Aleph 30 post-mortem

[spzzzzkt]

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]

Burnt output board



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:

[es44]

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.
When i look at the back of these boards, the C+ strip is very very close to chassis.

My condolences
Ebbe

[spzzzzkt]

Thanks Ebbe.

Where do you think the flux bridge was? there was no flux near the mounting screw if thats what you meant?

Paul

[es44]

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

[Maousse]

Can you post pictures of your caps?
Balooned tops means dead caps usually, although they didn't exploded yet ...

[Nelson Pass]

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.

[Maousse]

Ah, yes Nelson, you are right (as usual :P) I was thinking of aluminium can pushing the plastic top, but if the metal bottom is still flat, OK, I haven't said anything

Nelson, are you always testing aging and failure causes thourously like that?

cheers,
Trung

[GRollins]

Originally posted by Nelson Pass


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.

It's possible that different manufacturers use different plastics. Some might shrink more than others.

Grey

[spzzzzkt]

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