Definately let the magic smoke out... - Page 1
Forum section: electronics / beginners

author: jmacqueen
author_rank: Contributor
timestamp: June 04, 2013, 02:42:35 am
content: Opened up my old Kenwood receiver, a while back I turned it on and it appeared some gremlin had taken up arc welding in there. Big light show, big puff of smoke.. I unplugged it and put it in the closet and plugged in an old cheap RCA receiver.Anyway, this is the damage, and it didn't even blow a fuse.. Blew a hole right out of the IC and burned a leg clean off.Question is should I try to repair it, I'm not exactly a beginner but it would be a bit over my head I think. That chip is available for about $7, I guess there is a pair of transistors in there.

author: David_AVD
author_rank: Super Contributor
timestamp: June 04, 2013, 03:08:58 am
content: My guess is the transistor went short (collector to base ?) and passed a large voltage (current) through that resistor. You'd be looking at a new pair of output transistors as well as any parts (drivers, etc) that copped a hiding.

author: jmacqueen
author_rank: Contributor
timestamp: June 04, 2013, 04:36:14 am
content: Here's a datasheet on that part.http://www.sanken-ele.co.jp/en/prod/semicon/pdf/2sb1570e.pdfSome kind of darlington arrangement in there, partnered across a -61v rail with similar one. 160v 12a rating, I guess there is supposed to be that much current available to i to arc out if it shorted, no fuse or protection stopped it.. Glad it didn't throw it out through my speaker coils.I think I'll go ahead and pull that main amp board, clean it up, and poke around for more damaged goods. I need the practice and learning even if the old receiver isn't probably worth the effort value wise.Got a free service manual downloaded as well and it looks pretty comprehensive. Having a little trouble trying to figure out where it dumped all that current looking at the schematic.

author: poorchava
author_rank: Super Contributor
timestamp: June 04, 2013, 05:14:14 am
content: Getting that transistor may be a problem. I remember when my father's rather hi-end Kenwood amplifier blew a transistor, the genuine factory matched pair of transistors for replacement was like $300... But that was like 10 years ago. Try finding them now.

author: jmacqueen
author_rank: Contributor
timestamp: June 04, 2013, 05:39:19 am
content: Well that would be out of the question, but $14 for a pair from digikey isn't..

author: kripton2035
author_rank: Super Contributor
timestamp: June 04, 2013, 06:01:55 am
content: that's happening often in denon amplifiers.some low ohm resistor gains value with age, and when too much value the big transistor gets shorted(not in that way but the result is the same)change that transistor, and check for low ohm resistors beside, some must be cutted.

author: jmacqueen
author_rank: Contributor
timestamp: June 04, 2013, 07:08:58 am
content: Looking at the schematic it has a current sense on the transistor output and a protection circuit, that must have failed as well. I do also see a rather cooked looking transistor that might be part of one of the protection circuits as well, but I am not sure if that one should have tripped on this fault anyway.Evidently that left channel output is driven with +61v to -61v B+ and B- and the chip is rated for 12 amps. I guess that is plenty for some gremlin arc welding in there.I forgot this thing was so chunky, 130w per channel for left, right and center, 80w per channel for surrounds. I was thinking it was more like 60 per channel.I'm thinking now this won't be so bad if the damage is limited to the the circuits on the main amp board, and didn't travel up the cable to the preamp board.lets see if I can get this schematic attached, for amp board, the fried package is q17.. upper left of second page

author: jmacqueen
author_rank: Contributor
timestamp: June 05, 2013, 06:35:39 pm
content: I decided to go ahead and spend a few bucks and see if this thing will resurrect. Nothing else visibly destroyed, but I'll be checking the other components around it in the circuit.Pulled the transistor chip and the dead resistor that connect to it, looks like it might have arced the air gap to the resistor right in front of it, nice hole in the resistor as well.Violent silicon destruction fascinates me...

author: David_AVD
author_rank: Super Contributor
timestamp: June 05, 2013, 10:30:15 pm
content: When parts are burnt, it's always a good idea to trace back where that destructive current flowed from/to. One component self destructing in isolation is not all that common. There's usually something else in series with it that copped a hiding too.

author: jmacqueen
author_rank: Contributor
timestamp: June 06, 2013, 07:53:14 am
content: One leg of that chip is on a -61v rail, pretty sure that's where it came from, another leg (I believe the leg that blew half away) goes through some device I'm not sure what it is yet and on to the speaker relay with some current sensing and protection relay circuitry attached, and the third I believe brings in the preamp signal through that popped resistor, from a transistor circuit from a ribbon from I assume a preamp board.I'm going to check the couple of transistors that are in the circuit coming into it, whatever is on the other end of that ribbon cable, and then there's the current sense and protection circuit that failed to kick in apparently that I suspect will have some crispy critters in it if anything does.Would have done it today but I got a 115wpc Yamaha receiver in today for $35 with a display backlight out, and was working over a free teac 10 band equalizer A friend gave me yesterday he picked out of a dumpster, been grooving on tunes since I got it hooked up and running.

author: David_AVD
author_rank: Super Contributor
timestamp: June 06, 2013, 08:48:01 am
content: That "chip" you keep referring to is just the output transistor. I'd be checking the driver transistors as that's probably where the current flowed when that resistor burnt up.

author: jmacqueen
author_rank: Contributor
timestamp: June 12, 2013, 01:49:18 am
content: Quote from: David_AVD on June 06, 2013, 08:48:01 amOk, I called it a chip because the datasheet shows a pair of transistors on the equivalent circuit.Finally had time to get back to it, found one bad transistor upstream of that popped resistor. It seems to be an obsolete part that can be had from China maybe, but I ordered a cross referenced part instead, actually a pair so I'll just replace the one in the other channel to keep em matched.