[tradja]

I don't know how many of you might have been following the catastrophic saga of the Little Dot II tube amp group buy over at ipodstudio (http://ipastudio.com/forums/showthread.php?t=1862), involving a ~75% failure rate of the amps in the group buy. (The failures start cropping up around page 22 of that thread)

In any event, I want to upgrade the caps in my amp before they blow. I am reasonably competent with a soldering iron, but would probably just have it done by a shop here in town or at least let the caps discharge for days. The caps that are blowing are rated 330uF @ 250 volts, photos of blown caps here.

My question is, based on what's happening to these caps, what would be a good cap to replace them with? Any info is greatly appreciated!

 

[Pars]

Not knowing the exact size of these caps or whether they are snap mount (likely in this size) or not, a good bet would be something like a Panasonic TSHA 330uf 400V (Digikey P6854-ND or P6855-ND) which are 30 and 35mm diameter radial snap mounts. Not having read the whole thread, are these blowing because of overvoltage? The builder won't do anything about it?

 

[johnas]

Quote:

Originally Posted by Pars Not having read the whole thread, are these blowing because of overvoltage?

This is the key question that no one seems to know. There was some speculation that they were defective caps or heat was the issue.

The builder doesn't speak English there hasn't been any word from him as to what he thought the problem was.

Is there a way that we could tell if it was overvoltage that caused the caps to blow?

 

[Pars]

Quote:

Originally Posted by johnas This is the key question that no one seems to know. There was some speculation that they were defective caps or heat was the issue. The builder doesn't speak English there hasn't been any word from him as to what he thought the problem was. Is there a way that we could tell if it was overvoltage that caused the caps to blow?

The caps in the pic are 105 deg. C caps, which are about as good as you're going to do for electros... most (including the boutique BGs, etc.) are only 85 C. As far as telling if its overvoltage, measure it (make sure you know what your doing around high voltages though, and that your meter/leads are appropriate for this). If these are part of a tube PSU design, they very well could be over 250Vdc working or possibly a surge at start-up is the problem. Apparently the builder spoke good enough English to take your money

 

[blip]

Oooh! Those caps go off like firecrackers! But don't worry, Head-Fi's here to help!

Seriously, good advice in this thread thus far... Getting a voltage read is probably the first step towards fixing this.

Do you have a schematic for this? It would probably help. (Failing that, is it a single sided PCB? Maybe someone could trace the lines so we can get a better idea exactly what is going on here.) A description of any other known problems with these amps would help as well.

As for heat... I dunno, it might be the cause but I find it hard to believe that a high enough temp built up in a case to cause THAT to happen. 'Course I could be wrong... I've just never seen 105C caps do that!

Looks to me more like over-voltage or defective caps. High inrush would seem like a good candidate... Has anyone tried a soft-start circuit of some kind?

If you had just shown me the image, I would have assumed it was reversed polarity... But if it was, it would have gone off the first time you gave it power!

 

[HiGHFLYiN9]

The problem is the existing caps take up pretty much all of the board space, if you were to try and upgrade them to something of higher specs they probably wouldn't fit. Also I've seen an instance of caps other than the 3 big ones in the back blowing up AFAIK the builder is offering to send replacement caps out to the affected parties, so you may just want to try whatever he has because they evidently fit. If you do let me know how the process goes, I may be in the same boat at some point in time.

I ended up drilling some holes in the chassis to see if that might help heat dissipation, but it still seems like it's not enough. Probably some strategically placed heatsinks or a small 40mm fan would help matters.

 

[blip]

If some how it does wind up being heat you can always rig up a small temperature controlled fan. Very easy to do (though it will probably require a seperate power-supply). Anyway, if anyone wants them I have some linkage on that somewhere.

As for the board space problem... You might consider moving them off-board. I don't know if there is enough space for it in the case but there might be someplace they could be mounted. (Put them on a PC board and then glue it to the case... Simple and painless).