Another QSC K8 troubleshooting adventure! (Long)

[Reezekeys]

I guess I cant be much of a salesman for QSC K speakers anymore, as my second one has now started acting up. A little over a year ago I started what turned into a pretty long thread about my first K8, describing an issue that turned out to be an op-amp gone bad. I am still amazed at all the time and effort some of the knowledgeable folks here spent troubleshooting and giving me advice on how to fix it, especially MIDI2XS. Thanks again for all the help.

 

Now my second one has started cutting out while I play! It comes back on a while later but as soon as I start to play through it, it cuts out again. It just shuts down all lights go off. I did some troubleshooting in my semi-scientific manner. With program material playing but the input level turned fully CCW (signal present LED is lit but no sound being output), the speaker stays on. As soon as I raise the level past a certain point it shuts down. The last time this happened, I gave the speaker a nice whack on the side and it turned back on!

 

I did some googling and as it happens I found this post on Reddit which describes my symptoms exactly, along with a possible cause:

 

I often see K and KW series boxes with cracked leads between the PCB and output transistors or large caps, this will shut off the box intermittently during peaks in the signal, it's a documented problem and the first suggested fix in the service manual.

 

Further along in the thread he add this:

 

If you're experiencing power that is intermittent with vibration than your issue is likely a cracked lead in one of the larger components. Note that the cracks in the power transistor leads can be almost impossible to see visually, you must unsolder the broken connection before the break becomes obvious. (Insert obligatory warning about working on power amps, charged caps, death, bodily harm)

 

I took the speaker apart and removed the amp board with the large caps and power transistors. Visually I see nothing that looks cracked. No broken traces, or traces coming off the PCB. Trying to wiggle or move the caps, there is no play at all that I can feel. I also wiggled the power transistors a little to see how solid their connection was to the PCB. All appears normal. Then again, I really dont understand what this poster means when he says that a crack in the power transistor lead can be almost impossible to see without unsoldering.

 

At this point my plan is to just get my soldering pencil out and heat every connection point for the power supply caps and power transistors while applying some new solder. This is as good a fishing expedition as I can come up with. If anyone else has some ideas, or can explain how exactly an invisible cracked lead might show itself, Id like to hear that. I dont think I want to go down the road I did with my first K8, just to spare everyone the drama if this turns into a long hunt for a defective component I may just bite the bullet, bring it to a QSC service center and pay for a new board. Or, the tech at QSC I talked to actually suggested I buy the entire basket assembly which is everything except the speakers and the cabinet for $400. That might make more sense than what a service center might charge for replacing the main amp board plus their labor.

 

As alway, thanks in advance for any words of advice!

 

 

 

[mate stubb]

"Cracked" in this context generally means a crack in a solder joint.

 

Given the information you have, I would do exactly the same thing you propose - resolder all the connections you can.

[Delaware Dave]

Isn't a solder issue what took the Ensoniq VFX down for the count 25 years ago?

[Bif_]

I might suggest using a low wattage soldering iron as opposed to a soldering gun so as not to potentially overheat critical components. I use a trusty SP-32.

[mate stubb]

Lead free solder. Yay.

[aronnelson]

Can you reach those board and part when mounted? Here's a stupid simple, but may work thing to try. Take chopstick and with the speaker powered on, you can poke at components (the larger ones) and maybe it will repeat the same problem when you poke at the problem component.

 

Your idea of touching up the larger leads is good! Also check that the connectors are secure. I think almost all of these type of speakers are subject to these problems. Just a little drop or hit can crack the solder joints.

[Reezekeys]

I might suggest using a low wattage soldering iron as opposed to a soldering gun so as not to potentially overheat critical components. I use a trusty SP-32.

What I have is far from soldering-station quality, but should be OK for this a Rat Shack pencil with a switch for 15 or 30 watt operation.

[Reezekeys]

Can you reach those board and part when mounted? Here's a stupid simple, but may work thing to try. Take chopstick and with the speaker powered on, you can poke at components (the larger ones) and maybe it will repeat the same problem when you poke at the problem component.

This might be doable if a little risky the connecting cables don't have much slack so it could be hard to get everything connected & powered up and still allow room for getting to the inside of the "basket" (which is used as a heatsink for the power transistors). Good idea though thanks!

