Aleph 30 post-mortem

spzzzzkt

Disabled Account
2005-05-21 4:24 am
Melbourne
Hi All,

Just had the PSU in one of my BrianGT based Aleph30 monoblocs let out the smoke in a big way.

Initial inspect reveals I've blown 6 10,000uF Pana TSHA caps and the guts have blown out the end of one of the two alu bodied arcol resistors used for CRC filtering.

The amp has been running pretty happily for 12months so it's come as a bit of a surprise that it suffered such a major melt down now.

Any suggestions as to what is the best method for checking the rest of the supply and amp for damage etc??

edit: the damage is more extensive than I first saw. One of the 3W resistors on each output board is toasted, and one has scorched the output board quite badly. Looks like a rebuild is going to be in order.

cheers
Paul
 

spzzzzkt

Disabled Account
2005-05-21 4:24 am
Melbourne
Burnt output board

[IMGDEAD]http://www.mactrix.com.au/files/images/IMGP0206.JPG[/IMGDEAD]

On further investigation it seems like the caps are ok. The plastic trim on the tops are ballooned but the caps are actually physically intact.

The 2R arcol is toast:

[IMGDEAD]http://www.mactrix.com.au/files/images/IMGP0212.JPG[/IMGDEAD]
 
It looks as if there might have been a "bridge" of flux from the top of the resistor in picture 2.
That would have let current flow from "+C" to chassis, and slowly build up a lot of "coal", which eventually shorted for good. :hot:
When i look at the back of these boards, the C+ strip is very very close to chassis.

My condolences
Ebbe
 
Oh, i'm not sure, it was just the way the burn looked, that set my thoughts in that direction.
I thought that there could have been a bridge between the leftmost resistor, and the upper left mounting screw ( at the soldering side). But if there's no evidence of flux there, then i have no clue, sorry.

best regards
Ebbe
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
I've seen several instances of "ballooned" plastic tops on
power supply caps where the customer has operated the amp
in an enclosed cabinet or on top of another hot component.

It sees to be a case of the heat shrink around the body of the
cap shrinking further so that the little plastic disks held by the shrink warp.

The caps test good, but I replace them as a matter of course if
I see this. The first time I saw this, I tried an experiment, putting
the cap in the oven to duplicate it. At 225 deg F. the warpage
did not occur. It's possible that this occurs over longer spans,
or perhaps higher temperatures.
 

spzzzzkt

Disabled Account
2005-05-21 4:24 am
Melbourne
[IMGDEAD]http://www.mactrix.com.au/files/images/IMGP0209.JPG[/IMGDEAD]

I just noticed while editing the pic, that the dust pattern on the caps seems to indicate that the ballooning of the plastic is pretty recent. The amp covers don't have any venting - i know bad bad practice - so that is probably a contributing factor which will be rectified asap.

Paul
 
My first Zen amp shown similar behaviour :
The TSHA caps had lost their top plastic caps due to excessive heat inside the housing, so that the plastic tubing around the body has shrunk and freed the cap. It's like putting on a T-Shirt that have been over heated in the washing machine :)

It's more likely due to long term heating than the absolute temperature IMHO, since the T° inside te case was not that high, but the air was not circulating that much. BTW, the caps were still OK from the eletronic point of view.

Cheers
 

spzzzzkt

Disabled Account
2005-05-21 4:24 am
Melbourne
The caps on the other monoblock are the same and it was easier to pull the plastic discs on those caps. The metal on the top of the caps doesn't appear to be deformed at all, so it simply the plastic ballooning. The plastic discs on the dead amp push down easily so I'm sure they are ok also. Cheff your words are reassuring!!

I'm still puzzled as to the exact cause of the meltdown. The CRC resistor on the + rail obviously blew up in a big way, and one resistor on the + output board got hot enough to burn the board down to the fibre core. One resistor on the - output board appears to have overheated too, but may just be a poorly dipped example. I'm not sure what to make of this.

Anyway I guess replacing the CRC resistors and verify that the PSU functions is the next step. The rectifier diodes seem to be ok, so I'm hoping that will be fairly straight forward.

I'll upgrade the current source and output resistors the amps are on the bench. I'm thinking about using Mills MRA 1% instead of the current 5% silicone wire wounds.

cheers
Paul
 
Talking about the cause of failure, I see that you used rosin solder and also didn't cleaned the flux after soldering. That's obvious at Q11. After some time, rosin can get slightly conductive, that may explain the over current that burnt the 0R47 resistor, and BTW the power supply.

After checking your power supply (alone), may I suggest a test of the ouput IRFs one by one, out of the PCB?
 

spzzzzkt

Disabled Account
2005-05-21 4:24 am
Melbourne
I agree looking at the output boards that the rosin could well have caused problems between the legs of the IRF's. I guess I was in too much of a hurry to get the boards running when I put them together and skipped the basic house keeping. I'll give all the boards a good clean before I reapply power to either amp.

I'm in the process of pulling the IRF's at the moment and I'm progressively checking them all. I'm going to replace the existing 140's with a matched set of 240's from Tarasque as it's cheaper than buying enough to match myself!?!

I was planning to test the PSU before connecting anything to it. I'll also check the main board before applying power.

cheers
Paul
 
spzzzzkt said:
I agree looking at the output boards that the rosin could well have caused problems between the legs of the IRF's. I guess I was in too much of a hurry to get the boards running when I put them together and skipped the basic house keeping. I'll give all the boards a good clean before I reapply power to either amp.


Rosin or flux as i call it is the same in my world. That was what i was talking about.;)