Mackie 1640i with explosive PSU

[wub]

I think I've destroyed the PCB getting the rectifier out :(

I removed and tested the C6 capacitor and it measured quite a bit out of spec, should be 0.22uf and it measures at 0.16uf which is more than 20%

Because the board had been working it was probably a good sign that the rectifier was ok but I thought it would be safer to test it anyway. However, despite removing all the solder carefully it just wouldn't budge. Eventually with a lot of heat it came out, ripping all the copper from the PCB with it.

rectifier.JPG

I'm guessing this is the end of the world? Any advice would be really appreciated, on repair or where to look for a replacement PSU. I've found sellers online but nothing in stock as it's such an old mixer.

[exxos]

Really depends on if its a double or single sided PCB. I mean you can bend the legs over on a new rectifier and scratch the solder resist off and solder to the tracks. I've done that before. Harder if the tracks are on the top of the PCB but still not impossible.

[wub]

PSU topside.JPG

PSU underside.JPG

The PCB is double sided. The track for the positive leg is on the top side, the other three on the bottom side.

Could I solder the positive pin on the topside, where there is still something to solder to, and the other three legs on the bottom side? That could be a lucky escape..

[rubber_jonnie]

I think you might just get away with it, assuming the square pad on the underside matches the square + one on the top, so yes if you can solder on the top it should be fine and the rest, either bend over to make contact with the traces as exxos suggested. You did get lucky there as although the traces run both sides, they are only doing anything on one side and all you did was pull out the vias.

Might I ask how you desoldered it? With wick or a desolder gun? or just heating the individual legs?

For thick/heavy items that need a lot of heat due to the thermal mass of the item and traces, I tend to boost the temp of my iron as it can mean you spend less time on the solder as it melts quicker. It may also be lead free solder which can be a bugger to shift. Pre-heating with hot air or a warming plate is also often helpful in these scenarios.

It's worthwhile investing in something like ChipQuik in that case, which is a very low melting point solder and when mixed with other solders, it reduces the overall melting temp for easier removal.

[wub]

I think you might just get away with it, assuming the square pad on the underside matches the square + one on the top, so yes if you can solder on the top it should be fine and the rest, either bend over to make contact with the traces as exxos suggested. You did get lucky there as although the traces run both sides, they are only doing anything on one side and all you did was pull out the vias.

Yes, you have it right, the positive terminal on the topside is that one at the bottom on the other side that isn't connected to anything. I noticed it when I was taking the pictures. A ray of hope! Also, thanks to this experience I now know what a via is, I've heard the term used here but I've never been completely sure. :oops:

I desoldered using a soldering iron and wick. I have loads of flux and a flux pen so I add flux directly to the wick periodically to help it flow. I have one of those Chinese TS100 variable temp irons which can go up to 400 degrees C. It's always worked fine for me but the pins on the rectifier here are almost the same diameter as the hole so I couldn't draw the solder out as successfully as I usually do with capacitors.

I'd love to get a desoldering gun of some sort but I don't have the budget for it at the moment, it's not been a good year work-wise, but that ChipQuik stuff was mentioned on the forum here a while ago and I watched some youtube vids about it. I think I'll take your advice on that as soon as I can as it may have been the answer here.

I'm doing my best to treat this as a learning experience instead of freaking out that my beautiful mixing desk is possibly ruined! It's terrible timing when money is tight but that's always the way it goes! There's just so much to learn! I have a lot of experience with production soldering but that's far easier as you're working with brand new parts and bare PCBs. Learning to take things apart has been quite the adventure!

Thanks for taking the time, I really appreciate it. Sometimes you just need to hear someone confirm what you suspect as a sanity check :)

[exxos]

For large parts like that you need a good desoldering station unfortunately. The fallback is to cut pins so you can just heat and pull out one at a time.

Though I think you learned a valuable lesson that brute force isn't going to work. Parts should drop out of the PCB freely if all the solder is removed. Sometimes its not so easy.

The legs on a new rectifier are basically a via. Though like I said, you need to remove solder resist from the tracks and if lucky can solder blob from the legs to tracks. Otherwise use the legs to reach tracks. Failing all, resort to wires to fix it. Where ever the tracks go, you could probably use wires to link to other parts on the bottom of the PCB. But only if tracks are not salvagable.

