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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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modem holds line..
even when out of the PC-what could be the cause?
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modem holds line..
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modem holds line..
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modem holds line..
the relay (soldered) has approx 20kohms between the coil and 200k
between the output, when dis/reconnecting the dialtone returns but then again goes busy by itself! |
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modem holds line..
In message
Franc Zabkar wrote: the relay (soldered) has approx 20kohms between the coil and 200k between the output, when dis/reconnecting the dialtone returns but then again goes busy by itself! That's the same behaviour you get when you take your phone offhook and allow the dialtone to time out. It means that something is placing a load across the line, but not a dead short. If the relay contacts are open, then the only other culprit that comes to mind is a MOV, but IME these usually fail OC. I'd measure the voltage between the tip and ring inputs. In my case it is 53V when on-hook and about 10V when off-hook. Disconnect various components until the voltage is back to normal. Many phones, and some modems, have a diode bridge across the line to power part of the electronics (in a non-DECT phone, all of it). These tend to go leaky when a surge, e.g. lightning somewhere near, causes damage short of total destruction ... -- Peter Duck |
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modem holds line..
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modem holds line..
On Wed, 19 Oct 2005 00:31:37 +0100, Peter Duck
put finger to keyboard and composed: In message Franc Zabkar wrote: the relay (soldered) has approx 20kohms between the coil and 200k between the output, when dis/reconnecting the dialtone returns but then again goes busy by itself! That's the same behaviour you get when you take your phone offhook and allow the dialtone to time out. It means that something is placing a load across the line, but not a dead short. If the relay contacts are open, then the only other culprit that comes to mind is a MOV, but IME these usually fail OC. I'd measure the voltage between the tip and ring inputs. In my case it is 53V when on-hook and about 10V when off-hook. Disconnect various components until the voltage is back to normal. Many phones, and some modems, have a diode bridge across the line to power part of the electronics (in a non-DECT phone, all of it). These tend to go leaky when a surge, e.g. lightning somewhere near, causes damage short of total destruction ... AFAICS the diode bridge (in a transformer based modem) would always be on the host side of the relay contacts, not the line side. The only circuitry on the line side would be the ringer, and this would be isolated by a 1uF 250V DC blocking cap. -- Franc Zabkar Please remove one 'i' from my address when replying by email. |
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modem holds line..
["Followup-To:" header set to sci.electronics.basics.]
On 2005-10-19, wrote: Jasen Betts wrote: On 2005-10-18, wrote: the relay (soldered) has approx 20kohms between the coil and 200k between the output, when dis/reconnecting the dialtone returns but then again goes busy by itself! That first figure sounds way too high, 200 ohms would be the low side of believable can you check again? seing as it doesn't look like a short also can you measure the current through the modem when it's connected to the phone line (but on hook/powered off) a ohm meter running at 1.5-9V soetimes sees things differently to the phone line (48V on-hook) Is it possible you've measured the ring detector and not the relay? took the modem out of the pc, relay in:0.5kohms both ways (the flyback diode seems to be out but not sure if it can be tested without desoldering) and out:approx 500k ; That sounds much better. when I plug in the line the modem goes offhook but not right away and the relay output closes. ooh... you hear the relay closing? that doesn't sound like a relay fault. sounds line a fault elsewhere caa you use a terminal program to "talk to" the modem if so run all the tests. at&t1 at&t2 etc... see how it responds to commands like ath (go on hook) ath1 (go off hook) atxdp9 (dial nine pulses) both with the modem connected to to the phone line and disconnected. There is a diode bridge but cannot really tell if the relay is on the host side though it is rated 0.5A 20V/0.1A 100V; could the bridge be for the modems power supply? at 20V rating it'll be atleast half switched by the modem. otherwise behing it MOV is open at all times, How do I test OC and can I pull it out if I do not expect any calls? huh? Bye. Jasen |
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modem holds line..
