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biggsd  
#1 Posted : Saturday, June 27, 2015 8:28:07 AM(UTC)
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Hi there all

Ok so bit of a long story here, but I have:

2 x Buffalo SE with Placid PSUs (both in 1 case), 160W Transformers. Legatos. R1-R4 on the legatos has bene set to 187 ohms to get a little more output voltage (as the spdif signal i get in one setup scenario is to low level as attenuated at the source)

So, I was having problems with the Bridge Rectifier blowing up on the old Placed BP units. I then purchased 2 x new units (version 2.1) and replaced the Legato where this was happening. Its been perfectly stable for the last 12 months.

Now, last weekend at a gig the DAC apparently had loud white noise, no solution was found and it was bypassed. (i was not there)
Turned on now at home and the Placied BP Bridge Rectifier Blew up again on the Negative Rail for the second DAC when turning on. This was the DAC being used last weekend where the issue happened.

So, i really need to understand what can cause the rectifier to blow up.. is it overloading? Can the legato when run extremely hard draw too much current? In the case where it happened it is quite likely that it was being pushed pretty hard... I don't want to just replace the rectifier without some understanding of root cause, as its fairly inconvenient during an event (I use the DAC to run a PA system).

I was not there unfortunately, but in the setup where the issue happened just one of the two DACs in the case was being used, and that is the one where the issue happened.

Need to know
1. What parts should i replace on the Placid 2.1 to ensure that its fixed - i.e. just the rectifier? Or could it have caused downstream damage?
2. Keen for some understanding on what causes overloading.. is it the bigger 160W transformers? they sound SOOO much better so keen to make it as robust as possible.. What parts can i upgrade to make more robust

Really need some help as it makes my pa sound incredible!

avr300  
#2 Posted : Saturday, June 27, 2015 3:16:59 PM(UTC)
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Using a shunt power supply like the Placid first thing to remember/understand is, that it's running "class-a". No matter what the consumer (the Legato) does, the current passing the power supply is constant. Either the current is used by the consumer or the current is wasted into heat. That's the nature of a shunt supply.

(Derived from this, it is easy to understand why the shunt supply has a by nature built-in short circuit protection)

Again:
"No matter what the consumer does, the current passing the power supply is constant."

So, rule out the consumer, the Legato.


Back to the Placid, no matter how big tranny you put on, remember the capacity of the transformer is the ability to deliver the VA. If the consumer (here the Placid) doesn't demand the power, the tranny doesn't deliver. As long as you keep the AC voltage at the specified value you're safe.

The first thing that came to my attention was heat.

How is your implementation? Can you show a picture?


(Here I have quite a few Placids (all sort of versions). I never had a blown rectifier, though they do tend to get hot)
biggsd  
#3 Posted : Thursday, August 6, 2015 12:20:02 PM(UTC)
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Hey there

Been super busy so didnt get a chance to reply. Thanks for your help.

See Implementation here - please note I have connected the working PSU to the other legato here simply to ensure all working ok - and it is.

https://static1.squaresp...=1500w&storage=local

So, anyway if its assumed that its not the load or the tranny, could it be heat? I have fans running on the case but I could upgrade them.

Also - now I have a blown rectifier (and likely further damage down the PSU) what parts should I replace? I cant fire it up because it just shorts and blows fuses. Can you make a recommendation?

Also note that when i have had this fault with the old boards (not placid 2.1) the fault only happened when a legato was connected, not when there was no load. Historically i would replace the rectifier, turn it on with no load and it was fine. Then connect a load and bang it would blow up.

So, it could be related to the parts that regulate the shunt current.. i dunno... keen for some help though :-)


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biggsd  
#4 Posted : Saturday, August 8, 2015 12:58:47 AM(UTC)
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OK so i purchased a new Placid board and its installed all working perfectly.

One thing I have noticed is that the negative side of the legato draws 20ma more than the positive, is this normal?
Hence the negative side of the Placid is set higher in terms of CCS, but the shunti is still 50ma

So, really keen to fix the failed board so I have a backup, and understand WHY this is happening. Plus ensure it doesnt happen again.

I have attached pics of the underside where is shorted and a more hi res pic of the implementation.