[rubber_jonnie]

...but the pins on the rectifier here are almost the same diameter as the hole so I couldn't draw the solder out as successfully as I usually do with capacitors.

Unfortunately the only real solutions in this scenario are, as @exxos says, snip the legs and remove one at a time (Sacrifice a cheap component, not the PCB), or remove as much solder as possible with the wick, then use a heat gun to heat what's left evenly to melt it before trying to remove a component.

You just have to be careful that you don't burn the board by applying too much heat and use a lot of flux, if you can the thicker gooey stuff as the flux pen type can evaporate quite quickly being thinner so has less time to get where it needs to be.

One last note is that many Chinese soldering irons come with fine tips of low quality. If you can, get one with interchangeable tips, as a larger tip will help you when trying to get heat into areas of a larger thermal mass.

I bought a set of these as I was really struggling with my iron: Soldering iron tips as many have a very generic iron tip that works with these.

It revitalised my soldering iron, and despite their low cost have actually been an excellent set of tips that I use for many purposes.

De-soldering guns are fantastic, but they aren't the 'silver bullet', and in some scenarios don't work well, they're just a tool in the box. For many situations I still use wick, though it's starting to get expensive now.

[wub]

For large parts like that you need a good desoldering station unfortunately. The fallback is to cut pins so you can just heat and pull out one at a time.

Though I think you learned a valuable lesson that brute force isn't going to work. Parts should drop out of the PCB freely if all the solder is removed. Sometimes its not so easy.

You're right, and I feel pretty stupid for doing it now. I wish I had thought to cut the pins like you suggest, that would have been the perfect solution. The irony of all this, of course, is that the rectifier was in perfect health. The pins will be too short now if I need to bend them on to the tracks so I will need a new one anyway.

One last note is that many Chinese soldering irons come with fine tips of low quality. If you can, get one with interchangeable tips, as a larger tip will help you when trying to get heat into areas of a larger thermal mass.

The seller I bought my TS100 from let you pick which tip size you wanted with the order, and I can always get a couple of bigger sizes in the future. They are way more expensive than those you linked to though! It might be worth putting the money towards a better soldering iron compatible with tips like yours, although I do like the TS100 because it's really small and easy to manipulate. Lots to think about!

Thanks again for your advice guys, I'm a lot more hopeful that this is salvageable now! :)

[rubber_jonnie]

The seller I bought my TS100 from let you pick which tip size you wanted with the order, and I can always get a couple of bigger sizes in the future. They are way more expensive than those you linked to though! It might be worth putting the money towards a better soldering iron compatible with tips like yours, although I do like the TS100 because it's really small and easy to manipulate. Lots to think about!

Thanks again for your advice guys, I'm a lot more hopeful that this is salvageable now! :)

I think the likes of the T100 uses a tip that has the element built in, hence why they are more expensive, the items I linked to are just tips, the element is part of the iron itself. Whilst the T100 is no doubt ok for general lightweight tasks, there will come a point where you just need something heavier that can cope with a bit more.

As for the legs being short, the only one that's a problem is the +tive (You can probably solder the others as they were as they are on the underside) and you could like just use a reasonably sized wire to bridge it. I've done that sort of thing before and it works just fine.

[wub]

The parts arrived for this yesterday so I've had a chance to repair the PSU finally!

As for the legs being short, the only one that's a problem is the +tive (You can probably solder the others as they were as they are on the underside) and you could like just use a reasonably sized wire to bridge it. I've done that sort of thing before and it works just fine.

This is a good idea, a little hook joint on the leg would have been easy enough and I could have connected the wire to the pad first so the rectifier could sit as close to the board as possible. However the PCB bolts onto a metal base and then a shield goes around the whole thing so the link wire would have had to be quite long to go around the PCB length-wise. In the end I decided to do it like this:

rectifier bodge.JPG

I half shrunk the heatshrink on the other three legs so it kept itself level and by the time it was all soldered up the sleeves had shrunk down pretty evenly. I replaced the out of spec capacitor and the exploded thermistor and after re-assembly I'm very happy to say that it's working perfectly again!

:girldance:

Thanks for your help guys!