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#41
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
Try disconnecting the driver transistors of the bad channel, and measuring (with the DC Volts scale of your DMM) if there is DC on the speaker terminals. If it's not, then probably the output transistors are in good shape, and the trouble may be cheaper to repair. On the bad channel with no output transistors, I'm measuring -0.5VDC output. That's pretty substantial. I measured the output with the transistors in place, and it was sitting at -20VDC! Next steps? It looks like the transistors might be OK. Colin The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Change the bad ones. --- Ing. Remberto Gomez-Meda http://ingemeda.tripod.com/ INGE - Ingenieria Electronica. Puerto Vallarta, Jalisco, Mexico. |
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#42
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HELP! Hafler DH-200 misbehaving!
El Meda wrote:
The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. Argh! I can't even read. Where you said driver, I read output; and then interpreted the rest of your post with that misinterpretation. Anyways... With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Will do this over the weekend. I'm visiting the parents, so my dad gets to help out on this one. :-) Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Thanks! I'll post with updates after the weekend. Colin |
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#43
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HELP! Hafler DH-200 misbehaving!
El Meda wrote:
The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. Argh! I can't even read. Where you said driver, I read output; and then interpreted the rest of your post with that misinterpretation. Anyways... With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Will do this over the weekend. I'm visiting the parents, so my dad gets to help out on this one. :-) Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Thanks! I'll post with updates after the weekend. Colin |
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#44
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HELP! Hafler DH-200 misbehaving!
El Meda wrote:
The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. Argh! I can't even read. Where you said driver, I read output; and then interpreted the rest of your post with that misinterpretation. Anyways... With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Will do this over the weekend. I'm visiting the parents, so my dad gets to help out on this one. :-) Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Thanks! I'll post with updates after the weekend. Colin |
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#45
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HELP! Hafler DH-200 misbehaving!
El Meda wrote:
The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. Argh! I can't even read. Where you said driver, I read output; and then interpreted the rest of your post with that misinterpretation. Anyways... With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Will do this over the weekend. I'm visiting the parents, so my dad gets to help out on this one. :-) Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Thanks! I'll post with updates after the weekend. Colin |
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#46
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
El Meda wrote: The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. Argh! I can't even read. Where you said driver, I read output; and then interpreted the rest of your post with that misinterpretation. Anyways... With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Will do this over the weekend. I'm visiting the parents, so my dad gets to help out on this one. :-) Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Thanks! I'll post with updates after the weekend. Colin Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. |
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#47
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
El Meda wrote: The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. Argh! I can't even read. Where you said driver, I read output; and then interpreted the rest of your post with that misinterpretation. Anyways... With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Will do this over the weekend. I'm visiting the parents, so my dad gets to help out on this one. :-) Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Thanks! I'll post with updates after the weekend. Colin Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. |
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#48
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
El Meda wrote: The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. Argh! I can't even read. Where you said driver, I read output; and then interpreted the rest of your post with that misinterpretation. Anyways... With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Will do this over the weekend. I'm visiting the parents, so my dad gets to help out on this one. :-) Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Thanks! I'll post with updates after the weekend. Colin Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. |
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#49
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
El Meda wrote: The driver transistors that I was talking about weren't the output transistors, but the smaller transistors that are connected to them. Argh! I can't even read. Where you said driver, I read output; and then interpreted the rest of your post with that misinterpretation. Anyways... With the output transistors in place disconnect the smaller ones, power on the amplifier, and check for DC on the speaker terminals. If there is not DC, then the output transistors are probably OK. Will do this over the weekend. I'm visiting the parents, so my dad gets to help out on this one. :-) Even If the output transistors are good, check the disconnected drivers. Check in circuit (with the amplifier turned off) also every diode and Zener on the bad channel, and every resistance of less than 1 Kohm. Check in circuit the even smaller transistors for short circuits. Disconnect and test off-circuit everything suspicious. Thanks! I'll post with updates after the weekend. Colin Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. |
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#51
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HELP! Hafler DH-200 misbehaving!