[Reezekeys]

Lead free solder. Yay.

Is that what I should be using? Looking at my spool here, it's 62/36/2 rosin-core solder, says "Silver-Bearing", whatever this all means!

[mate stubb]

Lead free solder. Yay.

Is that what I should be using? Looking at my spool here, it's 62/36/2 rosin-core solder, says "Silver-Bearing", whatever this all means!

 

No no a thousand times NO! Lead free solder is the reason why electronics have become disposable. It simply does not work as reliably or for as long as leaded solder. I bought a lifetime supply of 64/40 rosin core solder before it was outlawed.

 

Use leaded solder if you can find it. Not sure if yours has lead in it.

[aronnelson]

That RS pencil will work as long as the tip is OK.

Just use lead solder, it's your equipment.

[Reezekeys]

OK I am getting it now! Thanks. I'll look for leaded solder though mate says it's now outlawed? I guess I'm out of luck then.

[Markyboard]

Deleted

[Markyboard]

RoHS applies to newly manufactured goods, not repair work. You can still buy spools for general use - check Amazon.com.

[Markyboard]

These type intermittent problems can be a bear. It may seem strange but it's much easier to troubleshoot if it's in a failed state. Just keep n mind that it MAY be a cracked lead or solder joint, but maybe not. Pushing on a component may be flexing the board enough to recreate the failure condition but may not have anything to do with that component. These type problems will give you some gray hairs- trust me.

 

I do think touching up solder joints on suspect parts connections is a good first course of action. But if you can recreate the failed condition for any length of time that's when you want to get a scope or meter on it to try to localize where the failure is occurring. Not trying to be a downer here but as I said these type problems can be a bitch. Or was it bear?

[Delaware Dave]

Lead solder

 

https://www.zoro.com/worthington-leaded-solder-dia-0062-in-1lb-334430/i/G1905461/?gclid=CK_kmoqMlc8CFZElgQodz4UExA&gclsrc=aw.ds

[Reezekeys]

Lead solder

 

https://www.zoro.com/worthington-leaded-solder-dia-0062-in-1lb-334430/i/G1905461/?gclid=CK_kmoqMlc8CFZElgQodz4UExA&gclsrc=aw.ds

I'm so far from knowledgeable on this it's not funny but you're linking to acid-core solder, which AFAIK is not for electronics work. Am I wrong?

[Reezekeys]

These type intermittent problems can be a bear. It may seem strange but it's much easier to troubleshoot if it's in a failed state. Just keep n mind that it MAY be a cracked lead or solder joint, but maybe not. Pushing on a component may be flexing the board enough to recreate the failure condition but may not have anything to do with that component. These type problems will give you some gray hairs- trust me.

Thanks Mark. I'm gonna touch up the solder joints and hope for the best. The fact that I can whack the side of the speaker and it comes up again, plus while it's on it sounds fine, seems to indicate a mechanical issue or failed (or maybe dirty?) connection. In the past I've seen cheap PC boards where the solder pad or trace lifts off the board IOW, something easily identifiable. Not this time though. My lucky streak with electronics may be coming to an end. Anyway, I've had these K8s almost seven years, so if I need to replace the amp board or bucket I'm not gonna cry too much.

[Markyboard]

Lead solder

 

https://www.zoro.com/worthington-leaded-solder-dia-0062-in-1lb-334430/i/G1905461/?gclid=CK_kmoqMlc8CFZElgQodz4UExA&gclsrc=aw.ds

I'm so far from knowledgeable on this it's not funny but you're linking to acid-core solder, which AFAIK is not for electronics work. Am I wrong?

 

No - not wrong. I'm sure Deleware Dave's intent was to steer you to this, just as an example. It was listed directly under the acid core solder.

[Reezekeys]

Right, that makes sense.

 

Q for anybody note the thermal grease on the power transistors in my picture above. Should I reapply new/more grease when reinstalling this board? The part of the metal bucket they screw into has what looks like a pretty healthy coating of the stuff too, so maybe I'm good I just don't know what best practices are here.

[Markyboard]

I wouldn't worry about it for now. When you're all done you might add some more for thermal related longevity but for short term you should be fine.

[AnalogGuy1]

Just a word of warning here.

