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Old 18th July 2004, 11:15 AM   #1
LarryN is offline LarryN  Hong Kong
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Join Date: Jul 2004
Location: Hong Kong
Default Help required !

Dear All ,

In attampting to repair my switching power supply , I met with the following problems , I have no circuit diagram , Barco is not able to provide me one , the switiching power supply uses the following chips , TDA4601 . BU508A and BD237 plus one 11v zenor and several diodes , my problem being that the output
voltages , 5 of them , drop to 1/3 when a loading is apply , for example , there is one output , @8-9 voltage , supplying votage to one 7085 for regulation , it drops to 2.6 volts and needless to say , nothing comes right ! I have replaced all said chips , zenor diode and test all capcitors & resistors , they are all OK . It is just one resister I am not sure , it should be a current limiting resistor as it is mounted 1/2" high from board , it is with the following color code :

yellow / grey / black / gold / yellow*

what confuses me being this 2nd yellow , what does it mean ?

I have detached the resistor and it measured several Mega ohms , it seems not possible to have a current limiting resistor at this value but my knowledge is basic and I am not sure what being its purpose , it is situated in the major circuit path before the small transformer that output several voltages !

AS Barco had discontinued this TV demodulator for long and I am not able to get any support , I would appreciate helps & hints from you all where should I dig further into .

Again , thanks in advance !
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Old 19th July 2004, 12:04 AM   #2
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Default resistor

Hello

Quote:
yellow / grey / black / gold / yellow*

This resistor to the best of my knowledge would be 48 ohms
with a tolerance of 5% the 5th band being the grade of resistor



* disclamer - I WOULD DOUBLE CHECK -


Jeff
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Old 19th July 2004, 01:05 AM   #3
johnnyx is offline johnnyx  United Kingdom
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Join Date: Jan 2004
Location: manchester
It could be that the resistor got very hot, it went open - circuit, and the bands changed colour. This is a common failure in psu or power circuits. Get a second opinion on the colours then guess what the value should be. If it is raised above the pcb, then it is supposed to get warm, it is raised to avoid scorching the pcb. For current sensing, I would think a low resistance is most likely. I would draw out the circuit around that resistor to establish it's purpose, and improve on the accuracy of the guess. I've guessed the values of burnt resistors many times, and it works ok.

Good luck
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