Hot issue with Adcom GFA-555 MKI

[Ahaudio]

Hi guys.

I pulled my Adcom GFA-555 out to play with and noticed a funny smell when it was on. I popped the cover and noticed the PCB on top of one of the caps looks burnt. I measured the temp from the top of the board at 160f. All the others were around 105f. Specifically it is the resistor running to the power LED that is running hot. If I disconnect it the temperature is normal. I swapped in another LED of questionable pedigree and the temp went back up.

Voltage at the main caps is 82v. Bias is 13.3mv cold. DC offset is .005v. It sounds fine.

Is it normal for this resistor to run hot or do I have a problem?

Thanks.

Ashley...

 

[indianajo]

81 v / 3.9 kohm is 20 ma. About right for a power LED. That is 1.68 W on a 2 watt resistor. That would run pretty hot. If you want is to run colder, replace with a 3 or 5 watt resistor. wirewound is okay. The parts list says this one is metal film which is the quietest, but the power LED does not affect the sound.
One of the reasons Adcom is worth less used than a Peavey Crown or QSC, they used paper boards. The others used FR10.

 

[Ahaudio]

Thanks for the suggestion I'll give that a try.

 

[Ahaudio]

I stuck a five watt resistor in there. Still runs just as hot. LED seems brighter. I placed on top of the board instead of underneath where it originally was so I guess it won't continue to cook the board. I think I am going to just run it and see what happens.

 

[akimmet]

You can consider increasing the value of the resistor. A modern LED is already too bright at 2mA let alone 20mA. Try 47K 2W out.

 

[Ahaudio]

Won't that run even hotter? I'm having a hard time accepting that this resistor generates so much heat by design just to illuminate a panel LED.

Thanks for the reply. I do appreciate it.

 

[DCE1198]

That resistor is always toasty in the original 555.

Dan

 

[akimmet]

The higher resistance value would decrease the current flowing through it. Less current with the same voltage drop equals less power dissipation.

 

[Ahaudio]

Got it, akimment. It turns out the brightness was a figment of my imagination. And thank you, Dan. That's what I needed to hear. I left the 5 watt resistor on top and buttoned it up. I am now enjoying listening to it.

Thanks everyone for your input.