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Old 29th August 2013, 08:45 PM   #1
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Default MTX Thunder 502

Anyone know the value of R324??

It sits right behind the speaker terminal block..

1 side connects to the 3rd leg of the outputs the other side connects to a mylar cap 100N J100
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Old 29th August 2013, 09:31 PM   #2
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Probably 4.75 ohms.
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Old 29th August 2013, 09:54 PM   #3
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Ok replaced that..

Amp will not power up..

Heres what i get on the 494

Pin 1:0.00
Pin 2:4.62
Pin 3:0.06
Pin 4:4.96
Pin 5:1.50
Pin 6:3.33
Pin 7:0.00
Pin 8:13.84
Pin 9:0.00
Pin 10:0.00
Pin 11:13.84
Pin 12:13.84
Pin 13:4.97
Pin 14:4.97
Pin 15:4.97
Pin 16:0.00

I have no voltage present on the gate leg of the power supply fets..

Any ideas?
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Old 29th August 2013, 09:58 PM   #4
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Join Date: Nov 2003
Location: Louisiana
The LM339 is holding the 494 off via pin 4.

In a power supply driven by a transformer, you won't see any significant DC on the gates of the power supply FETs. You have to check the signal with a scope or a multimeter capable of reading high frequency AC.
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Old 29th August 2013, 10:07 PM   #5
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The voltages on the LM339 are

Pin 1:0.07
Pin 2:0.07
Pin 3:13.35
Pin 4:4.97
Pin 5:0.00
Pin 6:0.00
Pin 7:4.97
Pin 8:0.64
Pin 9:3.10
Pin 10:0.07
Pin 11:3.11
Pin 12:0.00
Pin 13:4.97
Pin 14:0.07


So do i check whats connected to pin 4 of the Lm339 or what should i check?
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Old 29th August 2013, 10:15 PM   #6
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Location: Louisiana
I meant pin 4 of the 494.

Pin 5 of the 339 should be at about 8v. It's driven by the remote turn-on, likely via a diode and resistor. I'm working from notes for a different amp but the circuits seem similar.
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Old 29th August 2013, 10:32 PM   #7
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Pin 4 of the 494 goes thru some resistors and connects to the small yellow transformer..

Pin 5 of the 339 goes thru a resistor then a diode and goes to the small yellow transformer..

I get no voltage on the diode that connects to the small transformer..

Do thoose transformers go bad or do i need to look further back in the circuit?
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Old 29th August 2013, 10:38 PM   #8
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I think you took a wrong turn at some point. Follow it from the remote terminal.
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Old 29th August 2013, 10:52 PM   #9
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Thanks Perry,,

Amp is fixed..

From the terminal block to the board had a bad solder connection not making contact..
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