On 19 Oct 2005 09:25:23 -0700, put finger to
keyboard and composed: Jasen Betts wrote: On 2005-10-18, wrote: the relay (soldered) has approx 20kohms between the coil and 200k between the output, when dis/reconnecting the dialtone returns but then again goes busy by itself! That first figure sounds way too high, 200 ohms would be the low side of believable can you check again? seing as it doesn't look like a short also can you measure the current through the modem when it's connected to the phone line (but on hook/powered off) a ohm meter running at 1.5-9V soetimes sees things differently to the phone line (48V on-hook) Is it possible you've measured the ring detector and not the relay? Bye. Jasen took the modem out of the pc, relay in:0.5kohms both ways (the flyback diode seems to be out but not sure if it can be tested without desoldering) and out:approx 500k ; when I plug in the line the modem goes offhook but not right away and the relay output closes.There is a diode bridge but cannot really tell if the relay is on the host side though it is rated 0.5A 20V/0.1A 100V; MOV is open at all times, How do I test OC and can I pull it out if I do not expect any calls? So it's an internal modem, and you've taken it out of the PC? If so, then this means that the hook relay's coil would not be getting any power. If the relay contacts are closing under these conditions, then this means that the relay itself is faulty. To confirm this, desolder one of the contacts and monitor the tip and ring voltage. -- Franc Zabkar Please remove one 'i' from my address when replying by email. |
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modem holds line..
Ambient Ham chipset Modem ATI diagnostics OK.Disabled 'wait for dial tone' and It dials but it cannot connect and after that it just 'arrests' the line; tried at&t1 in Hyper terminal-modem 'connects' at 115200 and spits some characters, ath/h1 ok; checked line-around 50 volts but when modem plugged in, it goes to 13v, desoldered MOV is ok but low resistance measured at it's modem board contacts;bridge output goes to darlington...?; as mentioned-in or out of the PC when I plug in the line first you can hear the dial tone &then after 1-2 min. modem is holding the line |
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modem holds line..
Franc Zabkar је напиÑ?ао On 21 Oct 2005 12:55:26 -0700, put finger to keyboard and composed: Ambient Ham chipset Modem ATI diagnostics OK.Disabled 'wait for dial tone' and It dials but it cannot connect and after that it just 'arrests' the line; tried at&t1 in Hyper terminal-modem 'connects' at 115200 and spits some characters, The AT&Tn commands test the data path to the DSP. They are not really helpful when troubleshooting a DAA fault such as yours. ath/h1 ok; checked line-around 50 volts but when modem plugged in, it goes to 13v, If you are measuring 13V on both sides of the relay contacts, then the relay is stuck. desoldered MOV is ok but low resistance measured at it's modem board contacts;bridge output goes to darlington...?; Yes, that's the loop current stabilisation circuit, or whatever it is called. It would be on the host side of the relay contacts. If you are measuring any voltage there, then the relay is stuck. as mentioned-in or out of the PC when I plug in the line first you can hear the dial tone &then after 1-2 min. modem is holding the line Change the relay, or at least desolder it. -- Franc Zabkar Please remove one 'i' from my address when replying by email. Shouldn't I suspect the transistor i.e. the circuit feeding the coil-looks like upon line plug-in voltage buids up gradually to switch the relay. You mentioned OC-if it's only function is ring detection can i remove it, though OC's input diode seems to be connected also to relay coil... |
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modem holds line..
If you add: &C2&D2 to your modem init string, it should hang up. On 21 Oct 2005 12:55:26 -0700, wrote: Ambient Ham chipset Modem ATI diagnostics OK.Disabled 'wait for dial tone' and It dials but it cannot connect and after that it just 'arrests' the line; tried at&t1 in Hyper terminal-modem 'connects' at 115200 and spits some characters, ath/h1 ok; checked line-around 50 volts but when modem plugged in, it goes to 13v, desoldered MOV is ok but low resistance measured at it's modem board contacts;bridge output goes to darlington...?; as mentioned-in or out of the PC when I plug in the line first you can hear the dial tone &then after 1-2 min. modem is holding the line |
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modem holds line..
