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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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Help - RCA CTC169CA Fuse Troubleshooting
I'm hoping somebody can help me with this TV set. I am new to
electronics repair and have read quite a bit on the net but the recommendations are fairly generic. This set blows the fuse instantly upon applying power. I have checked the board for hairline cracks with no avail (at least to my eyes). I believe I have verified everything up to the HOT (Q4401), but anything is possible for a newbie. The HOT is high on C-E in both directions, but on B-E it reads 0.4 ohms in both directions (on 200 ohm DMM scale). Does this mean the HOT is bad? If so, what are the next steps (I do not want to simply replace the HOT and see if it happens again, which I've been told previously). Thanks in advance for any help! |
#2
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Obviously you did not verify the power supply which is where you should
have started when diagnosing a blown fuse. It looks like from you information that you have tried several fuses. Time to call on someone who knows what they are doing and has a basic understanding of proper troubleshooting technique to have a go at the set, in person. You may need to have someone who has more experience working in your shop give you a hand with the repair and help to bring you up to speed on troubleshooting and repair. Continuously replacing a blown fuse is not proper troubleshooting. Newbie wrote: I'm hoping somebody can help me with this TV set. I am new to electronics repair and have read quite a bit on the net but the recommendations are fairly generic. This set blows the fuse instantly upon applying power. I have checked the board for hairline cracks with no avail (at least to my eyes). I believe I have verified everything up to the HOT (Q4401), but anything is possible for a newbie. The HOT is high on C-E in both directions, but on B-E it reads 0.4 ohms in both directions (on 200 ohm DMM scale). Does this mean the HOT is bad? If so, what are the next steps (I do not want to simply replace the HOT and see if it happens again, which I've been told previously). Thanks in advance for any help! |
#3
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I apologize for not being more clear in my original post. I believe I
have verified everything up to the HOT (Q4401) beginning with the main power supply. Because the set went out while the TV was running, I figured it was worth a shot to replace the fuse once and apply power. Because the fuse blew immediately, I started by looking at the components beginning with the PS. Everything appear normal until the HOT... |
#4
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The fuse is only there to protect against hard shorts on the primary
side of the power supply (on sets like this using a true SMPS). You missed something there, try again. Proper technique is to replace the fuse, bring the unit up slowly on a variac while monitoring current. If the total current the tv would normally draw is exceeded before you reach 120V ac, then stop and do not cause additional damage. |
#5
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On 6 Feb 2005 13:13:45 -0800, "Newbie" wrote:
I apologize for not being more clear in my original post. I believe I have verified everything up to the HOT (Q4401) beginning with the main power supply. Because the set went out while the TV was running, I figured it was worth a shot to replace the fuse once and apply power. Because the fuse blew immediately, I started by looking at the components beginning with the PS. Everything appear normal until the HOT... If the fuse blows instantly, then most likely the short is on the primary side of the switching power supply, ie the chopper output is shorted, or one or more of the diodes in the bridge rectifier is shorted. If the chopper is shorted, then there are probably multiple other parts that are bad as well. Alan Harriman |
#6
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Thanks for the replies guys. I know that the bridge rectifiers are in
good shape (with no power on the board, CR1,2,3,4 all show around 130k ohm resistance). What's the best way to proceed in diagnosing the chopper output and any other damaged components withhout introducing any further damage? |
#7
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#1 do a google groups search for ctc169 in sci.electronics.repair for
lots of information. The diodes could very well be breaking down with full voltage applied, so the ohm meter reading may not tell the whole story. Capacitors are common failures in the SMPS and cause multiple additional parts failures. C4108 is a critical value so use an OEM part only. Best way is to assume a part is bad if you cannot accurately and with 100% confidence be assured it tests good, replace it. All the small parts cost very little in that set. Even with the replace it to be sure the total parts count should still be under 12 after finding the parts that are known bad. |
#8
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Newbie wrote:
I apologize for not being more clear in my original post. I believe I have verified everything up to the HOT (Q4401) beginning with the main power supply. Because the set went out while the TV was running, I figured it was worth a shot to replace the fuse once and apply power. Because the fuse blew immediately, I started by looking at the components beginning with the PS. Everything appear normal until the HOT... Hey, that's what happened to my set also.. RCA ctc169ca also. I turned it on and 2 minutes later it went out. Replaced the fuse and it immediately goes out. It was a virus set for a certain time and date |
#9
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Remove the HOT. If the B-E still mesures .4 ohms in both directions,
it's shorted. Normally reads between 15 - 30 ohms or more. Newbie wrote: I'm hoping somebody can help me with this TV set. I am new to electronics repair and have read quite a bit on the net but the recommendations are fairly generic. This set blows the fuse instantly upon applying power. I have checked the board for hairline cracks with no avail (at least to my eyes). I believe I have verified everything up to the HOT (Q4401), but anything is possible for a newbie. The HOT is high on C-E in both directions, but on B-E it reads 0.4 ohms in both directions (on 200 ohm DMM scale). Does this mean the HOT is bad? If so, what are the next steps (I do not want to simply replace the HOT and see if it happens again, which I've been told previously). Thanks in advance for any help! |
#10
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Arnold stewart wrote: Remove the HOT. If the B-E still mesures .4 ohms in both directions, it's shorted. Normally reads between 15 - 30 ohms or more. (But it shouldn't cause the fuse to blow.) Newbie wrote: I'm hoping somebody can help me with this TV set. I am new to electronics repair and have read quite a bit on the net but the recommendations are fairly generic. This set blows the fuse instantly upon applying power. I have checked the board for hairline cracks with no avail (at least to my eyes). I believe I have verified everything up to the HOT (Q4401), but anything is possible for a newbie. The HOT is high on C-E in both directions, but on B-E it reads 0.4 ohms in both directions (on 200 ohm DMM scale). Does this mean the HOT is bad? If so, what are the next steps (I do not want to simply replace the HOT and see if it happens again, which I've been told previously). Thanks in advance for any help! |
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