Amp overheating - thoughts?

[howlindawg]

Had a strange morning here in the kennel.

Started the morning as I usually do browsing emails and the forums, coffee in hand, before the days work begins in earnest. I mosly work from home y'see.

After reading a post in the gallery I was motivated to connect up my spare speakers, a pair of aurum Cantus Leisure 2SE. These speakers have been my mains in the past but were loaned to a friend for a while and have been sitting idle ever since.

I had just got everything connected up when the phone rang so I turned the amp down to a background listening level and went to take the call. I got distracted and completely forgot the ACs were hooked up until I went into the kitchen to make a fresh pot of coffee (I'm a total caffeine addict!). It was only then that I noticed a distortion to the sound and an odd smell.

Closer inspection revealed the tweeters distorting badly at all but minimum volume levels and a VERY hot amplifier (MF X-A2 which normally runs quite cool). We're not talking burn your hands Class-A hot but uncomfortable to touch nonetheless.

I immediately turned everything off, checked all connections, swapped back to may main speakers and gingerly powered back up... Thankfully normal service was resumed so I powered everything off again and left the amp to cool fully.

So now the conumdrum... Where does the fault lie.
Both speakers are affected and the distortion only appears through the (ribbon) tweeters.
These speakers worked fine with this amp in the past.
The friend that borrowed the speakers didn't mention any problem with the speakers when he returned them.
The speakers have been idle and unused for at least 12 months.

I won't get a chance to open them up to check the crossovers until next week at the earliest but in the meantime, any thoughts as to what the fault is likely to be?

[StanleyB]

The first thing to do is to check the amp on another set of speakers just to make sure the amp is working OK. Try it at low and normal level.
The heat generated is very likely from a low load at the output. That could mean a partially short on the speakers. You might be able to check that with a multimeter on the Ohms range across the speaker terminals. Both left and right side should be the same within 0.5 Ohm if it is an 8 Ohms speaker. If they are different by more than than that amount, you'll have to measure the individual drive units at the point where the are connected to the crossovers.

IF you are lucky it could be just a blown cap in one of the cross-overs.

[Dr Bunsen Honeydew]

Sounds like the amp has latched up (look it up) and gone unstable. Ribbons can give a virtual extreme HF short circuit due to capacitance in their load. The amp obviously doesn't like it.

[howlindawg]

Thanks for responding gents.
The amp is now working fine with my regular speakers, no distortion and cool to the touch.

I just went out to check the speakers with a DMM and can report that the root cause was an IO (Idiot Operator) error.

When my friend was using them he used short bare wire speaker cables running hidden through the speaker stands. These were still in place but I didn't notice them as they were tucked up inside the stands and my cables use bananas. :doh:

I'm relieved but feeling just a little dim. :o

[Yoga]

...the root cause was an IO (Idiot Operator) error

Marvellous! :lol:

[howlindawg]

Well at lunch time I decided to open the Amp to ensure that all was well inside (remember that smell I mentioned in the OP?).

So this is what I found
http://img714.imageshack.us/img714/1262/capsfb.jpg (http://imageshack.us/photo/my-images/714/capsfb.jpg/)

Oh well, I'd been meaning to upgrade them at somepoint anyway.
Ho Hum. :rolleyes:

At this point they've been removed from the PCB and all traces of electrolyte cleaned from the board using IPA.

Now need to source some replacements.
Any suggestions?

[Alex_UK]

Unlucky Martin. :( Still at least you're seeing the positive in the upgrade potential! Good luck with that. And don't worry about the "IO error" - I've done far worse, I can assure you!

[howlindawg]

Unlucky Martin. :( Still at least you're seeing the positive in the upgrade potential! Good luck with that. And don't worry about the "IO error" - I've done far worse, I can assure you!

It's not too bad Alex. Thankfully I still have my DIY power amp and am using the Caimen as a pre for the moment. (What a versatile little box it is.)

I'm not too mad at myself for the IO error. The cables my mate had left in place were exiting from the bottom of the binding posts and were then stuffed into the insides of the metal stands. They can't be seen at all from above.

Replacement caps ordered so should be back in business by the weekend and lesson learned. :)