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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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20" White Westinghouse...no power
20" White Westinghouse
TV Model WVT-61901 Year Jan 1999 Will not power up. Just stopped working one day. Brought down to workshop, plugged it in and it worked. Back upstairs and wouldn't work next morning. Now won't power up at all. Checked bridge...ok. Found 5W 1.8K power resistor that reads 2.5K. Is this normal for these large resistors to be off by so much? It feeds a zener and capacitor circuit so i'm thinking it's part of some low voltage circuit which might explain the problem ?? Don't have one handy and not close to town so wondering anyone's thoughts on this. thanks. |
#3
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(George) writes:
20" White Westinghouse TV Model WVT-61901 Year Jan 1999 Will not power up. Just stopped working one day. Brought down to workshop, plugged it in and it worked. Back upstairs and wouldn't work next morning. Now won't power up at all. Checked bridge...ok. Found 5W 1.8K power resistor that reads 2.5K. Is this normal for these large resistors to be off by so much? It feeds a zener and capacitor circuit so i'm thinking it's part of some low voltage circuit which might explain the problem ?? Did you disconnect at least one end of the resistor? It might be open and you're the resistance of other parts in the circuit. Resistors shouldn't be that far off. However, I doubt the incorrect resistance is the problem. Did you actually check that the resistor is marked 1.8K? Often, the manufacturer will put in a part with a different value than in the schematics or labeled on the PCB as a design fix or improvement. Assuming the resistor itself is marked 1.8K, then it's likely changed value due to overheating or some other cause. Have you checked the zener and any other parts it's feeding? --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header is ignored. To contact me, please use the feedback form on the S.E.R FAQ Web sites. |
#4
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Hi Sam...thanks for the response.
Did you disconnect at least one end of the resistor? It might be open and you're the resistance of other parts in the circuit. I completely yanked it and get this 2.5K reading. It is very clearly labeled on the resistor as 1.8K Ohm 5W...it's a good sized ceramic thing. Assuming the resistor itself is marked 1.8K, then it's likely changed value due to overheating or some other cause. Have you checked the zener and any other parts it's feeding? This power resistor is fed from a diode giving a half-wave rectified 120V voltage into one side of it...and on the other it connects into a zener and then a a 680 ohm 1/4W resistor....those check out fine with a reasonable resistance. I think i'll source to try and source one of these 1.7K 5W resistors...can't cost that much. Maybe it's dropping too much voltage? I don't have a schematic or a part number for the zener to really tell. Any more ideas more than welcome...thanks!! --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header is ignored. To contact me, please use the feedback form on the S.E.R FAQ Web sites. |
#5
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(George) writes:
Hi Sam...thanks for the response. Did you disconnect at least one end of the resistor? It might be open and you're the resistance of other parts in the circuit. I completely yanked it and get this 2.5K reading. It is very clearly labeled on the resistor as 1.8K Ohm 5W...it's a good sized ceramic thing. Assuming the resistor itself is marked 1.8K, then it's likely changed value due to overheating or some other cause. Have you checked the zener and any other parts it's feeding? This power resistor is fed from a diode giving a half-wave rectified 120V voltage into one side of it...and on the other it connects into a zener and then a a 680 ohm 1/4W resistor....those check out fine with a reasonable resistance. I think i'll source to try and source one of these 1.7K 5W resistors...can't cost that much. Maybe it's dropping too much voltage? I don't have a schematic or a part number for the zener to really tell. It's certainly worth trying, easy and cheap enough. What else is it feeding? --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/ Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/ +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html Note: These links are hopefully temporary until we can sort out the excessive traffic on Repairfaq.org. Important: Anything sent to the email address in the message header is ignored. To contact me, please use the feedback form on the S.E.R FAQ Web sites. |
#6
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20% tolerance power resistor. Could have been as high as 2.2K ohm. Was
the meter properly calibrated recently? If not, the assumption about it being 2.5K could be wrong. That tv is a Daewoo made elcheapo. The 2K resistor is a dropping resistor for the standby power source circuitry. If the proper voltage is measured on the zener diode with the tv safely plugged into an isolation transformer for disassembled testing, there is no problem in that part of the circuit. A bit more standard tv set troubleshooting is in order to narrow down the problem. David |
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