Burnt out resistor possible cause?

Thread Starter

jaybefr

Joined Aug 21, 2017
10
Hi,
I am completely new to this field. Due to illness I am no longer able to work and have decided to try to fix some of my old audio equipment as a hobby. I have just begun a audio electronics course but am impatient to try to effect my first repair. This is a vintage Sansui Receiver. When powered up there is a burning smell and a wisp of smoke. I have identified the resistor involved and am seeking to replace it but obviously don't want the same to happen to a new one. I wonder if someone can give me an idea of what may have caused it to burn out in the first place. I attach some images and a schematic. On the images, you will see the burnt ( I think its gold, black, orange, orange) resistor, some marking to the edge of the board and, underneath, there is something brown over some of the pins. Could this be the cause, or is it a symptom of the burnt resistor? Any advice would be gratefully received.
 

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Reloadron

Joined Jan 15, 2015
6,129
The resistor is getting hot and burning as a result of excessive current flow through it. So what you are seeing is a symptom, the trick is finding the cause. This is where a schematic comes in so you can identify the resistor in the big picture and determine why it would see excessive current.

Your post reminds me, I think I have an old Sansui 8000 tuner amplifier in the attic somewhere I bought around 1972. Had distortion on one channel. :) I want to think with those receivers and amps that when bought they included good schematics and documentation.

Ron
 

Ylli

Joined Nov 13, 2015
1,053
Sure would be nice if the board was labeled with Rxx numbers, and/or the schematic had resistor values. Without that it is very difficult to determine exactly which resistor that is.

In any case, that is a 33 ohm 5% resistor. Most likely, one of the transistors in the circuit with this resistor is bad. I'd unsolder/remove each transistor (one at a time) and check the junctions with an ohmmeter.

That black stuff on the back of the board is probably just toasted rosin.
 

AlbertHall

Joined Jun 4, 2014
11,525
I believe that resistor is either R826 or R828. If so, then the cause of it burning is likely to be one or both of the two transistors indicated below.
Clipboard01.jpg
 

DickCappels

Joined Aug 21, 2008
7,877
I once replaced most of the parts in a similar looking output stage. The cause of the failure that burned the parts was that the amplifier was use to play music at full volume 24 hours a day until it stopped working.
 

Thread Starter

jaybefr

Joined Aug 21, 2017
10
I once replaced most of the parts in a similar looking output stage. The cause of the failure that burned the parts was that the amplifier was use to play music at full volume 24 hours a day until it stopped working.
I think you may have a point here. I think I remember the person I got it from saying it didn't have the "oomph" to drive his speakers.
 
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