MTX TNA251

S Merlington · 2014-07-26 5:47 pm

Amp is using STP75NF75'S as the power supply fets with 100 ohm gate resistors..

Would the 3205'S or 1010E's work as a replacement for them?

Perry Babin · 2014-07-26 6:07 pm

Does this amp use a driver transformer?

S Merlington · 2014-07-26 6:44 pm

Perry Babin · 2014-07-26 6:48 pm

I'm not familiar with it but if you reduce the gate resistor value, either of those should work.

S Merlington · 2014-07-27 9:31 pm

I put 47 ohm gate resistors in the amp..

With just power and ground hooked up the fets blow..

I replaced the 2N3906 drivers but the amp still blows fets with just power and ground hooked up..

If i remove the fets Do i put a 1K resistor from Drain to Source or gate to drain or gate to source ??

Just wanna look at the drive signal loaded down to see if i get a nice square wave

ultra · 2014-07-27 9:47 pm

Normally it should be between gate to source.
I personally use a 10nF capacitor.

S Merlington · 2014-07-27 9:50 pm

Thanks

S Merlington · 2014-07-27 10:02 pm

Ok i looked at the gate signal with the 1K resistors in place then i replaced them with the 10NF cap gate signal looks perfect..

Anyone know what else would cause the fets to blow within 1 sec with just power and ground hooked up?

ultra · 2014-07-27 10:22 pm

When you say perfect, you mean that square wave signal becomes a bit "curvy" on the edges when the capacitor is connected?

Did you try the capacitor in each fet to eliminate a broken trace?

S Merlington · 2014-07-27 10:38 pm

I'll take a pic of the square wave and post it

S Merlington · 2014-07-28 1:44 am

Here is the square wave im getting on the gate pads with a 10NF cap hooked between gate and source..

Scope settings are 5us 1v/div

Dont understand why this amp blows fets with just power and ground hooked up

Anyone have any ideas?

ultra · 2014-07-28 2:13 am

this looks ok.

Perry Babin · 2014-07-28 9:04 am

3.5v of amplitude seems a bit low.

vwdave · 2014-07-30 5:15 am

Not sure if this helps but I have had a similar problem on a completely different amp but it turned out to be a bad PWM. Changed it out and the amp now runs my components flawlessly.

S Merlington · 2014-07-30 7:10 am

Thank you for the info I will try that to see if that helps and let you know

S Merlington · 2014-08-01 4:26 am

Ok the 494 wasent the problem

There is a IC in the power supply marked HS108P-J

It connects to the gate leg of the power supply fets I'm guessing it's defective and cannot find a replacement for it through any suppliers

Is there a way of testing it or jumping it to see if it's defective for sure or a suitable replacement for it?

Do I need to supply more info on the amp?

luigidedom · 2014-08-01 8:29 am

post some pics

MTX TNA251

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