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#52
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HELP! Hafler DH-200 misbehaving!
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#53
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HELP! Hafler DH-200 misbehaving!
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#54
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HELP! Hafler DH-200 misbehaving!
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#56
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HELP! Hafler DH-200 misbehaving!
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#57
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HELP! Hafler DH-200 misbehaving!
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#58
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HELP! Hafler DH-200 misbehaving!
Robert Gault wrote: lid wrote: snip Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. snip Dick Dick, Could you send me a circuit diagram for this DC nulling mod for the DH-200 by e-mail? The DH-200 DC nulling circuit was described by Walt Jung in The Audio Amateur 1/83, page 56. It is simple and mounts nicely in the circuit card by drilling 3 small holes with a wire gauge drill (like for drilling holes in a PCB). Just attach a 100Kohm pot between the two power supply rails and feed the output of its wiper through a 2 megohm resistor to where R3 (or R3) touches the input trace. I used a miniture 1/2 Watt multi-turn cermet variable resistor. I suppose a small film cap across the legs of the pot would be a good idea and will do this soon and give it an "ear" test. This little circuit solves a bunch of problems and I don't know why more people don't use it. It is not a destructive mod, just a simple add-on. The pot allows inserting through the 2 meg ohm resistor a very small "sample" of either + or - voltage to the amp's input. I've seen more than one schematic for other amps that routinely do the same thing. Hope this helps. Spread the word! Dick |
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#59
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HELP! Hafler DH-200 misbehaving!
Robert Gault wrote: lid wrote: snip Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. snip Dick Dick, Could you send me a circuit diagram for this DC nulling mod for the DH-200 by e-mail? The DH-200 DC nulling circuit was described by Walt Jung in The Audio Amateur 1/83, page 56. It is simple and mounts nicely in the circuit card by drilling 3 small holes with a wire gauge drill (like for drilling holes in a PCB). Just attach a 100Kohm pot between the two power supply rails and feed the output of its wiper through a 2 megohm resistor to where R3 (or R3) touches the input trace. I used a miniture 1/2 Watt multi-turn cermet variable resistor. I suppose a small film cap across the legs of the pot would be a good idea and will do this soon and give it an "ear" test. This little circuit solves a bunch of problems and I don't know why more people don't use it. It is not a destructive mod, just a simple add-on. The pot allows inserting through the 2 meg ohm resistor a very small "sample" of either + or - voltage to the amp's input. I've seen more than one schematic for other amps that routinely do the same thing. Hope this helps. Spread the word! Dick |
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#60
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HELP! Hafler DH-200 misbehaving!
Robert Gault wrote: lid wrote: snip Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. snip Dick Dick, Could you send me a circuit diagram for this DC nulling mod for the DH-200 by e-mail? The DH-200 DC nulling circuit was described by Walt Jung in The Audio Amateur 1/83, page 56. It is simple and mounts nicely in the circuit card by drilling 3 small holes with a wire gauge drill (like for drilling holes in a PCB). Just attach a 100Kohm pot between the two power supply rails and feed the output of its wiper through a 2 megohm resistor to where R3 (or R3) touches the input trace. I used a miniture 1/2 Watt multi-turn cermet variable resistor. I suppose a small film cap across the legs of the pot would be a good idea and will do this soon and give it an "ear" test. This little circuit solves a bunch of problems and I don't know why more people don't use it. It is not a destructive mod, just a simple add-on. The pot allows inserting through the 2 meg ohm resistor a very small "sample" of either + or - voltage to the amp's input. I've seen more than one schematic for other amps that routinely do the same thing. Hope this helps. Spread the word! Dick |
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#61
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HELP! Hafler DH-200 misbehaving!
