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Repairing SMPS

 
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Repairing SMPS

Post by Amrad » Wed Mar 05, 2014 6:51 pm

Can anyone recommend a good book about repairing small SMPS, such as those used in computers, etc.? Thanks.

Regards,

Dave.

 
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Re: Repairing SMPS

Post by rob t » Wed Mar 05, 2014 8:02 pm

the biggest problem I find (for computer psu's) is a lack of service info even circuits.
if it has not gone bang with a shorted chopper transistor then most faults tend to be electrolytes going high ERS . or secondary circuit diodes.
I don't know of any books that would be of use in the real world.
Rob T

 
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Re: Repairing SMPS

Post by Michael Watterson » Wed Mar 05, 2014 8:15 pm

Nor do I and I have been designing and repairing these evil things for a very long time. I like it when the caps are all bulging and/or bits blown off casings of semis. Gives a starting point.

They rarely ever partially work, either
Working
Dead
Blowing fuses / transistors etc

 
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Re: Repairing SMPS

Post by crustytv » Wed Mar 05, 2014 8:26 pm

ppppenguin wrote:Evingar (not sure if he's here at VRAT or on one of the other forums)


Yes he's a VRAT evinger

 
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Re: Repairing SMPS

Post by boyblue » Wed Mar 05, 2014 8:40 pm

I got very familiar with the UC3842 type of switched mode PSU used in CRT monitors, they now hold no horrors but PC type psus, as mentioned, are too cheap unless odd/rare types to be worthwhile.

Peter

 
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Re: Repairing SMPS

Post by Amrad » Wed Mar 05, 2014 11:04 pm

Thanks, everyone, for you replies.

I've not had much to do with SMPS, so I'm in somewhat unfamiliar territory! The lack of service data is a problem but if manufacturers of control ICs for them have any application data, it might help.

The SMPS in question,is not one for a PC, but is in an external HDD case that I bought at a computer fair some years ago, so getting a replacement for it is probably unlikely, even if I knew from whom I bought it.

I have seen one or two books on the net, but it's difficult to know if they'd be any use, without being able to look through them, hence my question. A recommendation would be very useful. There are also some 'guides' to repairing SMPS on the net, though I haven't found one I like, yet.

Regards,

Dave.

 
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Re: Repairing SMPS

Post by Michael Watterson » Wed Mar 05, 2014 11:21 pm

They are not likely.

Also I'd simply get another PSU (or complete case) from Hong Kong. Likely the PSU wasn't a good one in the first place.

 
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Re: Repairing SMPS

Post by Amrad » Wed Mar 05, 2014 11:42 pm

Michael Watterson wrote:They are not likely.

Sorry, what are not likely?

Also I'd simply get another PSU (or complete case) from Hong Kong. Likely the PSU wasn't a good one in the first place.

The PSU is quite small, so the difficulty would be in getting one that would fit in the case. A new case would solve the problem, but I'd like to have a go at repairing it, if only for the experience. ;) It goes against the grain to just throw it away, as it should be possible to repair it.

Regards,

Dave.

 
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Re: Repairing SMPS

Post by hamid_1 » Thu Mar 06, 2014 12:57 am

In the last decade I used to scavenge the local household waste dump for electronic items which I used to repair and sell as a hobby. I repaired dozens of faulty switch-mode power supplies in things like satellite boxes, DVD players and LCD TVs.

By far the most common fault was bulging / burst electrolytic capacitors (typically 470-1000uf 16-25v). It's very easy to spot a capacitor that's gone dome-shaped on top. Most of the time, replacing the failed capacitor(s) will get the power supply working again. It's always worth a look, before throwing it away.

If the mains fuse has blown, check the bridge rectifier, then the chopper transistor, though if the latter has gone short circuit, often the controller chip will have blown too, and repair may not be worthwhile.

Diagrams and repair manuals for modern electronics are harder to find than for vintage stuff. Sometimes the power supply will have a part number marked on it. Putting this into a search engine like Google occasionally comes up with some info. Likewise you can sometimes find replacement PSU boards on ebay. If you can't find a schematic, look at the controller chip on the power supply, then type its number followed by 'datasheet' into a search engine. The datasheets normally give a typical circuit diagram - in my experience the whole power supply is usually very much like the typical circuit given in the controller datasheet.

Finally, remember that all a power supply does is give out voltage and current. One power supply can be substituted for another. In the case of your HDD enclosure, it's almost certainly a +5v and +12v supply at about 1 amp max. If you really can't fix it but still want to use the enclosure, you can cut off the output lead from your dead PSU and wire it up to another one, as long as it provides the same voltages. Power supplies can be salvaged from other modern junk equipment. Of course, as Michael points out, these days HDD enclosures can be ordered very cheaply online. Sadly our throwaway society means it's often cheaper to order a whole new HDD enclosure from China instead of just buying a replacement power supply. But it's still fun to try and fix things!

 
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Re: Repairing SMPS

Post by Michael Watterson » Thu Mar 06, 2014 9:04 am

How much is the HDD and data worth?

 
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Re: Repairing SMPS

Post by boyblue » Thu Mar 06, 2014 10:37 am

Hi. Most smpsus have a start up resistor, a high value maybe around 2meg or so, sometimes 2 in series. Its there to give the PSU a start before the main device conducts. They often go OC. Its easily found, one end always goes to the + of the big cap after the mains bridge rec. beware of the large charge held in said cap!

