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Old 10th June 2012, 04:39 AM   #1
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Default Problem with Biema FW900

I am a new member of this site, i was happy when i regestered this diyAudio site... I have my Biema FW900 which is 2x1350w in 4 ohms i hook up with 2 spkr per chn. and operate to parallel mode in below clipping, after an hour the channel burn out 10 power transistor broke without any signal to protect. the 2sn1943 area of amplifier was burnout. does anybody can help me to fix this problem? thanks
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Old 10th June 2012, 06:12 AM   #2
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Hi djelectro, I've moved your post to the solid state forum in it's own thread.

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Old 10th June 2012, 04:12 PM   #3
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Looks like SOA was exceeded, and the output transistors fell victim to secondary breakdown. PNPs are the more critical transistors anyway, and well-performing clones are not as common as with NPNs. Plus, even nominal "8 ohm" loudspeakers have stuff like 3 ohm impedance minima sometimes.

It would be interesting which manufacturer / brand of 5200/1943 was used in this amp. Cheap fakes would explain a lot. (This is an amp advertised for exceptional value.)

Otherwise all you can do for now is poke around with a multimeter, find out which transistors are dead short or do not pass a diode test, note them down and look out for replacements.

It would also be interesting to know how high the supply voltages are. (What type are the big power supply electrolytics?) 10 power transistors per channel may seem like a lot (that's probably 1 pair driving 4 pairs), but 5200/1943 don't have great SOA at high voltages and a 1350 W / 4R amp would definitely need those - more than +/-100 V. Even with genuine / well-performing parts, that configuration does not seem generous. You get what you pay for, I guess.

OK, so it's a Class G/H amp, presumably with something like +/- 55 V and +/-110 V rails. That seems more doable with 5200/1943s, but you'd still want 4 pairs x2 per channel if it's supposed to be bulletproof... 2 pairs is generally doable though.

Last edited by sgrossklass; 10th June 2012 at 04:25 PM.
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