ATX PSU Repair

As noted in the HTPC build I used a repaired Ultra D0408 ATX PSU. I was using it in the P4 and it would work fine for a long time, then just start randomly rebooting, then it would be fine again. At first I thought it was the motherboard or ram, but then discovered if i bumped the desk or case it would almost always start acting up. Of course that screams "Something's Loose!".

So I had the opportunity to remove it and inspect it as I did my upgrade. Here's what I found:

Caps all look ok, no bulges, etc.

Cover Off

Be very careful when taking these apart, Voltages may remain for a very long time. I held it by the wires to remove the board, flipped it over and checked those 2 big caps (upper left). They had less than 2VDC in them, but depending on the failure it could be over 300VDC.

Bottom of board

Now we can see something, I don't know if you can see it in the above picture but in the lower right corner there is a capacitor that has come loose from the solder. It is on the 12V rail. The heatsink on the left of the unit also has a bad solder joint, but it appears to be isolated.

Bad solder joint (about in center)

So I soldered it up and will try it out. These units are cheap, so it doesn't pay to spend much time fixing them, but sometimes it's something simple like this. It's also practice for things like the Parrallax. Time will tell if that was the problem or if it's something more serious.

Parallax 7345 SMPS Repair Tips

I started this as a reply in the Parallax 7345 RV Power Converter (SMPS) post. It quickly grew to the point I decided to tidy it up and make a new post out of it. Most of the specific info is for the Parallax unit though it may apply to other SMPS's.

Most of what I know about SMPS repair comes from reading: http://www.repairfaq.org/sam/smpsfaq.htm If you intend to work on a SMPS I strongly suggest reading and digesting the contents of it first.

Possible Problem Areas

Quick Disclaimer or "Safety Third":
Be very careful! There can be potentially lethal voltages all over the board even when disconnected. At least one heatsink is live! Proceed at your own risk!

Again, be careful, It's hard to diagnose a SMPS without powering it up. Usually a isolation transformer is used. I've toyed with the idea of using an inverter or generator for this. Basically you don't want the neutral tied to ground, you want it floating, then it's only dangerous between the line and neutral rather than line and ground (that you're standing on). All household / shore  power is referenced to ground at some point.  Depending on the wiring of your generator it may or may not be. I'm not sure how to advise you on this, no matter what you do there is a risk. Try to minimize it.

Check the obvious and make sure the problem lies inside the unit.
That said, I'd follow the power through the unit as much as possible. Start where the 120v line connects, I've spent hours searching boards for a problem only to find it wasn't powered to start with. make sure you have 120v going into the board from your generator / line connection before disassembling the unit.

After you can disconnect power, let it sit for a couple hours then disassemble it. Still be careful what you touch, use the one-handed method until you get the board out. turn it over and check the voltage across the two big caps, it should have bled off, but could be over 150VDC (I don't recall the voltage spec on them, assume it's fairly close to it). If they are zero (or at least below 20V) check the continuity from each power-in line to the bridge rectifier (just after the MOV, Square thing with a heatsink) it should read almost zero ohms (if it's several thousand your problem lies here).

Always look for obvious damage,burnt components, etc. 
Sometimes a burnt component is caused by a faulty but perfectly good looking component, so don't assume you found it, you may replace it only to have it go again. (A failed triac on a Maytag Neptune washing machine will destroy a resistor, replace the resistor and it will destroy it again, replace both and you can wash clothes)


That's as far as I got on this one the problem was rather obvious, so I'm out of specific information on it. As I noted it's basically a big smps, The document at the beginning of this post should help you with the basic operation. I marked some things on the above image, you may need to check how to actually test these, but hopefully it will give you a start. As always Google is you friend, just looking for more generic SMPS info than specific Parallax info should help.