If someone can help me with:
- What parts to test / replace on the failed board
- Any other things I can do to the implementation. I have fans on the case but i was thinking to upgrade them.
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biggsd  
#5 Posted : Saturday, August 8, 2015 1:01:16 AM(UTC)
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Here is the underside of the board
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avr300  
#6 Posted : Saturday, August 8, 2015 6:21:49 AM(UTC)
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Yes it normal the negative half draws more current than the positive.

I would start by measuring the temperature on the surface of the rectifier when the DAC is operational.


Regarding the blown - start by changing the most obvious - the rectifier.
biggsd  
#7 Posted : Tuesday, August 25, 2015 12:03:05 PM(UTC)
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PLEASE HELP - its happened again!

So, I had the issue I described earlier, and I COMPLETELY REPLACED the Placid board with a brand new one, all working perfectly at home, no problem. Note that I replace the PLACID board but the legato and DAC still the same on that channel.

Now I took the DAC into the venue where the event was, turned it on and BANG rectifier blew up again, brand new supply smoked.GRR.

It was a perfectly new and hardly used board, with Placid 2.1 so it was the newest one. the only thing that is the same is the Tranny, the Legato and the digital board. Rule out the digital board and we are down to the tranny and the legato on either side of the Placid.

Like another user has said, when I have seen this fault in the past it only occurs when a legato is connected. Is there ANY chance that the legato might have been overloaded and overheated before, and then something failed in it or is now a bid dodgy that is intermittently shorting the output of the Placid when it turns on? Causing the inrush current to be too much when its turned on and thereby smoking the rectifier?

I really need to fix this this week and I can replace the rectifier but Im afraid it wont fix the problem, please help! I will do anything to get this fixed this week as desperately need it working for my event on Friday.

Thanks!




biggsd  
#8 Posted : Tuesday, August 25, 2015 12:29:05 PM(UTC)
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Trying to troubleshoot an issue but keen to get a response on exactly what the impact of a placid BP overheating is...

What parts are likely to fail? Could this cause shorts anywhere?
Brian Donegan  
#9 Posted : Tuesday, August 25, 2015 1:06:58 PM(UTC)
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This is only happening on one of the two Placid HD BPs? The other Legato/Placid are working fine?

Is the venue you mentioned the same place where this keeps happening?

There are two possible things that will kill a rectifier like that: over-voltage fro the transformer (doubtful) or excessive current draw (a short). "In rush" current is not your problem. There is not enough capacitance on the Legato to cause that sort of problem. It's more likely a plain old intermittent short, either in the wiring/soldering, or a bad part, such as a cap or transistor.

I would install a fuse between the transformer and the Placid to protect it moving forward. Something like a 1A slow-blow. Typically this is not needed, but it will save you from killing boards while you track this down.
Brian Donegan  
#10 Posted : Tuesday, August 25, 2015 3:44:53 PM(UTC)
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Quote:
Trying to troubleshoot an issue but keen to get a response on exactly what the impact of a placid BP overheating is...

What parts are likely to fail? Could this cause shorts anywhere?


QN1/2 and QP1/2 will be the parts to overheat. If they fail, they will fail permanently, probably letting out smoke. This is speculation, and depends on other factors, such as the failure mode causing the heat). I have never smoked one, so I do not know for sure.

Edited by user Tuesday, August 25, 2015 4:57:08 PM(UTC)  | Reason: Not specified

biggsd  
#11 Posted : Tuesday, August 25, 2015 9:18:42 PM(UTC)
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Yes I assumed it could have been QP1/QP2, but when i have seen the fault its only the rectifier that blows. Yes thats correct there is another whole Placid/legato assembly working perfectly off the same power source.

In looking at the underside of the board, on the output of the Placid between - and gnd its looks like it has heated quite a bit.. saying that possibly a lot of current has gone through the output on the side that blew up. the positive side doesn't have this. So, therefore, is it possible that the legato now has some kind of short/faulty part that could be shorting the output of the placid, thereby blowing the rectifier due to over current?

That is the big question because if that possibly the case, i will look at the legato

Good idea about the fuse, that will save me damaging boards... :-)
biggsd  
#12 Posted : Tuesday, August 25, 2015 9:22:00 PM(UTC)
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Yes I assumed it could have been QP1/QP2, but when i have seen the fault its only the rectifier that blows. Yes thats correct there is another whole Placid/legato assembly working perfectly off the same power source.

