Sansui 3900z caused a speaker to catch on fire!

[mcgrawver]

I just purchased it from an estate sale and hooked it up to my Marantz 7 speakers tonight with just the fm radio playing. The left speaker was humming so I switched speaker wires to the other channel and started seeing smoke from the left speaker. I turned it off right away but the speaker burst into flame before I could get water on it. Just as I was turning it off a tiny bit of smoke came out of the top of the receiver.

I took the cover off and nothing looks out of the ordinary and it was relatively clean inside. I'm definitely not an expert though. I really like the look of the receiver and everything worked great up until that point. Lights, controls ect.

Any ideas about this and is it worth getting serviced? Much appreciated!

 

[Sonytubes]

I wonder whether that is a class A receiver?
They run hot.

 

[Sansuiman]

Probably put excessive DC on the speaker output and overheated a voice coil. These smaller Z series models (3900-5900Z) typically have no speaker protection or DC voltage detection circuitry present. I've seen a couple that did have a factory fitted "crowbar" circuit that may have been a later production addition, but that was very rudimentary and not particularly effective, from what I could tell. Most units didn't even have that, so in the event of an output stage failure, any attached speakers are likely to get smoked.

 

[mcgrawver]

Thank you for your reply! Sounds like it's not worth getting serviced then? I really liked the look of it!

 

[Sansuiman]

These aren't Sansui's best gear, they show the effects of cost cutting that went on in the early 80's. So, whether or not it's worth fixing is kind of subjective. The 3900Z (as the lowest model in the series) is fairly low power, and lacks the digital power meters present on the 4909Z (and higher models) but is still a nice little unit when working. They are nice looking receivers, and I do like the styling. I don't know that I'd put much money into repairing it, as the cost is likely to be nearly as much as buying another one in working condition, but again, that's a judgement call. If you can repair it yourself it's worthwhile. If paying a tech to do so, perhaps not so much. And you still have a damaged speaker to repair to replace, as well.

 

[EastPoint]

Shorted outputs can lead to a receiver or amplifier spitting DC voltage out of the speaker terminals. It's important to test equipment carefully before you hook it up to your speakers, or you risk damaging them. This can lead to actual fires, though it's rare. Some equipment has a protection circuit to prevent this, but not everything, and sometimes the protection circuit can have problems too.