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Old 12th September 2006, 01:10 AM   #21
vypr is offline vypr  United States
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ok it seems like that resistor goes from the positive on the speaker output then it is connected to a tiny blue cap then it goes to negative....i put the ohmmeter across that resistor on the other side it read 5.15 ohms, and yes i have one of those magnifying glasses. ill check the other bias pot now too you want want me to turn it all the way counter clockwise too?
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Old 12th September 2006, 01:22 AM   #22
vypr is offline vypr  United States
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its a yellow violet gold thats 4.7 ohm if im not mistaken
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Old 12th September 2006, 01:37 AM   #23
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You're correct. 4.7 ohms.

If it's in series with a small capacitor and they're connected across the output, it's a zobel network. Unless the capacitor is shorted, the low resistance should not be causing any problems. Of course, it's out of tolerance and should be replaced.
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Old 15th September 2006, 12:11 AM   #24
vypr is offline vypr  United States
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ok i think i have isolated the problem i found an open mpsa06 that im going to replace, when i get this amp fixed perry how do i readjust the bias pots
Thanks for all your help it is greatly appreciated espically perrys help
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Old 15th September 2006, 10:23 AM   #25
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In a class AB amp, the biasing is used to prevent distortion as the signal is passed from one half of the outputs to the other half of the outputs. This is done by setting the outputs just at their threshold (where they are just begining to pass current). When you have the proper specs, you generally use the voltage drop across the emitter/source resistors as an indication of current flow. 1mv (0.001v) across the emitter/source resistors is generally a good starting point. Maybe 'amplifiers inc' can provide the exact factory specs.

If you don't have the proper specs, you can get very close by increasing the biasing until you see just a 'tiny' increase in current drawn from the power supply. If you set it too high, the amp will run hot. If you set it too low, the output may be distorted. You need to have the amplifier mounted back in the heatsink when you try to adjust the biasing. If the transistors are not clamped down and the bias current is set too high, the outputs will overheat and fail within seconds.
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Old 15th September 2006, 04:40 PM   #26
vypr is offline vypr  United States
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ok thank you perry, the left channel is set just a little bit less then the right, what i did was rotated the pot slowly till i saw that the current rose about an amp then backed it off a very small amount
it is idling at about 1 amp or close to that. I have a 4 ohm 200watt dummy resistor, sine wave generator and a scope im going to test it fully tonight.

Thanks alot for your help,
Mike
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Old 15th September 2006, 08:10 PM   #27
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A sine wave is good to check for distortion but not always good for long term testing. Many amplifiers, brand new, out of the box, will fail if driven with a sine wave to full power into their toughest rated load. The outputs overheat and due to poor design the thermal is too far from the outputs to protect the amp from failure. This amp (especially if it has the metallic insulators) may survive that type of test but you should be careful.

Dummy loads are much tougher to drive than speakers because they are a constant load. When driving speakers, the inductance of the voice coils and other properties cause them to have a higher impedance at various frequencies. This lightens the load for the amp.

When using music the output varies continuously so, even if you're driving the amp to clipping on the peaks, there are periods where the amp is less stressed due to the lower output.

If you're going to test the amp into a 4 ohm mono load, I'd suggest using music. Set the level to where it just clips on the peaks. If you have a 4 ohm load that can handle only 200 watts continuously, they may be OK at full power with music but you should monitor their temperature. A fan on the dummy loads can keep their temperature down and allow them to handle more than their rated power.

If your power supply and dummy loads are up to it, I'd suggest driving the amp until it goes into thermal protection. Leave everything as is and allow it to come out of thermal at full power. If it survives that, it's going to be reliable.
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Old 15th September 2006, 10:28 PM   #28
vypr is offline vypr  United States
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ok i have been testing the dummy load on the one channel(the left channel) im waiting on my friend to bring me a cd player with rca audio outs, i have a 30 amp power supply, i hook the dummy resistor up with a 60hz sine wave i can draw every bit of 30 amps before clipping and 30 volts ac, ok heres the problem when i bridge it at 4 ohm mono i only get 15 volts ac to the dummy load but still draw 30 amps off the power supply why is that is it because of the biasing?
thanks
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Old 15th September 2006, 10:48 PM   #29
vypr is offline vypr  United States
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oh i forgot to mention the right channel works fine also 30 volts ac to the dummy load with 30 amps drawn off the power supply
im confused because when you bridge this amplifier the channels are in series when bridged the amplifier gets warm really quick also, it is still showing a clean sine wave on the scope just not alot of power, if i remember correctly to find power output from the amp with just resistance and voltage the formula is voltage(squared) (divided by) resistance when i run it in single channel to 4 ohms at 30 volts out thats 225 watts and bridged i get 112 watts almost exactly half please help if you can
thanks
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Old 15th September 2006, 11:19 PM   #30
vypr is offline vypr  United States
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my friend forgot to mention to me that he was running it in bridged mode when it failed so i bet its the mosfets on the other channel they are probally ready to fail so im gonna replace them also
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