These are worth checking, roughly ordered from most likely to least (imho):

1. Bad Capacitors, look for bulged , burst or leaking caps, they cause all kinds of headaches
2. Output resistors. You can see these have been really hot, but are working fine on this one. However if one failed I'll bet the rest would go within seconds or the output would drop dramatically. They should be a low ohm value(0-1R). 
3. Switcher Mosfet(s) - handles all the power, it should switch on and off really fast (100khz to several mhz, depending on design) this is connected to one of those big heatsinks, probably the live one.
4. Logic / Startup power. I might be off the mark with this, but I think without this resistor the unit will not start. It requires some power to get thinks going, I think it comes from here. Not sure of value, but shouldn't be open and probably fairly high(100K+?).
5. Bridge rectifier - if this goes you will get low(or no) voltage on the caps, unit may not start.
6. The logic section, the most complicated part of the system. Controls the whole thing, depending what's wrong it may be un-repairable (bad microcontroller, even with a new micro you won't have the firmware) or could be something replaceable (op-amp, pwm controller, etc.) I put this last because I know of one that survived 240V on the input, so I assume it's pretty rugged.

Parallax 7345 RV Power Converter (SMPS)

See Also:
Parallax 7345 Repair Tips

A friend had a Parrallax 7345 power converter for a RV that didn't work. We know what happened, It's designed for 120VAC only and somehow got hooked up to 240VAC. So it released some of it's magic smoke. (and probably made a big BANG noise I imagine)

Basic Info:

• The big heatsinks may have 300VDC+- on them, Be careful! (obviously if it's on the heatsinks, it could be anywhere, like those 2 big caps) It doesn't just go away if it's not plugged it either.
• 45A@13.8VDC (7300 series 45A)
• 120VAC input
• Despite the fancy "converter" name it's just a big Switch Mode Power Supply (SMPS)
• Schematics are not available (Parrallax has on their page "We do not give schematics to anyone perod", or something to that effect), as with most things in the RV universe, they are expensive to replace and don't expect any help repairing them.

The Diagnosis:
It was pretty easy to locate the fault, as there was black stuff all over one spot. (Note: I'd already removed the heatsink from the bridge rectifier in the first picture and cleaned to bottom a bit when I took the second picture)

I looked it over good. It seems a MOV (metal oxide varistor) is missing (vaporized in fact), but that wouldn't prevent it from working, it just shorts the ac line together when they reach a certain voltage, with it gone it should power up, unless something else is broken. Sure enough the line filter next to it had melted a wire clean off, I didn't see it at first because the MOV had left a bunch of residue that hid it.
The other black spot on the circuit board (far left) is under the output resistors, they work fine, but must run a bit hot under prolonged load.

I bridged the broken wire on the filter, sheilded my face and plugged it in. It powered up nicely, made a moderately loud buzzing noise (no load), so I unplugged it and connected it to a 12V battery. Absolutely quiet now, but putting an amp or so into my battery. The battery is connected to an inverter I use for emergency power, so I plugged a toaster in and it jumped right to 45A. I ran a couple toaster cycles then let it fully charge my battery and all seems well.


The Repair:
I couldn't find an exact replacement for the filter, so I had to make a few mods to the board for the one I found. I had to guess on the MOV, I used a 175V MOV (datasheet said 150VAC max) I still have no idea what size would have been right, I even spent a couple hours trying to find how to choose the voltage and still didn't come up with an answer.

But a new MOV and line filter later it's back in service. The good news it the MOV did it's job, protecting all the electronics after it, it should have had a fuse in the AC line and the MOV and line filter may have survived. There was some minor damage to the traces around the line filter and MOV, but nothing too bad. The owner was instructed "Don't hook it to 240 again!", but I think he already knew that.

The other side of the board, nothing wrong here, just in case you want to see it:

Update:

I picked up a broken 55A version of this unit. As far as I can tell it's exactly the same board. I'm going to do a post and probably a video of diagnosing and repairing it, but that may be a while. 

I was going to post the correct value of the MOV, but it doesn't have one. The MOV footprint is not populated. I have the numbers off the filter, but google hasn't helped my find anything about it, so it may be a wild goose chase. In case it helps someone:

Filter:

06847475-000 843TC6

AXM EIA-17-0315-1A