematic
In article , EADGBE wrote:
On Apr 22, 10:34 am, (GregS) wrote: Where did you get this diagram. Is it completely from scratch ? Greg: No, it was copied, carefully, from the original schematic. I would have scanned the original schematic, but it is very fuzzy. I did check my schematic against the other channel's schematic, which was a bit clearer but did not have the expected voltages indicated, and was cluttered with the actual component values. I have checked, re-checked, and RE-checked the accuracy of my hand- drawn schematic, comparing it to the original, and there are no mistakes. The emitter of Q204 has a strange DC path. The amplifier appears non inverting. What is the voltage on Q204 collector ? greg |
#23
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Amplifier Problem - Revised Schematic
GregS wrote: In article , EADGBE wrote: On Apr 22, 10:34 am, (GregS) wrote: Where did you get this diagram. Is it completely from scratch ? Greg: No, it was copied, carefully, from the original schematic. I would have scanned the original schematic, but it is very fuzzy. I did check my schematic against the other channel's schematic, which was a bit clearer but did not have the expected voltages indicated, and was cluttered with the actual component values. I have checked, re-checked, and RE-checked the accuracy of my hand- drawn schematic, comparing it to the original, and there are no mistakes. The emitter of Q204 has a strange DC path. The amplifier appears non inverting. What is the voltage on Q204 collector ? greg Q204 is in a pretty simple DC feedback path with Q205 and the 207/8 combination. Its emitter currrent flows down through Q208 to ground. The amplifier is non-inverting from the base of Q204, but inverting from its emitter, which is where both the DC and AC feedback end up. d |
#24
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Amplifier Problem - Revised Schematic
On Apr 22, 11:29 am, (GregS) wrote:
The emitter of Q204 has a strange DC path. The amplifier appears non inverting. What is the voltage on Q204 collector ? greg Greg: The voltage at the collector of Q204 measures +11.09VDC. |
#25
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Amplifier Problem - Revised Schematic
"EADGBE" wrote in message ... In the interest of clarification, I have added some information to the schematic of the tape deck headphone amp that I'm having problems with. Some circuit paths are now labeled: http://i240.photobucket.com/albums/f...mplifier-1.jpg As always, the points at which voltage readings are marked with a star are NOT producing the voltage indicated. The +7V location is measuring 131mV instead. The other two points where a star is indicated are measuring between 20mV and 30mV (fluctuating slightly). I am starting to think that a circuit OUTSIDE of this circuit might be at fault, because NOTHING inside of this circuit seems to be faulty...but the darn thing still WILL NOT work. I haven't read every follow up message but have you tried removing the electrolytic caps and retesting the d.c.? I've seen the d.c. stability been thrown off in many amps due to bad coupling caps. Good luck. -- David Farber David Farber's Service Center L.A., CA |
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