MTX Thunder 502

[S Merlington]

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

 

[Perry Babin]

Probably 4.75 ohms.

 

[S Merlington]

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?

 

[Perry Babin]

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.

 

[S Merlington]

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?

 

[Perry Babin]

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.

 

[S Merlington]

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?

 

[Perry Babin]

I think you took a wrong turn at some point. Follow it from the remote terminal.

 

[S Merlington]

Thanks Perry,,

Amp is fixed..

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