Unusual resistor in power supply - help ?

[Dannyx]

Good day folks. A colleague of mine asked me to take a look at this ATX supply which he said is completely dead. I didn't plug it in myself, but instead started probing around to see if I can find anything with the board "cold".

I started with the stand-by circuit which is driven by a TNY278PN IC which is not shorted. In that area however, I found THIS resistor which clearly had a hole in it and it reads completely open - no resistance at all. It goes between a leg of the small STBY transformer and the + leg of the main capacitors and for some reason it popped. The fuse is good, so it seems to me nothing exploded violently here, but the resistor still decided to give up....

No problem, I said to myself, I'll just replace the resistor and see what happens....that was until I looked up the color code and this is where I got stuck: there's no value for a gold band in the second or third positions ! So we've got yellow (4), purple (7), gold (?), gold (?), white (10% tolerance ?). Normally a gold band signifies a 0.1 multiplier, but not in the second or third position like that, so what is this resistor ? I thought maybe it's an inductor, but not only is the board marked R50, but it also doesn't quite make sense from an electronic point of view to have an inductor in series with the HV line feeding the standby transformer. It should be a fairly low value resistor in my opinion, something like 0.47 or 4.7 at most.....what do you guys think ? If we omit the last white band, that would indeed make sense as it would be 4.7ohm, since we have valid values for the gold bands in those positions. Let's start with this and I'll keep you posted on the repair as we go.

[eccerr0r]

Re: Unusual resistor in power supply - help ?

looks like a typical 5% 4.7Ω resistor. Gold in band 3 divides the significant digits by 10. White is probably temp coefficient.

[Dannyx]

Re: Unusual resistor in power supply - help ?

looks like a typical 5% 4.7Ω resistor. Gold in band 3 divides the significant digits by 10. White is probably temp coefficient.

Yes, now that you mention it, it is. What threw me off is the additional white band at the end. Makes perfect sense if you omit that, like I said in the last few lines of my post.

[redwire]

Re: Unusual resistor in power supply - help ?

Replacing the resistor will fix nothing... it was overloaded for a reason.

[R_J]

Re: Unusual resistor in power supply - help ?

Follow the line through the transformer and it likely connects to the tny278, which would be shorted.

[mariushm]

Re: Unusual resistor in power supply - help ?

Scrape the gray crap to the right of the tny chip just in case it became somewhat conductive. You have high voltage on those pins.

Check the ESR of that capacitor to the right of the tny chip - it has no vent release so no easy way of knowing if it's cooked or not.

Check the to220 chip on the heatsink to the right of the resistor, it could be shorted mosfet or something.

What's with the diode to the left of the resistor in the third picture? Can't tell by the angle of the camera, is that diode going into the circuit board? Some reflection in the metal or what? Weird...

[redwire]

Re: Unusual resistor in power supply - help ?

If the resistor does power the standby SMPS, then the part might be there just as a cheap fuse (fusible resistor). The main fuse is too big, really.

Otherwise I would say it is a sense resistor for a MOSFET that shorted.

[Dannyx]

Re: Unusual resistor in power supply - help ?

Ok, some more info on this supply: true, I was about to make the most amateurish mistake ever: replace the "fuse" ! You never replace the fuse !

The TNY chip is indeed shorted, so thanks for opening my eyes. Reads 9 ohms D-S and around 26 ohms D-BP/M (pin 2)....not OK. I got the same readings on my first round of troubleshooting but it was with the TNY still in circuit, so I thought it measured through the transformer which is normal to show very low values. I removed the chip and these are the values. I would say it's dead.

Replaced the small 10uF cap next to the IC just in case.

About the diode: yes, it's an optical illusion There's actually TWO diodes there. The one that appears to be going into the board is sitting behind the other one which has one of its legs soldered onto a fat piece of metal (foreground). It appears to be disappearing into the board because the black part of the diode overlaps with the black diode symbol on the board Here are some other pictures of the areas in question.

