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PC Power Supply Repair

Posted on 2016-08-17
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Last Modified: 2016-08-23
Hi PC hardware experts,

I use an old HP Compaq DC7700 Small Form Factor desktop PC and a few days ago I accidentally left it running overnight (which I know should not be a problem, and it has been fine before), but the next day I went to use it and nothing was working, even after powering it off/on - not even a fan or light worked (not even the mother board light).

I think this is a power supply issue, because:
- I changed the mains lead - no go.
- The power supply fan doesn't even move (but I tested the fan by taking it out and feeding it 12V and that worked).
- I took the power supply out, and after watching some YouTube videos, I used a paper clip to short the green wire on the 24 pin connector to the adjacent black wire, but the fan still wouldn't go.

It may be not "worth" it, but if I can fix this power supply, I'd like to.  Could be good for experience, too.

Having inspected both sides of the power supply circuit board, I can't see any:
- bulging or leaking capacitors
- discoloured looking resistors
- dry looking solder joints.

I've tested that 236V (which is around the official mains voltage here in NZ) is getting through to the resistor which is close to where the mains comes in (just past the yellow rectangular component).
However, when I check for voltage across the long black component (presumably a fuse in shrink wrap?), I get nothing!
I also tested the resistance of that fuse (?) by unsoldering one end of it, and it's near 0, so I guess it's fine.

Questions:
Q1. Should I have seen voltage when I measured across that long black component that looks like a shrink wrapped fuse?
Q2. Would that long black component be the fuse?
Q3. Can you suggest any other things I can test with a simple multimeter to isolate the culprit?
Q4. Can you suggest any other components that either look suspect or are likely candidates?  If so, how can I test them?

See photos attached.  (Sorry they don't show everything and some are a bit unfocused.  Let me know if you want me to take more.)

Thanks.
Tel2
DSCF1956.JPG
DSCF1958.JPG
DSCF1962.JPG
DSCF1965.JPG
DSCF1966.JPG
DSCF1967.JPG
DSCF1970.JPG
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Question by:tel2
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Expert Comment

by:dbrunton
ID: 41759198
Cough.  Need photos.

Now does it have a standard ATX connector or does it have a mini ATX connector?
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Assisted Solution

by:Eirman
Eirman earned 50 total points
ID: 41759222
Did you check the fuse? (I can't see in your photos)
It's probably soldered to the main PCB, but you can still test it with a multmeter
and replace it if you are handy with a soldering iron.

Other than that, it's not worth the trouble.
Just recycle it and buy a replacement on eBay (My last replacement cost £20)
Make sure the wattage is equal or higher. I'd suggest 350 Watts.
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by:Eirman
Eirman earned 50 total points
ID: 41759239
Fuses are generally glass - Check F1 in the bottom right corner of DSCF1967.JPG
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Assisted Solution

by:IT-Expert
IT-Expert earned 70 total points
ID: 41759245
If we're on the same page here, that 'long black component' you refer to (that's sat on the edge of the cct board, looks like it says 'F1' above it, which if that's right, WILL be a fuse.  In which case, just do a simple continuity check on it.  You wont' hear a noise if it's failed, and if so, that definitely will cause the PSU to not work!
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Expert Comment

by:Sandeep
ID: 41759251
I would prefer leaving this job for the skilled Chip level expert to fix this than going to fix it on our own as it can give you some electric shocks if not handled carefully.

All the best for your try.
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Accepted Solution

by:
rindi earned 230 total points
ID: 41759267
Q1: No, if that is a fuse you are talking about.
Q2: It depends which component you are asking about. The fuse is shown on your 3rd jpg, lower right, labeled "F1", also in your 3rd to last jpg (this time upper left), 2nd last jpg (lower right again).
Q3: If it is a multimeter with diode measurement, you can also test the diodes and rectifier with it, and usually also the transistors or similar components.
Q4: With Powersupplies the main thing that went bad which I've seen were Electrolytic caps, and as you already said, there don't seem to be any on your power supply that look bad. But you can try wiggling them, if they seem loose they could have cracked open at the base where you can't see.

