So it is a mains voltage rated 750k resistor whose purpose is to discharge the interference supression capacitors on the mains input to prevent the user getting a shock when unplugged.

Thanks AlbertHall. So If I measure 240v on the one pin of the resistor and point my probe to the next pin, should there be voltage?

 

It is connected across the mains input so if it is plugged in then, yes there should be mains voltage across where it was.
If there is no voltage there then the most likely culprit is the nearby fuse.

 

It is connected across the mains input so if it is plugged in then, yes there should be mains voltage across where it was.
If there is no voltage there then the most likely culprit is the nearby fuse.

Yes I am thinking the same...thanks for all yor help.

 

As you are measuring close to zero ohms across that resistor it is probably one of the components in parallel with it has failed short circuit. I think the varistor VX801S is the most likely item to have failed. As there is item NT811S ( Probably a negative temperature coefficient thermistor.) between the mains input and the input to the bridge rectifier I would first measure the resistance across the bridge rectifier input. If this is a few tens of ohms greater than the reading across the 750K resistor then the fault is on one of the items directly in parallel with that resistor. If so then you will have to de solder one end of each of these components one at a time until you find the faulty component. If you can measure very low resistance values then you may be able to use the fact that the common mode filter windings will have some resistance to narrow down which area the fault lies.

Les.

 

If measuring directly across the two leads of the resistor, if you're not getting 240 VAC then the fuse is blown. As to why it's blown - I can't guess. There's not enough of the circuit depicted to determine what comes after the coils.

As for resistors failing short circuited - that doesn't happen. If it has then hold on to it real tight, it's got to be worth millions because someone is going to want a shorted resistor just for the sake of claiming to having one.

You said you measured the resistance with the resistor out of the circuit. You (if I read you correctly) said it still measured zero ohms (or nearly zero ohms). If it IS that low a resistance then you don't want it in circuit at all because it is basically a dead short and a fire hazard. As for anything else about it - again, there's not enough circuitry shown to be able to trace it out and determine a most likely cause.

Just for reference: A 1 ohm resistor across 240 VAC will see 240 amps (in theory). Your circuit breaker should click out LONG before that. And if it doesn't, the power cord should melt in about 5 one-hundredths of a second. If it doesn't, your circuit should display a spectacular light show for a very short moment. So you're telling us it reads zero ohms - none of us are going to buy that. I hate to say this but it sounds like you're doing something wrong with the way you're taking your reading.

In any case, the fuse should blow LONG before any spectacular light shows occur.

 

If measuring directly across the two leads of the resistor, if you're not getting 240 VAC then the fuse is blown. As to why it's blown - I can't guess. There's not enough of the circuit depicted to determine what comes after the coils.

As for resistors failing short circuited - that doesn't happen. If it has then hold on to it real tight, it's got to be worth millions because someone is going to want a shorted resistor just for the sake of claiming to having one.

You said you measured the resistance with the resistor out of the circuit. You (if I read you correctly) said it still measured zero ohms (or nearly zero ohms). If it IS that low a resistance then you don't want it in circuit at all because it is basically a dead short and a fire hazard. As for anything else about it - again, there's not enough circuitry shown to be able to trace it out and determine a most likely cause.

Just for reference: A 1 ohm resistor across 240 VAC will see 240 amps (in theory). Your circuit breaker should click out LONG before that. And if it doesn't, the power cord should melt in about 5 one-hundredths of a second. If it doesn't, your circuit should display a spectacular light show for a very short moment. So you're telling us it reads zero ohms - none of us are going to buy that. I hate to say this but it sounds like you're doing something wrong with the way you're taking your reading.

In any case, the fuse should blow LONG before any spectacular light shows occur.

Wow thats true...the circuit is running in parrallel. Thanks for the great tips. Will try that and come back.

 

If measuring directly across the two leads of the resistor, if you're not getting 240 VAC then the fuse is blown. As to why it's blown - I can't guess. There's not enough of the circuit depicted to determine what comes after the coils.

As for resistors failing short circuited - that doesn't happen. If it has then hold on to it real tight, it's got to be worth millions because someone is going to want a shorted resistor just for the sake of claiming to having one.

You said you measured the resistance with the resistor out of the circuit. You (if I read you correctly) said it still measured zero ohms (or nearly zero ohms). If it IS that low a resistance then you don't want it in circuit at all because it is basically a dead short and a fire hazard. As for anything else about it - again, there's not enough circuitry shown to be able to trace it out and determine a most likely cause.

Just for reference: A 1 ohm resistor across 240 VAC will see 240 amps (in theory). Your circuit breaker should click out LONG before that. And if it doesn't, the power cord should melt in about 5 one-hundredths of a second. If it doesn't, your circuit should display a spectacular light show for a very short moment. So you're telling us it reads zero ohms - none of us are going to buy that. I hate to say this but it sounds like you're doing something wrong with the way you're taking your reading.

In any case, the fuse should blow LONG before any spectacular light shows occur.

Yes you are correct I was measuring it wrong.