MTX Thunder 4160 Repair - diyAudio
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Old 21st September 2007, 12:53 PM   #1
ECM is offline ECM  United States
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Default MTX Thunder 4160 Repair

I have here an MTX Thunder 4160 amplifier that is defective. The two power supply fets were missing, so I replaced them with 60n06 types. The resistors were burned and those will get replaced with 62 ohm parts. I have an extra 4160 so it was easy to cross reference and replace the missing/burned parts.

My question is about biasing. Two BUZ11 outputs failed which I think led to the power supply failure (FEN/FEP diodes are good). I purchased some replacement BUZ11 but they are obviously from a different batch than the originals. Will I have to rebias the outputs after replacing them? Since three channels appear to be good, can I measure the voltage drop across the large drain resistors and then adjust the repaired channel to match?
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Old 21st September 2007, 07:32 PM   #2
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Before you apply power, turn the bias control for the defective channel to the fully counter-clockwise position. If the threshold voltage for the new transistors is lower than the old ones, the original bias settings may cause excessive current flow which could cause the new FETs to fail. Clamp the transistors to the sink in case there are other problems that could cause the FETs to overheat.

If the bias setting on the other channels is high enough to read with your voltmeter, using the same settings for the new channel would be OK. If the voltage is too low across the resistors, turn the bias clockwise until you just barely see an increase in idle current (on the amp meter of your DC power supply) and then back off of the biasing just a bit.
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Old 19th November 2007, 10:55 AM   #3
ECM is offline ECM  United States
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Thanks for the info Perry.

I ran the amp up with a battery instead of my power supply since I was having issues with it.
I was able to measure the voltage across both drain resistors of the good channels and adjusted the bias on the repaired channel to match.

The voltages across the resistors on the good channels are +/- 1.4mV and the repaired channel had +/-5.5mV. With the bias pot, it was easily adjusted to +/-1.4mV as well. Now they all match.

DC offset of good channels is -24mV, 13mV, and -10mV. Repaired channel is 8mV. All referenced to ground and measured at speaker terminal.

Power supply DC rails are a solid +/-24.0 volts. I don't have a scope to check out the waveform though. Transformer didn't buzz or get hot and neither did the fets.

After everything checked out, I connected a speaker and all channels produced clear sound. Amp worked in bridged mode too. Next up is a full power operational check with a couple 6.5's and a sub.
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