Mtx 3401

[scampo77]

i am fixing 2 of these amps simultaniously and i have run out of things to fix on one amp so i thought i could switch to this one.

this amp is the exact same amp as this thread

what is this?

with this amp i am getting 0v at the op amp and +- 2-3 volt at the regulators. the rail voltage is 2v and i cant see any obvious melt downs or burn marks. i checked the R106 location and surrounding components and everything measures fine.

where do i look now?

[Perry Babin]

It sounds like the power supply is inoperative. What is the DC voltage on each of the legs of FET102 and FET104?

FET102
1.
2.
3.

FET104
1.
2.
3.

Don't plug this amp's preamp board into the other amp. If there's something wrong with the other amp that's causing the op-amps to fail, it will damage the op-amps on this preamp board also.

[scampo77]

ok i wont plug the pre amp board in

when i check for power on those transistors i get +8v only on the middle leg of each transistor the other legs are 0v

[Perry Babin]

It sounds like the power supply you're using is set too low or is being dragged down by the amplifier. Does the voltage go higher (on the power supply) when you disconnect the amp?

The amp has low voltage protection (if I'm not mistaken) so it may not power up with only 8v.

Does the supply voltage remain at/near 12v without applying remote voltage to the amplifier?

[scampo77]

yes that is exactly what is happening. it is 13.5v till i hook it to the amp then it drops down. i am using a resistor to power the amp for testing but even without the resistor this still happens.

[Perry Babin]

Does the supply voltage get pulled down if you don't connect the remote turn on?

If not, check the output transistors (FET401-404) to see if any are shorted (no power applied, meter set to ohms or diode-check).

[scampo77]

no i dont get a short (really high resistance on some) but nothing to make my meter beep.

if i apply power to only rem or B+ then it doesnt drag down the power, only when the 2 of them have voltage does it happen.

[Perry Babin]

If the voltage drops the same with/without the resistor, your DC power supply would seem to be faulty.

What's the current rating of the supply?

What value resistor are you using?

[scampo77]

power supply is 5A 65 watt and im using 2 ceramic resistors 20W and 4 ohm they are tied parrellel and measure at 2 ohm

this power supply lights up the other amp which is identical to this one

should i get more power for this?

[Perry Babin]

5 amps should be enough to power it up.

How low does the supply voltage drop without the resistor?

How low does the supply voltage drop with the resistor?

Measure the voltage at the terminal block of the amplifier.

I think the amp may be drawing just enough current to trip the low voltage protection.