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Old 22nd July 2012, 12:24 AM   #1
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Default Neg rail voltage on two pads?

I'm working on my Memphis MCD4KW...

I have all the outputs removed. When I power up the amp and have a 4 ohm speaker attached no signal to amp there is a light hummm noise coming from the speaker.

I have 118.4VDC solid on the + rail (drain on the 360's) and no voltage on either of the other pads...

I have between -114.9VDC and -114.4VDC on both the gate and source pads that fluctuates roughly .5v up and down .1v at a time.

Now is it just me... or is something wrong here? I should have something around 5v on one pad too right? The power supply voltage was 3.??v. I finally got the output transistors, but there seems like there is going to be more to this.... don't wanna put them in and have them pop.

Amp powers up fine, no excessive draw, even with remote power. No protection light.
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Old 22nd July 2012, 12:48 AM   #2
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With no outputs installed, drive a strong 100Hz signal into the amp. Measure the DC voltage for both the high and low side FETs with the black probe on the 3rd leg and the red probe on the first leg.

Do you read ~5v on both the high and low side?

If you read it only on the low-side FETs, it could be because there is no supply for the high-side until the outputs start to produce output. In some amps, you can use a 9v battery to supply the high-side driver circuit with voltage but I've never tried it in one of these amps.
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Old 22nd July 2012, 11:03 AM   #3
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K, I'll get onto this soon. I don't have a signal generator yet so I have to use a deck. I did manage to finally buy a scope though should you need to see something else later.

Should I clamp the power supply transistors? or am I good cause of no outputs?

Oh... and thanks Perry.
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Old 23rd July 2012, 12:03 AM   #4
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I suggest clamping the PS FETs but if you're careful and monitor their temperature, they should be OK.
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Old 23rd July 2012, 10:07 PM   #5
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I applied power, ground and remote to the amp. I fed the amp a 100hz sine wave via my deck, RCA voltage was 2.6VAC. I placed the black probe on the 3rd pad (source) of several output transistor locations and the red probe on the 1st pad (gate) and I have no DC voltage on either the positive or negative rails.
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Old 23rd July 2012, 11:57 PM   #6
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Here are a couple pics of the power supply and output cards for the Memphis Mojo 16-MCD4KW, if anyones interested in them.
Attached Images
File Type: jpg IMG_0076.jpg (311.7 KB, 48 views)
File Type: jpg IMG_0081.jpg (368.1 KB, 47 views)
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Old 25th July 2012, 02:31 AM   #7
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Do you have the correct supply voltage on the various pins of the audio driver board?
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Old 25th July 2012, 07:05 AM   #8
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I'm not sure Perry. There are 3 chips and I have no idea what each one are doing, or what voltages I should be seeing. I'll get you readings first thing after work. I got all the reading from the 494's in the power supply last night

Last edited by Deadly Sones; 25th July 2012 at 07:13 AM.
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Old 25th July 2012, 07:13 AM   #9
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494C #1: With black prob on the ground terminal

Pin 1: 2.189
Pin 2: 2.184
Pin 3: 3.305
Pin 4: 0.103
Pin 5: 1.5
Pin 6: 3.647
Pin 7: 0.023
Pin 8: 12.15
Pin 9:1.6 to 1.7 fluctuating
Pin 10: 0.624 to 0.626 fluctuating
Pin 11: 12.15
Pin 12: 10.23
Pin 13: 4.95
Pin 14: 4.95
Pin 15: 4.95
Pin 16: 0.023
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Old 25th July 2012, 07:17 AM   #10
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494C #2: With black prob on the ground terminal

Pin 1: 0.025
Pin 2: 4.96
Pin 3: 1.9
Pin 4: .039
Pin 5: 1.51
Pin 6: 3.55
Pin 7: 0.025
Pin 8: 12.15
Pin 9: 3.0? fluctuating
Pin 10: 3.01 to 3.04 fluctuating
Pin 11: 12.15
Pin 12: 10.23
Pin 13: 4.96
Pin 14: 4.96
Pin 15: 2.482
Pin 16: 2.487
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