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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
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Sony KV-35S45 Power Supply
I acquired this set with a blown HOT and power supply, and
presumably a bad flyback as well. I replaced the flyback, HOT, and a number of power supply components including the MX0841 IC. Wouldn't you know it, I had missed a shorted zener in the power supply, and of course blew the brand new switching IC. Since I didn't have another IC in stock, and it appears to be nothing more than two switching transistors in one package, I tried replacing them with a pair of normal switching transistors, similar to those used in other Sony power supplies. This was of course after removing the shorted zener diode from the secondary side. The power supply now starts but the voltages are very low. There are three supplies on the secondary side of this switching transformer (a separate xfrmr & circuit is used for the 5V standby); the B+, which measures at just 50V, a 12V supply which measures at 3V, and a 9V supply which is at zero. Removing the 12 and 9 volt rectifiers brought the B+ up to about 100V, but the power supply makes a noticeable squeal and sounds like it's either straining itself or not running hard enough (the switching transistors don't seem to be getting warm though). I've gone through the power supply and can't find any bad resistors or diodes, which leaves me with just one question: is the problem related to my subbing the MX0841 IC, or is there something else wrong with this power supply? This is a difficult circuit to troubleshoot and I'm just afraid there might be something more serious at fault. Thanks for any help. |
#2
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Sony KV-35S45 Power Supply
On Wed, 2 Apr 2008 10:38:16 -0700 (PDT), packrat79
put finger to keyboard and composed: I acquired this set with a blown HOT and power supply, and presumably a bad flyback as well. I replaced the flyback, HOT, and a number of power supply components including the MX0841 IC. Wouldn't you know it, I had missed a shorted zener in the power supply, and of course blew the brand new switching IC. Since I didn't have another IC in stock, and it appears to be nothing more than two switching transistors in one package, I tried replacing them with a pair of normal switching transistors, similar to those used in other Sony power supplies. This was of course after removing the shorted zener diode from the secondary side. The power supply now starts but the voltages are very low. There are three supplies on the secondary side of this switching transformer (a separate xfrmr & circuit is used for the 5V standby); the B+, which measures at just 50V, a 12V supply which measures at 3V, and a 9V supply which is at zero. Removing the 12 and 9 volt rectifiers brought the B+ up to about 100V, but the power supply makes a noticeable squeal and sounds like it's either straining itself or not running hard enough (the switching transistors don't seem to be getting warm though). I've gone through the power supply and can't find any bad resistors or diodes, which leaves me with just one question: is the problem related to my subbing the MX0841 IC, or is there something else wrong with this power supply? This is a difficult circuit to troubleshoot and I'm just afraid there might be something more serious at fault. Thanks for any help. I don't recognise your Sony model, but I've seen Sony sets where a dried out B+ capacitor produced the same symptom. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#3
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Sony KV-35S45 Power Supply
On Apr 2, 5:45 pm, Franc Zabkar wrote:
On Wed, 2 Apr 2008 10:38:16 -0700 (PDT), packrat79 put finger to keyboard and composed: I acquired this set with a blown HOT and power supply, and presumably a bad flyback as well. I replaced the flyback, HOT, and a number of power supply components including the MX0841 IC. Wouldn't you know it, I had missed a shorted zener in the power supply, and of course blew the brand new switching IC. Since I didn't have another IC in stock, and it appears to be nothing more than two switching transistors in one package, I tried replacing them with a pair of normal switching transistors, similar to those used in other Sony power supplies. This was of course after removing the shorted zener diode from the secondary side. The power supply now starts but the voltages are very low. There are three supplies on the secondary side of this switching transformer (a separate xfrmr & circuit is used for the 5V standby); the B+, which measures at just 50V, a 12V supply which measures at 3V, and a 9V supply which is at zero. Removing the 12 and 9 volt rectifiers brought the B+ up to about 100V, but the power supply makes a noticeable squeal and sounds like it's either straining itself or not running hard enough (the switching transistors don't seem to be getting warm though). I've gone through the power supply and can't find any bad resistors or diodes, which leaves me with just one question: is the problem related to my subbing the MX0841 IC, or is there something else wrong with this power supply? This is a difficult circuit to troubleshoot and I'm just afraid there might be something more serious at fault. Thanks for any help. I don't recognise your Sony model, but I've seen Sony sets where a dried out B+ capacitor produced the same symptom. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. Nope, the caps are fine. I guess the next thing is to get an original MX0841 IC, in case I'm just wasting my time looking for other problems. |
#4
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Sony KV-35S45 Power Supply
I strongly suggest to use an original part. Sometimes the specs in the
service manual show a device like a simple transistor type, and it may be using Darlington pairs or something like that. -- JANA _____ "packrat79" wrote in message ... I acquired this set with a blown HOT and power supply, and presumably a bad flyback as well. I replaced the flyback, HOT, and a number of power supply components including the MX0841 IC. Wouldn't you know it, I had missed a shorted zener in the power supply, and of course blew the brand new switching IC. Since I didn't have another IC in stock, and it appears to be nothing more than two switching transistors in one package, I tried replacing them with a pair of normal switching transistors, similar to those used in other Sony power supplies. This was of course after removing the shorted zener diode from the secondary side. The power supply now starts but the voltages are very low. There are three supplies on the secondary side of this switching transformer (a separate xfrmr & circuit is used for the 5V standby); the B+, which measures at just 50V, a 12V supply which measures at 3V, and a 9V supply which is at zero. Removing the 12 and 9 volt rectifiers brought the B+ up to about 100V, but the power supply makes a noticeable squeal and sounds like it's either straining itself or not running hard enough (the switching transistors don't seem to be getting warm though). I've gone through the power supply and can't find any bad resistors or diodes, which leaves me with just one question: is the problem related to my subbing the MX0841 IC, or is there something else wrong with this power supply? This is a difficult circuit to troubleshoot and I'm just afraid there might be something more serious at fault. Thanks for any help. |
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