Resistors for USAMPS USA-400

smithc1986 · 8th October 2010, 03:40 AM

Im not sure if this is the right place to post but here it goes...

I have a 1999 US Amps USA-400. Two resistors are blown. I think they are carbon compositions, but im not sure. Im pretty sure the blown ones are the same as the two peach colored ones in the second pic. My question is what do I need to replace these with? And also do I need to replace the other two also?

Sorry for the poor picture quality. If it will help I can try to find a better camera.

Damon Hill · 8th October 2010, 05:32 AM

Those appear to be carbon film, not composition. Looks like at least 1 watt, if not two. I can't clearly make out the color bands; guessing at 21 ohms.

Metal oxide resistors at 2 watts would seem like a suitable replacement. The unburnt ones should be fine.

Resistors don't fail by themselves; they're a symptom of failures elsewhere.

smithc1986 · 8th October 2010, 12:33 PM

Any idea on what caused this to happen? Im pretty sure one of them has been blown since I bought the amp 5yrs ago. The other one just recently went up.

Damon Hill · 8th October 2010, 08:03 PM

Hard to say without a schematic; my first guess would be the main switching transistors. There could easily be more damaged semiconductors.

oddiofile · 12th October 2010, 06:33 PM

I have a few of those amps. I havent run into the problem personally, but could you let me know what impedance you were running and for what duration? Did you find that the amp was running hot ever?

smithc1986 · 21st November 2010, 04:58 PM

I have done some research and discovered those are bias resistors. Should i use cf or mf resistors?

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8th October 2010, 05:32 AM	#2
Damon Hill diyAudio Member Join Date: Mar 2001 Location: Auburn, WA (somewhere between Seattle and Tacoma) Blog Entries: 1	Those appear to be carbon film, not composition. Looks like at least 1 watt, if not two. I can't clearly make out the color bands; guessing at 21 ohms. Metal oxide resistors at 2 watts would seem like a suitable replacement. The unburnt ones should be fine. Resistors don't fail by themselves; they're a symptom of failures elsewhere.

8th October 2010, 12:33 PM	#3
smithc1986 diyAudio Member Join Date: Oct 2010	Any idea on what caused this to happen? Im pretty sure one of them has been blown since I bought the amp 5yrs ago. The other one just recently went up.

8th October 2010, 08:03 PM	#4
Damon Hill diyAudio Member Join Date: Mar 2001 Location: Auburn, WA (somewhere between Seattle and Tacoma) Blog Entries: 1	Hard to say without a schematic; my first guess would be the main switching transistors. There could easily be more damaged semiconductors.

12th October 2010, 06:33 PM	#5
oddiofile diyAudio Member Join Date: Oct 2010	I have a few of those amps. I havent run into the problem personally, but could you let me know what impedance you were running and for what duration? Did you find that the amp was running hot ever?

21st November 2010, 04:58 PM	#6
smithc1986 diyAudio Member Join Date: Oct 2010	I have done some research and discovered those are bias resistors. Should i use cf or mf resistors?

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