On 22 Oct 2005 04:54:15 -0700, put finger to
keyboard and composed: Franc Zabkar ?? ??????? On 21 Oct 2005 12:55:26 -0700, put finger to keyboard and composed: Ambient Ham chipset Modem ATI diagnostics OK.Disabled 'wait for dial tone' and It dials but it cannot connect and after that it just 'arrests' the line; tried at&t1 in Hyper terminal-modem 'connects' at 115200 and spits some characters, The AT&Tn commands test the data path to the DSP. They are not really helpful when troubleshooting a DAA fault such as yours. ath/h1 ok; checked line-around 50 volts but when modem plugged in, it goes to 13v, If you are measuring 13V on both sides of the relay contacts, then the relay is stuck. desoldered MOV is ok but low resistance measured at it's modem board contacts;bridge output goes to darlington...?; Yes, that's the loop current stabilisation circuit, or whatever it is called. It would be on the host side of the relay contacts. If you are measuring any voltage there, then the relay is stuck. as mentioned-in or out of the PC when I plug in the line first you can hear the dial tone &then after 1-2 min. modem is holding the line Change the relay, or at least desolder it. -- Franc Zabkar Please remove one 'i' from my address when replying by email. Shouldn't I suspect the transistor i.e. the circuit feeding the coil-looks like upon line plug-in voltage buids up gradually to switch the relay. The transistor is driven from the DSP chip which gets its power from the slot. The relay's coil voltage also comes from the slot. No slot = no power. If the transistor or coil are getting power from the phone line, then you have a *serious* isolation problem. Change the relay, or at least desolder it. You mentioned OC-if it's only function is ring detection can i remove it, though OC's input diode seems to be connected also to relay coil... OC = open circuit -- Franc Zabkar Please remove one 'i' from my address when replying by email. |
#18
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modem holds line..
Franc Zabkar wrote: On 22 Oct 2005 04:54:15 -0700, put finger to keyboard and composed: Franc Zabkar ?? ??????? On 21 Oct 2005 12:55:26 -0700, put finger to keyboard and composed: Ambient Ham chipset Modem ATI diagnostics OK.Disabled 'wait for dial tone' and It dials but it cannot connect and after that it just 'arrests' the line; tried at&t1 in Hyper terminal-modem 'connects' at 115200 and spits some characters, The AT&Tn commands test the data path to the DSP. They are not really helpful when troubleshooting a DAA fault such as yours. ath/h1 ok; checked line-around 50 volts but when modem plugged in, it goes to 13v, If you are measuring 13V on both sides of the relay contacts, then the relay is stuck. desoldered MOV is ok but low resistance measured at it's modem board contacts;bridge output goes to darlington...?; Yes, that's the loop current stabilisation circuit, or whatever it is called. It would be on the host side of the relay contacts. If you are measuring any voltage there, then the relay is stuck. as mentioned-in or out of the PC when I plug in the line first you can hear the dial tone &then after 1-2 min. modem is holding the line Change the relay, or at least desolder it. -- Franc Zabkar Please remove one 'i' from my address when replying by email. Shouldn't I suspect the transistor i.e. the circuit feeding the coil-looks like upon line plug-in voltage buids up gradually to switch the relay. The transistor is driven from the DSP chip which gets its power from the slot. The relay's coil voltage also comes from the slot. No slot = no power. If the transistor or coil are getting power from the phone line, then you have a *serious* isolation problem. Change the relay, or at least desolder it. You mentioned OC-if it's only function is ring detection can i remove it, though OC's input diode seems to be connected also to relay coil... OC = open circuit -- Franc Zabkar Please remove one 'i' from my address when replying by email. Thanks again checked the transistor driving the coil-ok as well as flyback diode Seems defective optocoupler(OC) could be connecting line to the relay even when modem out of PC? |
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modem holds line..
Franc Zabkar wrote: On 22 Oct 2005 04:54:15 -0700, put finger to keyboard and composed: Franc Zabkar ?? ??????? On 21 Oct 2005 12:55:26 -0700, put finger to keyboard and composed: Ambient Ham chipset Modem ATI diagnostics OK.Disabled 'wait for dial tone' and It dials but it cannot connect and after that it just 'arrests' the line; tried at&t1 in Hyper terminal-modem 'connects' at 115200 and spits some characters, The AT&Tn commands test the data path to the DSP. They are not really helpful when troubleshooting a DAA fault such as yours. ath/h1 ok; checked line-around 50 volts but when modem plugged in, it goes to 13v, If you are measuring 13V on both sides of the relay contacts, then the relay is stuck. desoldered MOV is ok but low resistance measured at it's modem board contacts;bridge output goes to darlington...?; Yes, that's the loop current stabilisation circuit, or whatever it is called. It would be on the host side of the relay contacts. If you are measuring any voltage there, then the relay is stuck. as mentioned-in or out of the PC when I plug in the line first you can hear the dial tone &then after 1-2 min. modem is holding the line Change the relay, or at least desolder it. -- Franc Zabkar Please remove one 'i' from my address when replying by email. Shouldn't I suspect the transistor i.e. the circuit feeding the coil-looks like upon line plug-in voltage buids up gradually to switch the relay. The transistor is driven from the DSP chip which gets its power from the slot. The relay's coil voltage also comes from the slot. No slot = no power. If the transistor or coil are getting power from the phone line, then you have a *serious* isolation problem. Change the relay, or at least desolder it. You mentioned OC-if it's only function is ring detection can i remove it, though OC's input diode seems to be connected also to relay coil... OC = open circuit -- Franc Zabkar Please remove one 'i' from my address when replying by email. checked relay-desoldered clicks just fine but is it possible that when defective it closes the output without stimulus at the coil? here is daa schematic http://www.geocities.com/anglomont/daa.jpg Since mov is ok I do not see what else can arrest the line... |
#20
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modem holds line..