Robert Gault wrote: lid wrote: snip Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. snip Dick Dick, Could you send me a circuit diagram for this DC nulling mod for the DH-200 by e-mail? The DH-200 DC nulling circuit was described by Walt Jung in The Audio Amateur 1/83, page 56. It is simple and mounts nicely in the circuit card by drilling 3 small holes with a wire gauge drill (like for drilling holes in a PCB). Just attach a 100Kohm pot between the two power supply rails and feed the output of its wiper through a 2 megohm resistor to where R3 (or R3) touches the input trace. I used a miniture 1/2 Watt multi-turn cermet variable resistor. I suppose a small film cap across the legs of the pot would be a good idea and will do this soon and give it an "ear" test. This little circuit solves a bunch of problems and I don't know why more people don't use it. It is not a destructive mod, just a simple add-on. The pot allows inserting through the 2 meg ohm resistor a very small "sample" of either + or - voltage to the amp's input. I've seen more than one schematic for other amps that routinely do the same thing. Hope this helps. Spread the word! Dick |
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#63
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HELP! Hafler DH-200 misbehaving!
lid wrote:
lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin |
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#64
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HELP! Hafler DH-200 misbehaving!
lid wrote:
lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin |
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#65
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HELP! Hafler DH-200 misbehaving!
lid wrote:
lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin |
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#66
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote in news:4070eb79
@news.nucleus.com: lid wrote: lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin Time for an oscilloscope I would say. I would check all of the resistors around Q9 and any preceding drive stages. r -- Nothing beats the bandwidth of a station wagon filled with DLT tapes. |
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#67
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote in news:4070eb79
@news.nucleus.com: lid wrote: lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin Time for an oscilloscope I would say. I would check all of the resistors around Q9 and any preceding drive stages. r -- Nothing beats the bandwidth of a station wagon filled with DLT tapes. |
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#68
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote in news:4070eb79
@news.nucleus.com: lid wrote: lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin Time for an oscilloscope I would say. I would check all of the resistors around Q9 and any preceding drive stages. r -- Nothing beats the bandwidth of a station wagon filled with DLT tapes. |
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#69
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote in news:4070eb79
@news.nucleus.com: lid wrote: lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin Time for an oscilloscope I would say. I would check all of the resistors around Q9 and any preceding drive stages. r -- Nothing beats the bandwidth of a station wagon filled with DLT tapes. |
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#70
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
lid wrote: lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin Colin, I doubt anything Killed Q9. Read my previous comments (above) about the 4 input transistors that make up the input differential amp. Did you monitor this channel for DC offset? You must do this. DC offset will blow the speaker output fuse (to protect the amp and speaker against the high DC offset). You must get your DC offset to be less than 50 mV. Note that DC offset can change as the amp warms up. AlsoI think you need to use a Variac and do some routine and tedious checking of voltages, especially around these 4 input transistors. Check their voltages around their CBE points. You need to correct the DC offset problem which is probably caused by one of these 4 transistors not matching its mate. The oscillosope stuff can come later to ferret out sneaky little problems with waveform analysis and distortion. It is possible that a high frequency oscillation may be stressing the output fuses, but I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Unless you install the little circuit I described in another previous message here. My comments are based on lots of experience fixing these amps................. Dick |
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#71
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
lid wrote: lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin Colin, I doubt anything Killed Q9. Read my previous comments (above) about the 4 input transistors that make up the input differential amp. Did you monitor this channel for DC offset? You must do this. DC offset will blow the speaker output fuse (to protect the amp and speaker against the high DC offset). You must get your DC offset to be less than 50 mV. Note that DC offset can change as the amp warms up. AlsoI think you need to use a Variac and do some routine and tedious checking of voltages, especially around these 4 input transistors. Check their voltages around their CBE points. You need to correct the DC offset problem which is probably caused by one of these 4 transistors not matching its mate. The oscillosope stuff can come later to ferret out sneaky little problems with waveform analysis and distortion. It is possible that a high frequency oscillation may be stressing the output fuses, but I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Unless you install the little circuit I described in another previous message here. My comments are based on lots of experience fixing these amps................. Dick |
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#72
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
lid wrote: lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin Colin, I doubt anything Killed Q9. Read my previous comments (above) about the 4 input transistors that make up the input differential amp. Did you monitor this channel for DC offset? You must do this. DC offset will blow the speaker output fuse (to protect the amp and speaker against the high DC offset). You must get your DC offset to be less than 50 mV. Note that DC offset can change as the amp warms up. AlsoI think you need to use a Variac and do some routine and tedious checking of voltages, especially around these 4 input transistors. Check their voltages around their CBE points. You need to correct the DC offset problem which is probably caused by one of these 4 transistors not matching its mate. The oscillosope stuff can come later to ferret out sneaky little problems with waveform analysis and distortion. It is possible that a high frequency oscillation may be stressing the output fuses, but I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Unless you install the little circuit I described in another previous message here. My comments are based on lots of experience fixing these amps................. Dick |
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#73
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HELP! Hafler DH-200 misbehaving!