Peter

 
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Re: Repairing SMPS

Post by Refugee » Thu Mar 06, 2014 1:53 pm

On older SMPSUs the start up resistor was about 300K and the chip was often equipped with a 12V zena diode.
If powered externally I would go for 15 volts and a 1K resistor to power it.
I used to repair boards that cost over £500 each and during manufacture they did not drill the heat sinks correctly. They used to go with a very big bang and I did manage to get the return rate will down by fitting factory insulated FETs and dumping all the insulating washers.

 
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Re: Repairing SMPS

Post by boyblue » Thu Mar 06, 2014 8:57 pm

Hi Dave, you,ve already got a 12v + 5V psu at hand, inside your PC. Make up an adaptor lead from an old CD drive or something and the jobs done!.
Peter

 
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Re: Repairing SMPS

Post by Amrad » Thu Mar 06, 2014 11:19 pm

Hello Hamid,


Many thanks for your very detailed reply.

hamid_1 wrote:By far the most common fault was bulging / burst electrolytic capacitors (typically 470-1000uf 16-25v). It's very easy to spot a capacitor that's gone dome-shaped on top. Most of the time, replacing the failed capacitor(s) will get the power supply working again. It's always worth a look, before throwing it away.

None of the caps are bulging, not even slightly.

If the mains fuse has blown, check the bridge rectifier, then the chopper transistor, though if the latter has gone short circuit, often the controller chip will have blown too, and repair may not be worthwhile.

When it died, the mains fuse was intact. It would run for a short time when switching on after a period of no use.

When I was checking it, there was a bang and a flash - the chopper transistor (SSS7N60B), TL3824P controller IC, one of the bridge rectifier diodes (1N4007) and the mains fuse (2A) are now dead! Luckily, I had obtained replacements for them, so I've changed them. I am now short of a zener diode of about 23V, I had removed it to check it but I can't now find it! The only markings on it were:

2
22

A search shows that it might be a GZ222. Until I get a replacement, I'm stuck.

The datasheets normally give a typical circuit diagram - in my experience the whole power supply is usually very much like the typical circuit given in the controller datasheet.

Unfortunately, all the data sheets I looked at only had block diagrams, no typical circuits.

Finally, remember that all a power supply does is give out voltage and current. One power supply can be substituted for another. In the case of your HDD enclosure, it's almost certainly a +5v and +12v supply at about 1 amp max. If you really can't fix it but still want to use the enclosure, you can cut off the output lead from your dead PSU and wire it up to another one, as long as it provides the same voltages.

The SMPS in the enclosure is quite small, so it might be difficult to fit a different one.

Power supplies can be salvaged from other modern junk equipment. Of course, as Michael points out, these days HDD enclosures can be ordered very cheaply online. Sadly our throwaway society means it's often cheaper to order a whole new HDD enclosure from China instead of just buying a replacement power supply. But it's still fun to try and fix things!

It is, and that's partly why I want to repair it.

Regards,

Dave.

 
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Re: Repairing SMPS

Post by Amrad » Thu Mar 06, 2014 11:23 pm

Hello Michael,

Michael Watterson wrote:How much is the HDD and data worth?

That's not really relevant, because, whether or not I can repair the SMPS, I can access the HDD data by fitting it inside my computer. As it happens, though, there was no HDD in the external enclosure at present, because I had removed it at the time the SMPS died.

Regards,

Dave.

 
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Re: Repairing SMPS

Post by Amrad » Thu Mar 06, 2014 11:30 pm

Hello Peter,

boyblue wrote:Hi Dave, you,ve already got a 12v + 5V psu at hand, inside your PC. Make up an adaptor lead from an old CD drive or something and the jobs done!.
Peter

That would be rather inconvenient. The external enclosure is connected to the computer by USB, so an additional cable would have to be made and some means of interfacing it to the computer and external enclosure would need to be devised. It would also mean that I wouldn't be able to connect the external HDD to a different computer, because it wouldn't have that connection to supply power to the external enclosure!

Regards,

Dave.

 
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Re: Repairing SMPS

Post by Michael Watterson » Fri Mar 07, 2014 12:01 am

Amrad wrote:Hello Michael,

Michael Watterson wrote:How much is the HDD and data worth?

That's not really relevant, because, whether or not I can repair the SMPS, I can access the HDD data by fitting it inside my computer. As it happens, though, there was no HDD in the external enclosure at present, because I had removed it at the time the SMPS died.

Regards,

Dave.


But what if there is data on drive and drive is expensive and a dodgy PSU fries the drive?

That's what I'm asking. I'd not risk my valuable data to a SMPSU I'd repaired unless I was very sure.

 
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Re: Repairing SMPS

Post by Amrad » Fri Mar 07, 2014 12:18 am

Hello Nichael,

Michael Watterson wrote:
Amrad wrote:Hello Michael,

But what if there is data on drive and drive is expensive and a dodgy PSU fries the drive?

I suppose a repaired SMPS might be less reliable than a new one but that's a risk with any electronic equipment. In any case, there is no HDD in the enclosure at present and I would take that into account when deciding to use it.

Regards,

Dave.

 
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Re: Repairing SMPS

Post by Michael Watterson » Fri Mar 07, 2014 10:56 am

It depends too why the original failed and how expert you are at repairing them.

Some designs are poor others are just using rubbish parts.


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