The mainboard of your PC on the other hand does seem to have at least 2 caps that seem to be bulging, provided the image isn't playing tricks to me. The first is next to the intel chip, to it's left, if you go further left there are some wires, and then an HP logo. The other is a little more to the left and further down, just next to the heatsink where the wires I just mentioned are over the heatsink. But it might just look as if they are bulging on the picture because of the flashlight reflections and perspective.

I've also seen bad caps on HP mainboards that are normally out of view because of the large CPU heatsink covering them, so I also suggest removing the heatsink to get a closer look.
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by:IT-Expert
IT-Expert earned 70 total points
ID: 41759271
Yes, I second the above comment.  If you're going to try and diagnose this, past the fuse point, i.e. if it's NOT the fuse, be VERY careful.  You're dealing with voltages and currents (when live), that could throw you across the room and KILL YOU if you're not careful.  Be safe!  (I'm sure you knew that, of course)
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Expert Comment

by:Obaid ur Rehman
ID: 41759473
Does the fan of the power supply turn on if you short PS_ON to COM?
See attached file.

Edited:

I see you have checked it and it does not turn on, I would suggest you to check the components visually and find the faulty component. If you have a voltmeter, you can check voltage values at different points to trace faulty area.
The best is to leave this job to an expert!
power-supply-check.jpg
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Expert Comment

by:David Johnson, CD, MVP
ID: 41759859
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Author Comment

by:tel2
ID: 41760405
Thanks for all your comments, guys!  (Apologies to any of you who may be gals - it's so hard to tell the difference these days.)

Hi dbrunton,
Sorry about the photos - I realised I hadn't attached them as soon as I posted it, and I finished attaching them about 15 mins later.
After a few mins of googling, I'm still not 100% sure if it's standard or mini ATX, but the large black connector has 24 pins, so I guess that's a standard ATX.  If that doesn't answer your question, please tell me how to tell the difference.

Hi Eirman,
To answer your questions about the fuse, do a few finds on the word "fuse" in my original post.
And yes, F1 is the long black component that I was referring to (I should have stated the "F1" label).

Hi rindi,
Thanks for your thorough response.
Re Q1, is that because there's no drop in voltage over a "wire"?
Impressive spotting re the motherboard bulging caps, and I think you're right, despite the perspective of the photo.  I had noticed that a couple of days ago too, but I just didn't expect that was the cause of my current issue because of the reasons I have given that the power supply seems to be faulty.  But I guess they're gonna die someday, so that's the first place I should look if something goes wrong after the power supply is sorted.

Hi Obaid ur Rehman,
> "I would suggest you to check the components visually and find the faulty component. If you have a voltmeter, you can check voltage values at different points to trace faulty area...".
See my original post for the visual inspection I have done, i.e. the paragraph starting with "Having inspected both sides of the power supply circuit board, I can't see any:".  As also mentioned I have tested a couple of places with my multimeter, but I don't know where else to test, hence my Q3.

Hi David Johnson,
Thanks for the eBay link.  They would probably take weeks to get to New Zealand though, and as I mentioned,
'It may be not "worth" it, but if I can fix this power supply, I'd like to.  Could be good for experience, too.'
Hopefully death by electric shock will not be part of that experience.   8)

Thanks.
tel2
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Expert Comment

by:dbrunton
ID: 41760455
Standard 24 pin ATX about 50mm and the mini ATX just under 40mm in length.

I was thinking you could connect an old ATX power supply to your motherboard for testing but the wiring scheme in your plug seems to be not the standard you get in normal power supplies.  So doing something like that would probably blow either the motherboard or the PSU or both.

I suspect it is probably going to be cheaper to wait for a DC7700 to appear cheaply on TradeMe to get the PSU.
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by:tel2
ID: 41760472
Thanks dbrunton,
It's about 37mm long so I guess it's a mini ATX.
Good to hear from another Kiwi who knows the local online sources.   8)
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by:tel2
ID: 41760567
P.S. dbrunton, look after that cough, will ya?  You know how viruses spread on the web.

Hi again rindi,
Re Q3, that's a bit more than I'm ready to take on with my current electronics skill level, but thanks for the info.
Re Q4, Tried wiggling the caps now, but none seem to be a problem.  I assume you mean put my finger on top of them and try to move them back and forth in the direction of each leg?  Most of them will obviously wiggle at right angles to the direction of the legs, unless they have that goo stuff holding them still.

