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"Hot" smell from Sony STR-D715

tyttuut

tyttuut

Active Member
Several weeks ago, I replaced a couple of dead power transistors and a ceramic power resistor (it's in the output path, I forget what it's called). I turned it on for basic testing, and all seemed well, but then I noticed a hot smell from the amp. It wasn't a burning smell, just "hot."

I had a fan pointing at me, so I shut it off and tried turning on the amp again. Without the fan, the hot smell was noticeable seconds after powering it on. No smoke, just that smell. I was able to figure out that it was coming from around the front right corner of the amp, behind the volume knob. The amp seems to function normally, although I haven't tried hooking up speakers yet. I just ended up setting the unit aside for a while, because it hasn't been in active use for years.

If it's relevant, the resistor I replaced was an unusual dual-element three-pin resistor. I replaced it with two power resistors of the same value of each element in the original resistor. They're rated for the same power dissipation as the original resistor.

I just pulled the daughterboard out of the amp, nothing looks unusual there or on the main board. No discoloration, sooty skid marks, or anything like that.
 
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Do you have an infrared gun that you can use to scan the amp and see what component is becoming that hot? Also, were the transistors a like-for-like replacement of the originals?

I replaced the outputs in my Realistic STA-78 recently, and had a similar issue where I could smell hot...then found the outputs were too hot to touch! Turns out that even though they were the same model transistor that the bias settings were now too high and it was idling at 700 mA. I think they call this "hot bias". After adjusting according to the service manual the hot smell and the actual heat went away.
 
vinylisfinyl said:
Do you have an infrared gun that you can use to scan the amp and see what component is becoming that hot? Also, were the transistors a like-for-like replacement of the originals?

I replaced the outputs in my Realistic STA-78 recently, and had a similar issue where I could smell hot...then found the outputs were too hot to touch! Turns out that even though they were the same model transistor that the bias settings were now too high and it was idling at 700 mA. I think they call this "hot bias". After adjusting according to the service manual the hot smell and the actual heat went away.
Click to expand...

The transistors are identical to the originals, same brand and all. I do have an infrared thermometer, but of course I took it apart and never put it back together! I guess I'll have to fix it... :biggrin:
 
Ha, sounds like most of my equipment too. :naughty:

I do recommend trying to see what component is getting that hot, perhaps with a finger test in worst case scenario? heat like that reminds me of something drawing a lot of current which could be a short circuit or a mis-biased transistor.
 
What killed the power transistors? Perhaps the component that is getting hot now? Definitely check idle current / bias etc and check for DC at your speaker outputs.
 
Markoneswift said:
What killed the power transistors? Perhaps the component that is getting hot now? Definitely check idle current / bias etc and check for DC at your speaker outputs.
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How should I go about checking idle current and bias? The service manual is near useless and I can't find anything better.

I did check for DC voltage on the speaker outputs, all outputs are at 10-15mV.
 
tyttuut said:
How should I go about checking idle current and bias? The service manual is near useless and I can't find anything better.

I did check for DC voltage on the speaker outputs, all outputs are at 10-15mV.
Click to expand...

Looking at the complexity of that unit, you probably can't adjust anything manually so don't worry abut it.
 
Markoneswift said:
Looking at the complexity of that unit, you probably can't adjust anything manually so don't worry abut it.
Click to expand...
You're right about that, there's pretty much no opportunity for adjusting things.

Now that I've turned it on again, it just kind of smells like audio equipment does when it warms up. I'll give it a test w/ some speakers at some point.

Random tidbit: it has LaserDisc and MiniDisc inputs. Blast from the past.
 
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