KB 60 resistor getting hot

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ic101
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KB 60 resistor getting hot

Post by ic101 » Wed Jan 08, 2020 2:28 am

I have an older KB 60 apart to replace some broken pots (shafts snapped off). On inspection the 22 ohm 1W resistor near the speaker out (R182) has a bit of blackening. I’m considering replacing this with a higher watt component. The amp seems to work ok, except for the pots.

Is this a common failure? Will there be side effects to upping the power handling on this part? Is the fact that it’s getting hot an indication of some other problem?

To my knowledge the speaker is original. The amp lived a few years in a very loud practice space, so all I can figure is it got hot from heavy use.
10213A02-006F-4182-9E36-ED92656F9E4F.jpeg
10213A02-006F-4182-9E36-ED92656F9E4F.jpeg (2.48 MiB) Viewed 609 times

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Enzo
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Re: KB 60 resistor getting hot

Post by Enzo » Wed Jan 08, 2020 11:19 pm

R182. it didn't burn up because it wasn't big enough, it burnt up because the amp went into oscillation.

Is it getting hot? Or does it just look like it was hot at an earlier time. The part itself does not look burnt in the photo, but it is hard to tell.

That resistor and the cap with at are your "zobel network."

ic101
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Re: KB 60 resistor getting hot

Post by ic101 » Wed Jan 08, 2020 11:53 pm

I had not considered oscillation, though it makes sense.

The resistor is reading 26 ohms, and the speaker reads 7.8 ohms. The capacitor appears unburnt, but I haven’t measured it. The circuit board is blackened and the color bands on R182 are not clear.

Once the pots get here I will put a scope on it. Hopefully everything is stable and cool.

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Enzo
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Re: KB 60 resistor getting hot

Post by Enzo » Thu Jan 09, 2020 12:38 pm

Oh, if the color bands are burnt up then definitely install a new resistor.. 22 ohm.

ic101
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Re: KB 60 resistor getting hot

Post by ic101 » Wed Jan 15, 2020 11:30 am

I replaced the R182 resistor with a 2 watt 22 Ohm. With the new pots and resistor I ran the amp a while and didn't find any run away heating. Decided to put it back together rather than poke around with the scope.

Cheers
-IC

Foxbat_25
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Re: KB 60 resistor getting hot

Post by Foxbat_25 » Wed Jan 22, 2020 8:39 am

Thanks for the little guide, I had the same issue and it's now all fine once I added a new 22 ohm resistor! Is there an issue with the original ones, or did it happen because the resistor was old? I've had my KB60 for 5-6 years.

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Enzo
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Re: KB 60 resistor getting hot

Post by Enzo » Wed Jan 22, 2020 10:11 am

The resistor is the symptom, not the problem. The resistor burns up when the amp goes unstable. That can happen for a number of reasons. usually, a new resistor is all that is needed.

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Re: KB 60 resistor getting hot

Post by Foxbat_25 » Fri Jan 24, 2020 9:37 am

Okay, thanks, I'll look for ways to keep tabs on the amp's consumption then.

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