"Colin B." wrote:
lid wrote: lid wrote: Q9 is a common weakness in this amp. It is part of the Vbe circuit that controls the bias setting. Replace this transistor. Most any garden variety NP2222 is OK, even an NPN from Radio Shack. Without a Variac your struggle is probably not going to be successful, however. Also, you have to understand that the most frequent problem with these DH-200 amps is DC offset, caused by imbalances amongst the 4 input differential amp transistors. The NPN Q1/Q2 pair must be hFe matched within 10% or less. Ditto for the PNP Q5/Q6 pair. The DH-220 included a small DC nulling circuit and there is a mod for the DH-200 to add a simpler version of it. I have installed a DC nulling circuit to two DH-200 amps and got DC offset down to less than 5 mV. Driver transistors fail and output MOSFETs fail (rarely) but I would look further upstream for your solution -- at least to begin with. Well well well. This weekend, we pulled and tested Q9. Sure enough, it was fried. One of the leads from the power supply to the amp board itself was also broken (cold solder joint, and melted insulation), so we fixed that. The result? Success! Sweet, blissful music again! For about two hours, after which it blew up again. This time it took the speaker output fuse with it. So it's not so much a question of Q9 failing, as much as something killing Q9. Back to the schematics. Sigh. Colin Colin, I doubt anything Killed Q9. Read my previous comments (above) about the 4 input transistors that make up the input differential amp. Did you monitor this channel for DC offset? You must do this. DC offset will blow the speaker output fuse (to protect the amp and speaker against the high DC offset). You must get your DC offset to be less than 50 mV. Note that DC offset can change as the amp warms up. AlsoI think you need to use a Variac and do some routine and tedious checking of voltages, especially around these 4 input transistors. Check their voltages around their CBE points. You need to correct the DC offset problem which is probably caused by one of these 4 transistors not matching its mate. The oscillosope stuff can come later to ferret out sneaky little problems with waveform analysis and distortion. It is possible that a high frequency oscillation may be stressing the output fuses, but I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Unless you install the little circuit I described in another previous message here. My comments are based on lots of experience fixing these amps................. Dick |
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#74
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HELP! Hafler DH-200 misbehaving!
In article , Dick West
wrote: I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Did the original poster ever remove the MOSFET's from the unit. If so, were they put back in exactly the same place? If not, it is possible that the matched sets were mismatched in the process. That would be enough to cause the problem that Dick is talking about. -john- -- ================================================== ================== John A. Weeks III 952-432-2708 Newave Communications http://www.johnweeks.com ================================================== ================== |
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HELP! Hafler DH-200 misbehaving!
In article , Dick West
wrote: I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Did the original poster ever remove the MOSFET's from the unit. If so, were they put back in exactly the same place? If not, it is possible that the matched sets were mismatched in the process. That would be enough to cause the problem that Dick is talking about. -john- -- ================================================== ================== John A. Weeks III 952-432-2708 Newave Communications http://www.johnweeks.com ================================================== ================== |
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HELP! Hafler DH-200 misbehaving!