If anyone else has any other suggestions on how I could isolate the culprit component(s)?

TRS
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Expert Comment

by:nobus
ID: 41760580
i also see the 2 caps on the mobo bulging, near the yellow wire going to the 6 pin AUX power connector
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Author Comment

by:tel2
ID: 41760620
Are they different from the ones mentioned by rindi, nobus?
If so, could you please crop the photo to show them, or draw on it, coz I can't see what you mean, sorry.
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Assisted Solution

by:rindi
rindi earned 230 total points
ID: 41760663
Q1: Yes, the wire has very low resistance, so you wouldn't measure any voltage difference between one end of the wire and the other. I would first change the caps on the mainboard, the Powersupply probably still is fine.
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Author Comment

by:tel2
ID: 41760828
Hi rindi,

Why do you think the power supply could be fine if, as I said in my first post:

"- The power supply fan doesn't even move (but I tested the fan by taking it out and feeding it 12V and that worked).
- I took the power supply out, and after watching some YouTube videos, I used a paper clip to short the green wire on the 24 pin connector to the adjacent black wire, but the fan still wouldn't go."


?
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LVL 87

Expert Comment

by:rindi
ID: 41760838
The fan is probably meant to only start running when it is needed, ie when temperatures reach a certain threshold. That will probably only happen when there is a certain load on the unit, and when you just use the shorting bridge like you mentioned, the PSU isn't connected to the PC so the only load you would have is from the PSU itself.
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Expert Comment

by:nobus
ID: 41761046
as long as it has bad caps - it likely won't startup even
so do as Rindi said, and yes they are the same caps i believe
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Assisted Solution

by:dbrunton
dbrunton earned 90 total points
ID: 41761517
The fan should start if the PSU has started.  You can place a dummy load on the PSU such as a hard disk to one of the external connectors.  Use your shorting bridge and if the PSU is OK the hard disk and fan should startup.
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Author Comment

by:tel2
ID: 41761635
Hi all,

A friend of mine who fixes PCs has given me a 2nd hand PSU out of the same model PC.
Before I installed it, I tested it by shorting the green & black wires with a paper clip, as described in my original post.  The fan started immediately after powering it on, with no external load.
I installed it and initially it didn't work, but I found that the paper clip had opened up the connector holes too much, so it was probably not contacting, so I closed them a bit and it now works, and I'm using that PC as I speak...er...type.

Q5. Is it worth removing dust from the new PSU?  I found quite a bit of dust in the old one but haven't opened the new one up to check it yet.

I know the 2 bulging caps on the mother board may die sometime, so I may have to replace them.
They are both brown and say this on their sides:
    KZG
    6.3V
    1000mF
Q6. I understand that I should install replacements with the same polarity, and they should have the same farad value, but I can use higher voltage caps if I want to, but does the colour (brown) or KZG (is that the brand?) matter?  I would have guessed not, but just thought I should confirm with people who know.

Thanks.
tel2
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by:dbrunton
dbrunton earned 90 total points
ID: 41761899
>>  Is it worth removing dust from the new PSU?  I found quite a bit of dust in the old one but haven't opened the new one up to check it yet.

Use a vacuum cleaner (with a soft tapered nozzle if possible) and apply to any large opening (usually the fan end but you can look at any large openings that exist) in the PSU and suck.  Apply narrow paint brush to fan blades to knock off encrusted dirt (hold PSU such that dirt does not drop back in).  That is usually good enough to clean the PSU.  If the vacuum can blow as well as suck then try the blow function.

>>  but I can use higher voltage caps if I want to, but does the colour (brown) or KZG (is that the brand?) matter?  

KZG is the brand.  Made by Chemi-Con.  Now there are various types of capacitors and I'm not familiar with what 1s suitable.  If none of the experts here don't know then try the forums at badcaps.net
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Author Comment

by:tel2
ID: 41761904
Thanks for that, dbrunton.

> "Now there are various types of capacitors and I'm not familiar with what 1s suitable."

Well I understand that the general type is "aluminium electrolytic".