 

I agree with all the posts that lead-free solder (the stuff that is also called RoHS-compliant) is crappy compared with old-fashioned lead aka 60/40 solder; it tends to be lumpy, and despite the low melting point of bismuth I find I need to turn up the heat on the iron much higher than for leaded solder for rework. It's known to be more brittle than leaded solder, and more prone to cracking from mechanical stress and thermal cycling.

 

And +1 on remelting the connections on the larger items that bear weight or that might get warm enough to cause thermal cycling stresses.

 

But I have a warning about adding the good old leaded solder to the new stuff. Leaded electronics solder is designed to be eutectic - that means it goes right from solid to liquid with no middle "pasty" phase. But lead-free solder is not eutectic. When you mix these together, you get a mess - tiny microfractures will form as it cools. No one knows for certain why, but the leading theory is that imperfect mixing causes regions of leaded solder to cool at different rates than the lead-free solder, pulling it while the lead-free is in that in-between phase of partial melt which is known to cause cracking.

 

So just see if you can duplicate the problem with some gentle prodding with a wood stick as already mentioned, and then remelt the joints as also mentioned, but not with extra leaded solder unless you clean off the lead-free junk first. If you do find the particular problem joint, that might be a good idea to do with a $10 solder-sucker, but you wouldn't want to do this for the whole board!

[Reezekeys]

Wow, my soldering pencil is heating up as I just read your post, AG! I was getting ready to start working on this.

 

I do have a solder-sucker so I'm going to remove the existing solder before redoing the connections. I found a youtube video last night about a QSC subwoofer (which uses the same amp module) where one of the large inductors had come off the circuit board (they're the ones at the bottom of my picture of the top of the board). Then I found a "troubleshoot note" in a QSC doc that said "Power Supply Failures: Check for broken/cracked lead of inductors L11, L12, L13." So I'll start with those, then redo the large caps and power transistors. Wishing myself some luck!!

[Markyboard]

Interesting AG1. I tend to work on much older equipment so this has never been an issue for me, but thanks for educating me on this lead free solder concern.

 

Btw, over a nice dinner would it be romantic to tell my wife she's looking rather eutectic? :idk

[Markay]

Btw, over a nice dinner would it be romantic to tell my wife she's looking rather eutectic? :idk

Hmm, is that without that middle pasty state she felt the first time she saw you?

[Reezekeys]

This thread seems to be in a state of flux.

[Reezekeys]

Progress report! I started re-doing the solder connections for the three large inductors in the power supply. Finished one, then started on the second. Surprise! I seem to have sucked the lead right out of the hole, so I guess I had a broken or cracked lead there?

 

Where it gets interesting is that I identified this inductor as L13, and I see this in a QSC doc I have:

 

f. Service Bulletin:

1. Service bulletin #KSE0001revA, Issue date: Oct 2009,

Title: K Series inductor reinforcement. L13

 

I bought these K8s in November of 2009 so it might be possible that my unit did get this reinforcement. Is that what that greyish blob is in my last pic here? In any case, it looks like QSC knew there was a problem as they wrote "1. Power supply failures: - No blown MOSFETs Q13 & Q14. Check for broken/cracked lead of inductors L11, L12, L13" right below this service bulletin.

 

I guess I will order this part from QSC, then deal with removing this inductor - the grey "blob" is rubbery, not hard but it seems like a glue-type substance. I hope it won't be that much of a bear to get this piece off.

 

 

 

[mate stubb]

Well diagnosed, sir.

[Reezekeys]

Why thank you - but it's not fixed yet!

 

And I have a part number but I'm not positive QSC will sell me just this one part.

 

[Edit - late breaking news.... the "grey blob" is not the reinforcement QSC implemented. I found this on their parts ordering page, it shows L13 held to the circuit board with a cable tie - something not on my board.]

 

[Markyboard]

Btw, over a nice dinner would it be romantic to tell my wife she's looking rather eutectic? :idk

Hmm, is that without that middle pasty state she felt the first time she saw you?

 

You know it, like melted butter ...MParkay.

 

Yes, well done Reezekeys. That inductor may be a common part that you can acquire from Mouser or equivalent. May be much quicker but QSC can advise you on this and removing the gray blob. Since its a known issue they may even be amenable to a board swap- never hurts to ask.

 

Either way glad you're getting it "ironed" out.