In article , Dick West
wrote: I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Did the original poster ever remove the MOSFET's from the unit. If so, were they put back in exactly the same place? If not, it is possible that the matched sets were mismatched in the process. That would be enough to cause the problem that Dick is talking about. -john- -- ================================================== ================== John A. Weeks III 952-432-2708 Newave Communications http://www.johnweeks.com ================================================== ================== |
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#77
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HELP! Hafler DH-200 misbehaving!
In article , Dick West
wrote: I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Did the original poster ever remove the MOSFET's from the unit. If so, were they put back in exactly the same place? If not, it is possible that the matched sets were mismatched in the process. That would be enough to cause the problem that Dick is talking about. -john- -- ================================================== ================== John A. Weeks III 952-432-2708 Newave Communications http://www.johnweeks.com ================================================== ================== |
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HELP! Hafler DH-200 misbehaving!
"John A. Weeks III" wrote: In article , Dick West wrote: I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Did the original poster ever remove the MOSFET's from the unit. If so, were they put back in exactly the same place? If not, it is possible that the matched sets were mismatched in the process. That would be enough to cause the problem that Dick is talking about. -john- John, if I recall parts of this thread correctly, the OP said that he also found one of the DC power supply wires to the circuit card was not attached. Obviously, this would cause a huge set of problems such as he described. Now, we do know that the two N-channel MOSFETs on a channel must be matched on Vge within 10%, which is why each MOSFET is marked with the Hafler grading number. Ditto for the P-channel MOSFETs. These devices are not the same 4 to which I referred. I referred to the 4 input differential amp circuit on the circuit card which must be matched, not the 4 MOSFETs attached to the heatsink. Let's wait to hear more from the OP before making anymore guesses about what is wrong. Besides, without the use of a Variac his efforts are probably doomed to failure. Dick |
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HELP! Hafler DH-200 misbehaving!
"John A. Weeks III" wrote: In article , Dick West wrote: I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Did the original poster ever remove the MOSFET's from the unit. If so, were they put back in exactly the same place? If not, it is possible that the matched sets were mismatched in the process. That would be enough to cause the problem that Dick is talking about. -john- John, if I recall parts of this thread correctly, the OP said that he also found one of the DC power supply wires to the circuit card was not attached. Obviously, this would cause a huge set of problems such as he described. Now, we do know that the two N-channel MOSFETs on a channel must be matched on Vge within 10%, which is why each MOSFET is marked with the Hafler grading number. Ditto for the P-channel MOSFETs. These devices are not the same 4 to which I referred. I referred to the 4 input differential amp circuit on the circuit card which must be matched, not the 4 MOSFETs attached to the heatsink. Let's wait to hear more from the OP before making anymore guesses about what is wrong. Besides, without the use of a Variac his efforts are probably doomed to failure. Dick |
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#80
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HELP! Hafler DH-200 misbehaving!
"John A. Weeks III" wrote: In article , Dick West wrote: I would first check those 4 transistors. Remember, they must be used in pairs matched for hFe, else DC offset results. Did the original poster ever remove the MOSFET's from the unit. If so, were they put back in exactly the same place? If not, it is possible that the matched sets were mismatched in the process. That would be enough to cause the problem that Dick is talking about. -john- John, if I recall parts of this thread correctly, the OP said that he also found one of the DC power supply wires to the circuit card was not attached. Obviously, this would cause a huge set of problems such as he described. Now, we do know that the two N-channel MOSFETs on a channel must be matched on Vge within 10%, which is why each MOSFET is marked with the Hafler grading number. Ditto for the P-channel MOSFETs. These devices are not the same 4 to which I referred. I referred to the 4 input differential amp circuit on the circuit card which must be matched, not the 4 MOSFETs attached to the heatsink. Let's wait to hear more from the OP before making anymore guesses about what is wrong. Besides, without the use of a Variac his efforts are probably doomed to failure. Dick |
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