Need A Resistor Value

swordfish689 · 10th April 2006, 11:44 PM

I received this amp, mtx 8100d. The problem was that it had both power supply resistors are shot. I replace them with 68ohms, but i think the value are wrong. Because i get a hissing sounding coming from the transformer. I know rockfords have this problem, but usually mtx are very quiet amps.

there r435 and r436

niteowl · 11th April 2006, 05:02 PM

Hi swordfish689

That value should work fine. You can almost use any value from 10 to 100 ohms. try pressing your finger on the transformer in differant spots and see if the sound go's away. I've had to do this with rockfords then if it quiets down, I would put some glue on the windings to suppress it. Does the amp work?

Martin

Perry Babin · 11th April 2006, 06:00 PM

In the 1501 that I have here, they are 39.2 ohms.The board layouts look the same and the resistors have the same designations so the value should be right.

Some of those amplifiers are extremely noisy. After repairing it, run it hard. The power supply and the output FETs should all operate at roughly the same temperature. If either set of FETs is running significantly hotter than the other, you may want to check it a little more thoroughly.

lndm · 12th April 2006, 12:07 AM

Not sure from the picture, but it looks as if the capacitor has expanded. Bad capacitors can hiss (not sure if this is what you meant).

swordfish689 · 12th April 2006, 05:34 PM

Thanks for the replies. The only thing that i replaced were the 8 power fets, 2 resistors near the fets, and one diode that seemed to be overheated, it still worked, but i didn't want to take any chances. It turns on and works, the only thing i don't like is the sound of the transformer, making a loud hissing sounding. Also it has no gain, its full all the time, the first time i connected it to my sub it rattled the windows. Scared the **** out of me. I need to check the op-amps. Thanks again.

Perry Babin · 12th April 2006, 06:34 PM

If the RCA jack shield 'bell' isn't soldered to the RCA jack 'frame', solder them together. Apply heat from the outside of the frame and apply solder to the inside of the frame. If the shield ground is not solidly connected, the amp will oscillate at ~100hz at full power. The newer RCAs are soldered.

The amps do have problems with failing op-amps that cause low frequency oscillation but I've never seen a failed op-amp cause full power oscillation.

lndm · 12th April 2006, 10:38 PM

Did you say it was the transformer making the sound? Is it a buzzing sound? If so, this is probably not a problem, many xfmrs buzz a little.

10th April 2006, 11:44 PM	#1
swordfish689 diyAudio Member Join Date: Jan 2005 Location: The Great Lakes	Need A Resistor Value I received this amp, mtx 8100d. The problem was that it had both power supply resistors are shot. I replace them with 68ohms, but i think the value are wrong. Because i get a hissing sounding coming from the transformer. I know rockfords have this problem, but usually mtx are very quiet amps. there r435 and r436

11th April 2006, 06:00 PM	#3
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	In the 1501 that I have here, they are 39.2 ohms.The board layouts look the same and the resistors have the same designations so the value should be right. Some of those amplifiers are extremely noisy. After repairing it, run it hard. The power supply and the output FETs should all operate at roughly the same temperature. If either set of FETs is running significantly hotter than the other, you may want to check it a little more thoroughly. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

12th April 2006, 06:34 PM	#6
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	If the RCA jack shield 'bell' isn't soldered to the RCA jack 'frame', solder them together. Apply heat from the outside of the frame and apply solder to the inside of the frame. If the shield ground is not solidly connected, the amp will oscillate at ~100hz at full power. The newer RCAs are soldered. The amps do have problems with failing op-amps that cause low frequency oscillation but I've never seen a failed op-amp cause full power oscillation. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

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11th April 2006, 05:02 PM	#2
niteowl diyAudio Member Join Date: Sep 2005	Hi swordfish689 That value should work fine. You can almost use any value from 10 to 100 ohms. try pressing your finger on the transformer in differant spots and see if the sound go's away. I've had to do this with rockfords then if it quiets down, I would put some glue on the windings to suppress it. Does the amp work? Martin

12th April 2006, 12:07 AM	#4
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	Not sure from the picture, but it looks as if the capacitor has expanded. Bad capacitors can hiss (not sure if this is what you meant).

12th April 2006, 05:34 PM	#5
swordfish689 diyAudio Member Join Date: Jan 2005 Location: The Great Lakes	Thanks for the replies. The only thing that i replaced were the 8 power fets, 2 resistors near the fets, and one diode that seemed to be overheated, it still worked, but i didn't want to take any chances. It turns on and works, the only thing i don't like is the sound of the transformer, making a loud hissing sounding. Also it has no gain, its full all the time, the first time i connected it to my sub it rattled the windows. Scared the **** out of me. I need to check the op-amps. Thanks again.

12th April 2006, 10:38 PM	#7
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	Did you say it was the transformer making the sound? Is it a buzzing sound? If so, this is probably not a problem, many xfmrs buzz a little.

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