But I'm still only 99% sure that colour doesn't matter.
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Assisted Solution

by:Obaid ur Rehman
Obaid ur Rehman earned 40 total points
ID: 41761988
Brands and colours doesn't matter. Capacitance matters.
Are you sure it is 1000mF and not 1000uF?
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by:tel2
ID: 41761995
Sorry - yes, 1000uF.  Any idea why they don't just write 1mF?
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by:nobus
nobus earned 20 total points
ID: 41762004
it's the usance, because the values have started with 10 -100 and now 1000uF or even 10.000uF
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Expert Comment

by:Obaid ur Rehman
ID: 41762007
To avoid replacement errors actually, electronic components are highly vulnerable to physical damages. That's why manufacturers write capacitance in Multiple of zeros 000+

See here I got an alarm when you said 1F.
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by:rindi
rindi earned 230 total points
ID: 41762104
The Voltage, Capacity, and max temperature matters. You need electrolytic caps that have at least the same voltage as the original (most mainboard caps use 6.3V). The capacity should be the same as the original (although sometimes you might get away with a larger capacity, that depends on what the circuit should do). As for the temperature rating, the higher the better.

The higher the voltage, as well as the higher the capacity, the larger the capacitor will be, so those values make it's size.
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by:tel2
ID: 41762166
Thanks rindi,

I didn't spot any max temperature on them when I was looking.  Could it be that it wasn't there, or do all electrolytic caps state that?  (It's possible I just missed it.)

These are called aluminium electrolytic caps, right?
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by:rindi
rindi earned 230 total points
ID: 41762224
Most of the should have a temperature printed on it, but sometimes it is hard to see, or the plastic cover where all those things are printed on, may have been cut off at the wrong place. This doesn't matter that much though. When you buy the caps you will see the details and temperatures of the products in their specs sheets of the catalog.

Most electrolytic capacitors (probably all) use an aluminum case, so that part is unimportant. What is important for you is that you can get different formfactors, like radial or axial. Radial is where both wires come out at the bottom (this is what you would need), while axial is where a wire comes out at each end.
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by:nobus
ID: 41762285
best replace them all
look for more info here :  www.badcaps.net
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by:tel2
ID: 41763186
OK, thanks rindi and nobus.
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by:tel2
ID: 41763204
Thanks to all of you for your advice!

Hard to split the points fairly over so many experts.  In general I gave more points to people who gave me new useful info (i.e. stuff that I hadn't covered in my original post, and hadn't been covered already by others).

Although I didn't end up fixing the power supply (just replaced it), I did learn some good tips from y'all along the way.

tel2
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Author Comment

by:tel2
ID: 41763378
Hi again experts,

I can ask these in another question if you like.  Just ask.

Although it was hard to read more text from the dodgy looking caps, because of their positioning, I was able to read it from a cap which is probably the same as them.  Here it is:
    (M) 105°C
    7 (k)
    7J

The "105°C" is obvious, but I have some more questions which I haven't yet solved after a quick web search:
Q7. What does the "(M)" mean?  Maximum?  If so, isn't the temperature marking always a maximum?
Q8. What does the "7 (k)" mean?
Q9. What does the "7J" mean?
Q10. Apart from the actual temperature (105C), do any of these other markings matter when it comes to selecting a replacement cap?
Q11. I've got some possible replacement caps which have this written on them:
    ltec (the brand), PET (what is that?), 7718D (what it that?) , 1000uF, 10V, LYZ105°C (M) (what is the "LYZ" & "(M)"?).  Looks as if the capacitance, voltage and temperature are compatible.  What about the rest?
Q12. I've seen some small electrolytic caps with 2 rows of 2 digit numbers cut into their aluminium tops.  What's that about?

Thanks.
tel2
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Expert Comment

by:nobus
ID: 41763405
you do not need all that info for replacing a cap
all you need is the capacity in uF - like 1000 uF which must be the same, and the voltage which must be the same also - or higher
be sure to place them in the same orientation - + to + and - to -
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Author Comment

by:tel2
ID: 41763437
I also need to have at least right max temerature, right nobus?

If anyone can answer all the questions in my previous post, that would be great.

