Help with PCB repair

Oily

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I'm trying to fix this circuit board,the problem is the part circled in red keeps blowing,it's a wire round resistor.It blew when it was plugged in with the plug switched on causing a power surge.I've also replaced the varistor,circled in yellow,this acts as some sort of surge protection.
Is there anything else i should be looking at?
 

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Spectre

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Was the wirewound thing a safety resistor? I can't see the part number.

Might be worth testing the ceramic capacitors to make sure they aren't short.

Looks like it generates two DC supplies from the mains, one being at the bottom left and the other being at the middle right of the PCB. I suppose you could take the full-wave bridge off to isolate the power supply at the bottom. The other one looks like it might be half-wave rectified by those two diodes next to the bridge.
 

Oily

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Thanks for the quick reply.Is there any chance you could highlight the components you mentioned above please.
Also how do I test the capacitors with a multi meter?
This is all a bit new for me
 

Oily

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The wire round resistor was rated at 27ohms but not sure if it was a ‘safety ‘ resistor?
 

dar1437

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Thanks for the quick reply.Is there any chance you could highlight the components you mentioned above please.
Also how do I test the capacitors with a multi meter?
This is all a bit new for me

To test the caps, you'd need to lift one side off the board and put the multimeter to ohms(resisitance) and put the probes either side of the caps and if the meter reads 0, or the needle goes all the way across, then its short circuit. (I think lol)
 

Spectre

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To test the caps, you'd need to lift one side off the board and put the multimeter to ohms(resisitance) and put the probes either side of the caps and if the meter reads 0, or the needle goes all the way across, then its short circuit. (I think lol)

I'd leave the ceramics in circuit as any low Ohm reading across them is bad news anyway :).

I was guessing they were safety resistors @Oily but it's a bit of a strange looking power supply anyway. Sometimes special wirewound resistors are used as fuses as they fail less catastrophically than normal ones.

If you isolate one of the supplies by taking the bridge rectifier off it will give you more of a clue. It looks like this below. There are a few supplies on that board judging from the silkscreen next to the testpoints.


1565370169422.png

The MELF diodes are probably rectifiers for another couple of supplies to the right middle and they look like this:

1565370320228.png

You could take all three of those off just to save blowing anything up further just to see if it is something closer to the input.
 

Oily

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To test the caps, you'd need to lift one side off the board and put the multimeter to ohms(resisitance) and put the probes either side of the caps and if the meter reads 0, or the needle goes all the way across, then its short circuit. (I think lol)
Cheers
I'd leave the ceramics in circuit as any low Ohm reading across them is bad news anyway :).

I was guessing they were safety resistors @Oily but it's a bit of a strange looking power supply anyway. Sometimes special wirewound resistors are used as fuses as they fail less catastrophically than normal ones.

If you isolate one of the supplies by taking the bridge rectifier off it will give you more of a clue. It looks like this below. There are a few supplies on that board judging from the silkscreen next to the testpoints.


View attachment 126045

The MELF diodes are probably rectifiers for another couple of supplies to the right middle and they look like this:

View attachment 126046

You could take all three of those off just to save blowing anything up further just to see if it is something closer to the input.
Could you show me which bridge rectifier to take off please?As i said I'm new to this so I'm learning as I go
 

Spectre

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1565374130935.png

Green is a bridge rectifier and seems to be some sort of main power supply.

Purple are diodes, possibly rectifying for a couple of smaller supplies as is the red diode.

Taking those off might isolate a lot of the circuit but it's difficult to tell what's going on. Is @chookey around? :)
 

Spectre

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I'm wondering what that opto-isolator is doing as the isolation gap doesn't exist...
 

Spectre

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Now you've completely lost me :p

Thingy to the left of the red diode.

They are usually used for feedback in switched mode power supplies but my guess would be that one is being a voltage level converter.
 

Oily

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It's a power supply out of a smart turbo trainer,it varies the amount of resistance to simulate hills etc just to give you an idea of what it does
 

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chookey

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View attachment 126049

Green is a bridge rectifier and seems to be some sort of main power supply.

Purple are diodes, possibly rectifying for a couple of smaller supplies as is the red diode.

Taking those off might isolate a lot of the circuit but it's difficult to tell what's going on. Is @chookey around? :)
I'm lurking Rob, got a lot happening at the moment.
Agree with all you've said. I would add that the bridge is supplying 2 different outputs through T1 and T5 either of which are possible culprits. The opto may be being used as a "servo" to control the current passing through T1. If T1 or associated component were faulty it could explain why R64 is exceeding it's wattage rating.
 

Spectre

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I'm lurking Rob, got a lot happening at the moment.
Agree with all you've said. I would add that the bridge is supplying 2 different outputs through T1 and T5 either of which are possible culprits. The opto may be being used as a "servo" to control the current passing through T1. If T1 or associated component were faulty it could explain why R64 is exceeding it's wattage rating.

No probs @chookey :).

Been doing this with @Oily and there is a low resistance between input pads of the bridge with the bridge removed. Looking again, is D11 attached to the output of that bridge? Strange to have a couple of extra diodes there and I first thought they were on the input.

There's about zero isolation in this design, although it might not need it. Are they linear regulators? They might get a bit warm (and fail) so yes, T5, is a good suspect too.
 

Oily

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No,D11 isn't attached to the output of the bridge :)
 
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