wrote: Franc Zabkar wrote: On 22 Oct 2005 04:54:15 -0700, put finger to keyboard and composed: Franc Zabkar ?? ??????? On 21 Oct 2005 12:55:26 -0700, put finger to keyboard and composed: Ambient Ham chipset Modem ATI diagnostics OK.Disabled 'wait for dial tone' and It dials but it cannot connect and after that it just 'arrests' the line; tried at&t1 in Hyper terminal-modem 'connects' at 115200 and spits some characters, The AT&Tn commands test the data path to the DSP. They are not really helpful when troubleshooting a DAA fault such as yours. ath/h1 ok; checked line-around 50 volts but when modem plugged in, it goes to 13v, If you are measuring 13V on both sides of the relay contacts, then the relay is stuck. desoldered MOV is ok but low resistance measured at it's modem board contacts;bridge output goes to darlington...?; Yes, that's the loop current stabilisation circuit, or whatever it is called. It would be on the host side of the relay contacts. If you are measuring any voltage there, then the relay is stuck. as mentioned-in or out of the PC when I plug in the line first you can hear the dial tone &then after 1-2 min. modem is holding the line Change the relay, or at least desolder it. -- Franc Zabkar Please remove one 'i' from my address when replying by email. Shouldn't I suspect the transistor i.e. the circuit feeding the coil-looks like upon line plug-in voltage buids up gradually to switch the relay. The transistor is driven from the DSP chip which gets its power from the slot. The relay's coil voltage also comes from the slot. No slot = no power. If the transistor or coil are getting power from the phone line, then you have a *serious* isolation problem. Change the relay, or at least desolder it. You mentioned OC-if it's only function is ring detection can i remove it, though OC's input diode seems to be connected also to relay coil... OC = open circuit -- Franc Zabkar Please remove one 'i' from my address when replying by email. checked relay-desoldered clicks just fine but is it possible that when defective it closes the output without stimulus at the coil? here is daa schematic http://www.geocities.com/anglomont/daa.jpg Since mov is ok I do not see what else can arrest the line... Hi... That relay is so small and flimsy it may well be that you can hear it click even though the contacts may be welded shut. Ken |
#21
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modem holds line..
I suspect a problem with your schematic. For example, the
NPR transistor that drives the off hook relay would be a poor design. Most use a PNP transistor that connects to +5V - not to ground. That would also ground the opto coupler circuit since the optocoupler should not affect off-hook relay. The typical destructive transient comes from AC mains seeking earth ground via the telephone line surge protection. A most common path to earth is through that PNP transistor, from relay coil to relay wiper, then on into phone line. If the transient is small, PNP transistor becomes shorted causing dial tone to time out when computer power is on. If the transient is larger, PNP transistors becomes open circuited resulting in the 'No Dialtone Detected' error message. Why a PNP transistor? Higher breakdown voltage between collector and base provides protection for the modem's computer. Your schematic provides no such protection - base to emitter. If I read everything correctly (and I have not been careful with the details), it is a classic 'PNP failed shorted' condition. A 'no dialtone' until computer power is removed so that relay can open. wrote: checked relay-desoldered clicks just fine but is it possible that when defective it closes the output without stimulus at the coil? here is daa schematic http://www.geocities.com/anglomont/daa.jpg Since mov is ok I do not see what else can arrest the line... |
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