Now, the diode with the metal strip goes between the D pin of the IC and the HV line on the main caps, reverse biased. It's a T2D60 - I could not find anything of the sorts.... It reads completely open in both directions. I don't know if that's normal or I should replace it as well...

[budm]

Re: Unusual resistor in power supply - help ?

When that TNY chip blows up it probably takes out the Opto and other parts with it also.
That TS02D-60 snubber diode may be back top back diode so it will shows as open circuit, you will need need high Voltage supply with current limiter to find out what the clamping Voltage is, similar to the application circuit here:

[Dannyx]

Re: Unusual resistor in power supply - help ?

I see you've edited the original reply. It was about that large diode which supposedly is a two in one sort of deal and it's pretty special. I was curious where you found that info, since I couldn't find anything...

The opto looks fine. Not having any real way of checking, I did a simple cross-check with the one next to it and they both read open circuit in both diode and resistance test, which is a really crude and rudimentary test, but it's good enough at this stage

[budm]

Re: Unusual resistor in power supply - help ?

The Photo transistor in the opto may be damage, you can see how it is being used in teh TNY application circuit.
I found ST02D-82 info:
And also the snubber diode as used in the application circuit:
If you have one of those LED tester that goes up to 300V, you can use it to find out what the clamping Voltage is.

[Dannyx]

Re: Unusual resistor in power supply - help ?

The Photo transistor in the opto may be damage, you can see how it is being used in teh TNY application circuit.

Correct. Like I said - no breadboard and no bench supply in this joint, so I can't test it, at least not easily and I'm not going through all that trouble for something I'm not getting paid for.

Off-topic. I had a look through that document you linked to and I was most interested in page 22, the thermal images of that demo board. What I find particularly interesting there is the input choke L11 on the left: notice how on a low mains voltage it's hot, but on higher input voltages it's cold and I can't quite figure out why that is. I'm guessing it has to do with power, classic ohm's law: more voltage means the current can be lower for the same 30w to be produced on the output of the supply (hence a colder choke), whereas a lower input voltage requires more current to be drawn from the primary to maintain the same output (so the choke gets hotter)...I could be wrong of course.

[budm]

Re: Unusual resistor in power supply - help ?

Off-topic: You are correct.

[stj]

Re: Unusual resistor in power supply - help ?

it's a flameproof resistor,
if you can find a gray or green similar replacement and you decide to bodge in a regular part - put it in fiberglass sleeving.

[Dannyx]

Re: Unusual resistor in power supply - help ?

Ok chaps, we have a winner I left that weird diode alone, replaced the TNY IC and the 4.7ohm resistor and with a bit of perspiration, I plugged the damn thing in. The series lightbulb I always use for this sort of thing did its thing, went bright then gradually went out, and sure enough I got standby on the purple wire. Removed the bulb, went straight on mains (because it's got APFC which always throws off me and the series bulb !) and with even sweatier hands shorted PS_ON to GND - because there's no way you could expect the joint to have a PSU tester *PSHAWW* - and I got a fan spin and stable voltages Thanks guys ! Cheers !

[stj]

Re: Unusual resistor in power supply - help ?

because there's no way you could expect the joint to have a PSU tester *PSHAWW*

lol
i have one of these sexy things on the table!
https://www.aliexpress.com/item/Chec...864168715.html

[Dannyx]

Re: Unusual resistor in power supply - help ?

I too had that at my former shop (of course) and have one at home (of course) , but there's no way I'm "donating" it to a million-euro company....... The one at work was actually metal

[Stefan Payne]

Re: Unusual resistor in power supply - help ?

Pic of the Backside??

Interesting unit...

[Dannyx]

Re: Unusual resistor in power supply - help ?

Pic of the Backside??

Interesting unit...

The supply or PSU tester ? The tester's back is flat - nothing to see there...

[stj]

Re: Unusual resistor in power supply - help ?

yea, just a plastic box with lcd on the front and connectors around the edges.
loads of reviews on utube.