MTX Thunder 81001

[Perry Babin]

What's the DC voltage on the 3 legs of FET192?

 

[mike49504]

Leg 1:-79.9
Leg 2:1.28
Leg 3:-82.0

 

[Perry Babin]

You probably have open resistors or defective transistors in the circuit driving this transistor.

 

[mike49504]

Im going to check the circuit and go get some resistors / transistors if needed...


will post results later..

 

[mike49504]

Do you have another pic of the area around FET401 Its right next to
Fet192..

Also a pic of the other side of the board around the rectifiers ?


Someone has worked on this before and wanna make sure they are the correct parts that are in it..


Fet 192 and Fet401 are 540's correct?

 

[Perry Babin]

I think FET401 is an N-channel FET but I'm not 100% sure. FET 192 is a 540.

 

[mike49504]

In the amp i have Fet191 has a diode solerd to leg 3 is that suppose to be there or did someone add it ?


Wondering if i should ake it out or not ?

 

[Perry Babin]

Leave the diode. I think it's to help protect the op-amps when the regulator fails.

 

[mike49504]

Ok will do..

Hopefully all the right parts are in this amp someone has worked on this before and has been all over the board..

The amp i have here Fet192 Fet401 Fet191 all have IRF540's in there locations..


Also i rebuilt the area around Fet192 Replaced all resistors transistors and the regulator..

As soon as i powerd it on the fans spun for 1 sec then Q109,Q111,D105,Q108 all failed again..

 

[mike49504]

Ok finally got the parts in for this amp..

I now have + - 15 volts on the op-amps..


Now i have 81 volts of DC measured directly across the speaker terminals..


All outputs test fine..


Any ideas where to go next?

 

[mike49504]

Ok i found Q405 to be defective..


I cant read the numbers off it.


Wondering if u have a pic of the board around the op-amps or do you know what Q405 is suppose to be ?

 

[Randy62]

Q405 =C1URA I believe it is a jfet that mutes the input of both A3120's.Pin 1&3 on Q405 go to pins 2&3 on the A3120's. Pin 2 of Q405 goes to R421.

 

[mike49504]

Q405 is marked C1U5A in this amp..

Cant find a data sheet on it but heres the reading i get


Top pin to left bottom pin :OL

Top pin to bottom right pin:OL

Left bottom pin to right bottom pin:0.015


Probes reversed

Top pin to bottom left:0.644

Top to bottom right:0.622

Bottom left to bottom right:0.015

Defective ??

 

[Randy62]

I THINK that part is OK.I have 3 of these amps here with blown power supply's and thats the same reading I get on all of them for Q405.They also test at about 25 ohms on pins 1&3.
And yes one of them I have is also marked C1U5A.
Does R417 test good?

 

[Perry Babin]

That transistor is a JFET. That's normal.

Post the DC voltage on all pins of the two op-amps near the opto-couplers.

Also post the DC voltage across the power supply pins (5 and 8) for the two opto-couplers.

Leave the preamp board disconnected.

 

[mike49504]

Ok i found C401 with a leg broken off the board..


Its marked 100 25A


Is that a 100uf 25 volt cap?

 

[Perry Babin]

That should be correct.

 

[mike49504]

The voltages on the 2 op-amps are


Pin 1:-6.46
Pin 2:-13.32
Pin 3:-13.25
Pin 4:-14.95
Pin 5:-6.75
Pin 6:-79.8
Pin 7:-7.62
Pin 8:15.74


Pin 1:15.47
Pin 2:14.03
Pin 3:14.39
Pin 4:-14.39
Pin 5:14.52
Pin 6:14.01
Pin 7:14.96
Pin 8:15.93


Voltage across opto-couplers

Pin 5 to 8:18.04

Pin 5 to 8:15.99

If i use ground terminal as reference for the opto-couplers i get

Pin 5:4.16
Pin 8:-63.1


Pin 5:-81.2
Pin 8:-65.3

I notice something in the power supply cycling i can hear a small squeak .. When amp is powerd up i get 81 volts across the output terminals ...

If i leave the amp idle for a lil while once i hear the squeak the voltage will drop to 4.25 volts across the outputs for a few secs the it will go back up to 81 volts..

The amp does this over and over

 

[Perry Babin]

Is the -79v on pin 6 a typo?

 

[mike49504]

Not a typo

There is -79 volts on pin 6