Help needed with power supply RC filter repair

[jfs]

Hello to you,

this is my first post and I'd like to point out i'm a newbie at electronics, so please be gentle with math (I guess my girlfriend can help, lol)


I am trying to repair a power supply, but cannot figure what value the resistor is in the RC filter.

Here is a picture:

**broken link removed**

the capacitor value is 2200uF/16v. This filter is in a 120 VAC 60Hz 31w input, 5VDC 1A +/- 12VDC 300mA power supply.

Thank you very much for your assistance

jf

 

[Sceadwian]

The picture doesn't show the colour bands on the resistors. Look on a google for resistor colour codes and use your own eyes to determine the resistors value.

 

[jfs]

Yes, I know the code but the thing is cooked from the eloctrolyte that oozed from the cap, can't figure the first band (might be brown), the second seems to be violet. I thought there might have been some kind of formula to determine the value.

Thanks for your help, I really need this psu back in order.

 

[Sceadwian]

You sure the failed component is the resistor? The burn marks on the board seem centered on the diode bridge, one of the diodes is visably cracked. What's the physical distention on the large vertical element next to the cracked looking diode? Looks like heat shrink over some component.

If you need the PSU up ASAP replace it with a working one, and figure out the failed one later.

 

[jfs]

Unfortunately I can't find another one

The diodes look ok in person, I think what you see as a crack (the line that splits the end in two) is the natural seam of the diode. I have some brand new 1n4007 here and that's what they look like. The diode region seems to have inherited most of the oozed out electrolyte from the busted (clearly) 2200uF cap, wich is why it seems pretty charred.

As for knowing if the resistor is the faulty component, I don't think so, but i intended to change all of the components, but can't make out the value of this one resistor.

If you need any other info, please ask.

Thanks again,

jf

 

[Sceadwian]

So check it on a meter? If it's not failed it'll read the appropriate value which will corespond with whatever colour bands you still can read.

 

[kchriste]

Yes, I know the code but the thing is cooked from the eloctrolyte that oozed from the cap, can't figure the first band (might be brown), the second seems to be violet. I thought there might have been some kind of formula to determine the value.

No real formula.... But, you can narrow it down if you know what the standard values are; there are only 2 combinations in 5 & 10% resistors where the 2nd band is violet. 27x and 47x where the 3rd band is x. So the first band is either red or yellow. The resistor doesn't look totally burned out to me, so you could remove it from the PCB and measure it to confirm.
Standard values:
https://www.elexp.com/t_eia.htm

 

[Pkhow]

I have worked on Power supplies for over 30 Years and this is not just a faulty resistor! The coating looks burnt and has blistered color bands so you can not read it. To blister like this something caused a large current to flow through the resistor. I agree the resistor is likley to be still functional not burnt out.
A wider Photo may be of more help if you still want to try.
pkhow!

 

[jfs]

Ok, i've pulled it out and measured it and get a stable reading of 175.3 ohms, wich would confirm the violet and posssibly the Brown that I suspected for the first band. I guess my question is now: Can a damaged resistor change value or is it a case of it either works or don't ?

And Pkhow, I suspect the resistor was splashed with electrolyte wich kind of cooked on it.

Also, if the RC filter is faulty in a psu, it still outputs the correct DC voltage, just full of ripples (wich causes my machine to constantly reboot ? Is this a good diagnostic ?

much thanks to you all

 

[Nigel Goodwin]

Ok, i've pulled it out and measured it and get a stable reading of 175.3 ohms, wich would confirm the violet and posssibly the Brown that I suspected for the first band. I guess my question is now: Can a damaged resistor change value or is it a case of it either works or don't ?

If it reads 175 ohms, and it's not chnaged value, then it's probably 180 ohms. Resistors can (and do) change values when overheated.

And Pkhow, I suspect the resistor was splashed with electrolyte wich kind of cooked on it.

Also, if the RC filter is faulty in a psu, it still outputs the correct DC voltage, just full of ripples (wich causes my machine to constantly reboot ? Is this a good diagnostic ?

What makes you think it's part of an RC filter?. Have you changed all the faulty electrolytics?, if ones duff the rest probably are as well? - you need an ESR meter or scope to check them.

 

[jfs]

To be honest, I don't know if this resistor is part of an RC filter, I assume it does since there should be one. Maybe the Rc filter cap is the 3300 uf wich has also leaked, althought not as much. i will be replacing them all.

If it reads 175 ohms, and it's not chnaged value, then it's probably 180 ohms. Resistors can (and do) change values when overheated.

Well, I don't know if it has changed value , what I meant is it NOW consistently reads 175.3, sorry if that wasn't clear, english is not my first language.

**broken link removed**

 

[Nigel Goodwin]

To be honest, I don't know if this resistor is part of an RC filter, I assume it does since there should be one. Maybe the Rc filter cap is the 3300 uf wich has also leaked, althought not as much. i will be replacing them all.

You don't normally have RC filters in PSU's, because they don't work - it limits the current far too much.

Replace all the electrolytics, and you may find it's repaired - electrolytic failure is probably the most common fault these days. Check and see where the resistor is connected to, it's not likely to be part of an RC filter.

 

[jfs]

I'll do that and let you know how it went. I'll be back monday night (too late for me to go to the store now)

As for the resistor, you are right, it is not connected to the cap, as far as i can tell, looking at the traces.

Thanks a lot,

jf

 

[kchriste]

Looking at the poorly ventilated box and all the heatsinks, you might want to replace those caps with ones rated for 105C instead of the standard 85C ones. Also, check the input rectifier diodes as one or more may be a shorted cap killer.

 

[erosennin]

It also looks as though the Electro on the left of the FET to the left of the central heatsink has also leaked. Might wanna check that out too.

 

[jfs]

Hello all,

well, I have replaced ALL the caps and put in a 180 ohms resistor and it all works lovingly.

Thank you very much for your help.

 

[Pkhow]

If your symptom is it keeps re-booting you have somthing putting a heavy load on the supply. Possibly the circuit that your Resitor(may be a current limit R) is connected too given it has run very hot. A method then is to lift different part of the circuit connected to the PSU. Sometimes unfortunatly there is a feedback loop and this may confuse the issue.
Pkh