Thanks.
tel2
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by:Obaid ur Rehman
ID: 41763444
Can you post a screen shot of the capacitor?
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by:Obaid ur Rehman
ID: 41763462
Tel2,

The model numbers vary among different manufacturers. The correct info can be derived from Datasheet. LTech LYZ series Datasheet is here:
http://www.compostar.com/Ltec/LYZ.pdf

Please make sure this is a reliable brand.
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by:rindi
ID: 41763522
If the temp goes up to 105° it should be fine.
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by:nobus
ID: 41763568
i have never worried about the temps of the capacitors - but of the temps in the PC
so make sure all fans are clean -  and running fine, and  everything is dust free

before you reach 100° in the pc, you r parts are melting down..
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Expert Comment

by:David Johnson, CD, MVP
ID: 41763705
I've never seen a temperature range on a component and I've been doing this since the tube days. Temperature ranges are only on the manufacturers data sheets. Always replace a component of the same value, the wattage and power rating should be equal or higher than the original item.  You can safely replace a 100K ohm 1/8W resistor with a 100K ohm 1W resistor. (it may not fit though). For diagnosing a switching power supply you really need an oscilloscope. prior to the choke/capacitor filtering you will see pulses of 12V on the 5V line which average out to 5V

You may want to view Louis Rossman's Basic Electronics Series
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by:rindi
ID: 41763714
I have aa cap in front of me and on it is printed 105°. If it is lower there is more wear and the cap will probably break sooner.
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by:IT-Expert
ID: 41763741
Rindi,

Just as a point of possible correction:

"The fan is probably meant to only start running when it is needed, ie when temperatures reach a certain threshold. That will probably only happen when there is a certain load on the unit, and when you just use the shorting bridge like you mentioned, the PSU isn't connected to the PC so the only load you would have is from the PSU itself. "

"probably"?

In my 20+ years of working on PCs, I've never once come across a PSU that only starts running "when its needed".   The fan plays a major part in keeping air flowing through the computer, and runs regardless of what the temperature is.
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Expert Comment

by:rindi
ID: 41763747
It depends on the PSU and PC type. There are cheap PSU's where that isn't the case, and also more expensive ones where it is. Particularly Manufacturers like HP and Dell etc. use such PSU's in small form-factor desktop PC's that should also be as quiet as possible. I for example have an HP PC where the PSU's fan only starts up after some time, and even then runs as slow as it can. The CPU fan usually starts up before the PSU fan.
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Author Comment

by:tel2
ID: 41766122
Thanks rindi & nobus for your comments about temperatures.

Hi Obaid,
Regarding the screen shot, I was just going to use old caps from something else, so it's not on my screen, and it's a bit hard to take a photo of it since I'd need to take the device apart again, but thanks.  I know used caps won't last as long, but they will probably last long enough for this old PC, and buying new ones is also a time user.
And thanks for that datasheet link!

Hi David,
I have a device here with a few brands of caps on it, and all of them have temperatures listed on them.
Thanks for the other tips and that YouTube series could be good!  Too bad I've already allocated points.

tel2
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Expert Comment

by:nobus
ID: 41766409
>>   I know used caps won't last as long  <<  i think you're wrong here; most good caps tend to continue infinitely; the bad ones fail because they were a bad product from the start

anyhow - you don't need to discuss this into eternity - just replace them and you're done
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Author Comment

by:tel2
ID: 41766553
Hi nobus,

I don't plan to discuss this into eternity.  In case you didn't notice, my last post was just tidying up lose ends (i.e. thanking people, responding to requests, etc).  I think that's the respectful thing to do.

However, despite my attempt to bring closure, I'm still learning things, i.e. what you just told me about the reliability of used caps.  Thanks for the tip.
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by:IT-Expert
ID: 41766597
I don't intend to discuss this into eternity either, but just for info:

David:

I've never seen a temperature range on a component and I've been doing this since the tube days. Temperature ranges are only on the manufacturers data sheets

I keep quite a few component drawers of capacitors of varying types (most not used), and have some electrolytics, WITH temperature ratings on them.  It possibly depends from manufacturer to manufacturer maybe?  But so far, it's not something I've paid much attention to tbh.
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