In looking at the underside of the board, on the output of the Placid between - and gnd its looks like it has heated quite a bit.. saying that possibly a lot of current has gone through the output on the side that blew up. the positive side doesn't have this. So, therefore, is it possible that the legato now has some kind of short/faulty part that could be shorting the output of the placid, thereby blowing the rectifier due to over current?

That is the big question because if that possibly the case, i will look at the legato

Good idea about the fuse, that will save me damaging boards... :-)
Russ White  
#13 Posted : Tuesday, August 25, 2015 9:27:22 PM(UTC)
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Legato is current limited by the CCS part of the circuit - so the only way the output could have seen a lot of current would have been if the CCS portion was somehow shorted or failed in a short.

Unfortunately if that happened you may have lots of dead actives.. :(

Sorry
Russ
biggsd  
#14 Posted : Wednesday, January 13, 2016 10:49:06 AM(UTC)
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Hi Team just thought i would share the issue was simply the rectifier as i was often plugging into a 32a supply. So put some big grunty rectifiers in and no more issues
biggsd  
#15 Posted : Tuesday, May 21, 2019 5:31:26 AM(UTC)
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Hi there everyone!

So i now have a new problem that is related to the first problem.

I have upgraded all the rectifiers and it has lasted a good 2 years .,... then POP.

Both boards in close to the same period. Smoke and high current. Seems to always be on the Plus side.

Got it out and put on some bench current limited supplies with 20v DC - so discounting the rectifiers now and we are past them.

Tests as follows:
Note: these issues only on positive side

Tests as follows (all is for plus side, negative rail is all good)

NO LOAD - Draws normal 250ma current, 15v DC - so all looks ok
ADD LEGATO LOAD - Voltage drops to 10v and
Then it tried to draw a lot of current 30 seconds later - supplies limited it at 300ma
NOW:
total current draw is 80ma.
0 voltage

So i now have a clearly faulted board. Other board had exactly the same fault.

I really want to figure out what parts are failing.

So far:
Replaced QN1 and QN2 - no change 0 - still 80ma current and no voltage. So its definitely not heat.

Keen to understand what else i can test, and i can also replace QP3-5 next.

Any other ideas as to what might have caused fault, or most loaded part?

Remember i have large trannies and massive rectifiers so its the next part in the chain that takes current when inrush happens on switch on.

Keen for thoughts and feedback.

Brian Donegan  
#16 Posted : Tuesday, May 21, 2019 1:49:11 PM(UTC)
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I think we have talked about this before, but I can't remember now...

What transformers are you using, and how do you have them wired to the Placids?

biggsd  
#17 Posted : Tuesday, May 21, 2019 7:30:15 PM(UTC)
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They are 80VA transformers. Why? Because it really makes a big difference to the low end in the legato.

Same circuit as per design but much larger rectifiers.

Transformers -> Rectifiers -> Placid

Just jumpered the inputs there the small rectifiers were. rectifiers supplying 20v DC.

this design has worked no problem for 2 years.

Keen to figure out what may have happened and what parts under the most stress after the recftifiers. so can upgrade. Happy to just have spares available if it happens again.

Thanks Brian, i also have some spare QP3-QP5 - worth trying those next? Or any measurements i could take?




Brian Donegan  
#18 Posted : Tuesday, May 21, 2019 7:34:47 PM(UTC)
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I was looking for specifics on the transformer secondaries and their connection to the Placid, just to rule it out as a possible cause of the issue.

Something like this:

UserPostedImage

Black/Red to positive side, Orange/Yellow to negative side.






.

Edited by user Tuesday, May 21, 2019 7:37:54 PM(UTC)  | Reason: Not specified

biggsd  
#19 Posted : Tuesday, May 21, 2019 8:13:12 PM(UTC)
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Hi Brian

it is definitely black/red on positive side. That’s where the fault originally occurred. Orange yellow on negative.

Again it was running perfect for 2 years with daily switch on / off - so maybe a part fatigued?

Right now replaced the placids with new ones and all working.

But reallly keen to fix these broken ones.





Brian Donegan  
#20 Posted : Tuesday, May 21, 2019 8:48:21 PM(UTC)
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Are you grounding anything to the case?
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