Hifonics Brutus BXi1600D intermittent and distorted sound - diyAudio
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Old 23rd March 2012, 06:47 PM   #1
Mote is offline Mote  Estonia
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Default Hifonics Brutus BXi1600D intermittent and distorted sound

I got another Brutus BXi1600D. This amp powers up fine, produces audio but often I have to turn the volume pot for it to start producing audio. Also 100 Hz sine often looks more like a sawtooth but sometimes I can see clean sine on speaker terminals.

Amp has been repaired before, PS FETs drivers have been replaced with BD139 and BD140. Waveforms on drivers look nice but on the FETs they do not look so good, is this something to be concerned about? FETs are IRFZ44Ns with 47 ohms gate resistors.

HIP4080 has been replaced, for some reason socket has been removed and HIP4080 is installed without it. Two banks of outputs have been replaced with IRF1310N, other two banks are IRF3710.

I have 10 good IRF3710s, which worked in another Brutus, left. Right now all I can think of is that those 1310Ns are causing this.
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Last edited by Mote; 23rd March 2012 at 06:51 PM.
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Old 23rd March 2012, 09:13 PM   #2
timwebb is offline timwebb  United States
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i think those transistors should be a1266
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Old 24th March 2012, 02:19 AM   #3
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The HIP4080 circuits are picky about which fets are used. I believe the IRF3710Z is the best choice here. A search for IRF3710Z might bring up some other threads.
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Old 25th March 2012, 06:31 AM   #4
Mote is offline Mote  Estonia
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Replaced the IRF1310N with IRF3710s. Do not get me wrong, I know that if I would order new parts and bought IRF3710, they would not work. Instead I used IRF3710s from another Brutus, in that amp some of the outputs had broken legs, it was for a paying customer so I replaced them all with IRF3710Zs. I did not throw out the old ones because I knew I could use them for troubleshooting in this kind of situation.

Anyway that did not solve my problem. I started to check for audio on opamps and measured DC voltage on their pins. DC voltage did not show anything unusual, each had very similar voltages. Last opamp before 330uF cap did not have any audio on any of its pins, replaced it and for now amp is working, producing clean and loud output.
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Last edited by Mote; 25th March 2012 at 06:39 AM.
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Old 25th March 2012, 02:04 PM   #5
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Is this the same amp that you were previously working on?
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Old 25th March 2012, 02:19 PM   #6
Mote is offline Mote  Estonia
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No, that was for a customer, this one I bought broken from eBay.
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Old 25th March 2012, 02:27 PM   #7
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If the outputs are swinging cleanly from rail to rail, even when the output is distorted, the class D section is likely OK.

I'm assuming that you've checked the zener and it's at about 6v, even when the signal is distorted.

Did you follow the signal through the op-amps (starting at the RCAs) to see where the signal becomes distorted?
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Old 25th March 2012, 02:42 PM   #8
Mote is offline Mote  Estonia
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Zener was OK, 6,1V across it. Waveforms on 4080 were also good.
I think there was something wrong in the preamp, and to get some signal on the speaker terminals I had to turn the gain pot very close to maximum, I think I drove it to clipping and that is why the output was not so clean. Could it be the case?

Then I replaced last opamp before 330uF cap because I did not see audio on it with a scope. After replacing the opamp amp worked fine for a while, I put it back to sink and I got to the same point where I was before: low output or distorted output.
I removed the board from sink, same story. When I crancked the volume to max for a second, then amp would start to produce loud and clean audio. After switching off and on - again low audio. Then I discovered that when I get the amp to produce loud audio and I tap with screwdrivers handle on the board (preamp section) I would lose audio. I replaced gain pot and 330uF cap, but still the same. Then I resoldered every solder pad in preamp section, and now I have not been able to hear low or distorted output anymore.
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Old 25th March 2012, 02:50 PM   #9
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That sounds like a dirty switch or an intermittent potentiometer. The plastic body pots are VERY sensitive to heat. It's almost impossible to pull one from an amp without damaging it. Did you have a new pot?

Pull one leg of the 330 ohm cap (the leg on the preamp side of the cap) out of the board. Plug one RCA plug into the RCA jack. Touch the center conductor of the other RCA plug to the pulled leg of the cap. Does the amp produce clean audio?

You may want to ground the pulled leg of the cap before touching the signal to it to drain any leaked DC off of it.
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Old 25th March 2012, 03:01 PM   #10
Mote is offline Mote  Estonia
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I pulled the pot from a amp I have for parts. I checked it with multimeter, measuring resistance while slowly turning the pot. It went nicely from 0 to 45k ohm.

For now it looks like the amp is fixed, but tomorrow I will see if I can make the clean audio go away by turning pots and tapping on them.
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