MTX Thunder 801D

[S Merlington]

Amp uses 51.1 ohm gate resistors with 75339P'S as the fets..

Will 47 ohm gate resistors work ??

 

[robbysankar]

for testing yes ...dunno if they will for long term use

 

[Perry Babin]

They will be OK.

 

[S Merlington]

All the 470uf 63v caps vented in this amp also..

What causes this ??

Is it rca jacks came in contact with power wire or something else?

 

[Perry Babin]

Too low of a load impedance, shorted output filter inductor, possibly something causing the output stage to operate at the wrong frequency...

 

[S Merlington]

Ok replaced the caps,gte resistors,drivers,power supply fets..

Amp will not power up..

Heres what i get on the 494

Pin 1:0.00
Pin 2:4.75
Pin 3:0.06
Pin 4:1..15
Pin 5:1.50
Pin 6:3.34
Pin 7:0.00
Pin 8:0.64
Pin 9:0.00
Pin 10:0.00
Pin 11:1.11
Pin 12:13.26
Pin 13:4.95
Pin 14:4.95
Pin 15:4.95
Pin 16:0.00

Any ideas??

 

[Perry Babin]

Look at pins 8 and 11 on the 494. Do you have a drive signal there?

 

[S Merlington]

No

 

[Perry Babin]

Do you see any drive signal on the small driver transformer?

If there is absolutely no drive signal, re-check the DC voltage on pin 4 of the 494.

 

[S Merlington]

Not sure but im thinking the 494 has failed.. Unless this is normal


If i place black probe on pin 8 and red probe on Pins 9,10,11,12 i get 23.4 ohms between the pins is this noraml or is it defective?

 

[Perry Babin]

Check the driver transistors and the 10 ohm resistors between the base and emitter.

 

[S Merlington]

Ok replaced them and as soon as i touch power and ground the burn up

 

[Perry Babin]

Do the windings of the driver transformer appear to have overheated?

 

[S Merlington]

No

R625,R626 burn up ..

Q625,Q626 A56's have been replaced also and are defective also..

Someone has been inside this amp before..

Do you happen to have a pic of the area around the 494 and right behind it all thoose transistors have been messed with and dont look right to me

 

[Perry Babin]

Which transistors do you need to see?

 

[S Merlington]

The A56's and 06's That sit by the rail caps there are 6 of them total that al sit right next to eachother in the middle of the board

 

[Perry Babin]

See attached.

If you don't find any other faults, replace the A56s and resistors, remove the driver transformer and connect a 1k ohm resistor from the center tap of it to each of the outer windings (all of this on the side of the transformer connected to the A56s).

To make sure that the power supply FETs remain off, connect 1k resistors to the other side of the driver transformer as well (same configuration).

Do you see a drive signal on the outer legs of the driver transformer connected to the A56s?

 

[S Merlington]

Power supply fets are out of the amp right now Do you want me to reinstall them forthis test??


Also here is a pic of the driver transformer removed what pads do i need to hook a 1k resistor to Do you happen to have a diagram ?

 

[Perry Babin]

Leave the FETs out for now.

Since the FETs are out, all you need to do is install two resistors. Looking at the bottom 4 pads with the left-most #1 and the right-most #4, solder one resistor from 1-2. The other resistor will connect between 3 and 4.

 

[S Merlington]

With the 1K resistors solderd in place on Pins 8,11 I get drive signal a nice square wave..