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Old 16th June 2011, 10:12 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default MTX RTP251D wont power up

Ive got another one of these little amps which will not power up. I removed both power supply fets from the board because my meter is measuring 32ohms across Legs 1&2, only to find the 32ohms is something on the board and the fets are likely OK. There is no voltage on the gates of the fets.

TL494:

Pin 1: 0.001
Pin 2: 4.74
Pin 3: 0.000
Pin 4: 4.98
Pin 5: 1.512
Pin 6: 3.368
Pin 7: 0
Pin 8: 13.24
Pin 9: 0
Pin 10: 0
Pin 11: 13.24
Pin 12: 13.24
Pin 13: 4.97
Pin 14: 4.97
Pin 15: 4.97
Pin 16: 0.000
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Old 16th June 2011, 10:28 PM   #2
ultra is offline ultra  Greece
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i happened to be online when i read your post.

Without wanting to take the chair of Perry Babin,

you could start looking why pin 4 has almost 5 volts.

Normally, voltage at that pin should be close to 0.
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Old 16th June 2011, 10:34 PM   #3
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If it has an LM339, post the voltage on all of its pins.
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Old 17th June 2011, 02:48 AM   #4
Dr Zeus is offline Dr Zeus  United States
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LM339

Pin 1: 0.072
Pin 2: 4.96
Pin 3: 13.22
Pin 4: 0.072
Pin 5: 3.117
Pin 6: 0.803
Pin 7: 3.117
Pin 8: 0
Pin 9: 4.99
Pin 10: 4.99
Pin 11: 0
Pin 12: 0
Pin 13: 0.073
Pin 14: 0.073
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Old 17th June 2011, 05:29 AM   #5
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It doesn't appear that you have remote voltage applied. Follow the remote from the terminal to see where it's being lost.
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Old 17th June 2011, 11:30 AM   #6
Dr Zeus is offline Dr Zeus  United States
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Found a broken trace under a rail cap. Now I've got to figure out why it happened. This trace is directly connected to REM.

Click the image to open in full size.

Last edited by Dr Zeus; 17th June 2011 at 11:35 AM.
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Old 17th June 2011, 11:38 AM   #7
Dr Zeus is offline Dr Zeus  United States
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Now I see what happened. Looks like the bottom/under part of the rail cap may have actually touched and shorted out this trace. Interesting.
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Old 17th June 2011, 11:44 AM   #8
Dr Zeus is offline Dr Zeus  United States
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I just realized that I can not successfully repair or build-up this trace under the cap for risk of it shorting again. Instead, I removed all the lifted trace material and added a jumped underneith the PCB as pictured. This should do the trick.

Click the image to open in full size.
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Old 17th June 2011, 11:57 AM   #9
Dr Zeus is offline Dr Zeus  United States
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The amp now seems to want and try to power up, but the power supply fets are getting hot with power attached and the amp is draining my power supply.

In post one I had found 31ohms across legs 1 to 3 of both fets. I think there is some problem here maybe like a shorted driver or something causing the amp to bridge the gate resistors which are 31ohms.
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Old 17th June 2011, 06:17 PM   #10
Dr Zeus is offline Dr Zeus  United States
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There is no signal coming off pins 9 & 10 of the TL494. The signal from pins 9&10 run through a tiny transformer and then through a pair of ~60ohm resistors in paralel (~30ohms) to the fet gates.

TL494

Pin 1: 0.099
Pin 2: 4.74
Pin 3: 0.067
Pin 4: 0.833
Pin 5: 1.516
Pin 6: 3.337
Pin 7: 0
Pin 8: 12.90
Pin 9: 0
Pin 10: 0
Pin 11: 12.97
Pin 12: 13.17
Pin 13: 4.97
Pin 14: 4.97
Pin 15: 4.97
Pin 16: 0

LM339
Pin 1: 4.74
Pin 2: 0.001
Pin 3: 13.16
Pin 4: 4.76
Pin 5: 3.413
Pin 6: 0.987
Pin 7: 3.411
Pin 8: 4.093
Pin 9: 4.99
Pin 10: 4.99
Pin 11: 8.62
Pin 12: 0
Pin 13: 4.78
Pin 14: 4.78

Last edited by Dr Zeus; 17th June 2011 at 06:40 PM.
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