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spooney 18th September 2009 06:31 AM

MTX Thunder 2300
I have an MTX Thunder 2300 here that I would like to bring back to life.It has a bad/shorted channel,blown ps fets,broken rca inputs,missing rectifier,and several other miscellaneous burned/missing components.I read another thread on here about an mtx thunder 2300 and it was very informative however it didn't tell me everything I need to know to try and tackle this repair.In the following picture there is two small square capacitors(i think) next to the transformer that are melted.I need to find the value of these parts:

I would also like to double check the value of the burned resistors to the left of the ps fets in this pic:

and the part number for the missing rectifier here:

any other info anyone could provide me with would be greatly appreciated.This amp is one of my all time favorites.I want her to be reliable should I be able to bring her back from the dead.I hear these amps are tricky to repair but I believe I am up to the task.

Perry Babin 18th September 2009 07:14 AM

The burned resistors are 22 ohms.

I don't know the value of the caps but one appears to be in reasonably good condition. If you're careful when removing it, you may be able to read the numbers.

The original rectifier was likely an FEN16CT. An MUR1620CTR would be a good sub. Mouser #:

Have you checked the outputs to see if any are shorted?

spooney 18th September 2009 02:58 PM

yes the outputs in one channel are shorted.the other channel appears to be ok.The channel with the shorted outputs had some of the source resistors break/fall out of the board. I've heard that irf3205's are good subs in PS for these amps.Will they work with the 22 ohm resistors that need to be replaced?Is it possible to use say an RFP70N06 instead or would the amp become unstable.

spooney 18th September 2009 03:00 PM

heres a shot of the bad channle that I forgot to post:

Perry Babin 18th September 2009 05:13 PM

I'd prefer to see the 3205s in this amp. There are only 4 power supply FETs. They need to be as rugged as possible. They should work with the 22 ohm resistors with no problems. Also check the two MPSA56s that drive the driver transformer (Q52/Q53?). The 10 ohm resistors connected between the base and emitter of those transistors rarely fail but you should confirm that they're OK.

If the outputs failed, expect at least a few of the smaller driver/protection transistors to have failed. Begin checking for shorted/leaking transistors. If that doesn't lead you to all of the defective ones, you will either have to compare the readings on one side to the readings on the other side or start pulling them to check them.

spooney 18th September 2009 05:48 PM

I may just replace all of those smaller transistors in the bad channel because they are cheap and I want to make sure it will be a winner.I'll get those 3205's as well.How come there is only two source resistors for the power supply fets?isn't there usually one per fet? maybe I am missing something or have just been out of the game for too long.Really surprised to see only four ps fets for an amp of this size.

Perry Babin 18th September 2009 06:21 PM

Yes, most amps have one gate resistor per power supply transistor but these amps use a significantly different drive circuit.

Most amps also have one emitter/source resistor per output transistor. This one doesn't. It has 6 resistors and 8 outputs.

spooney 2nd October 2009 04:14 PM

those little square capacitors read 123J100 on the top of them. I put the number into mouser's search and it came up with all these:

which one of these am I looking for or is it any of these at all? I'm guessing the 100 volt versions would be fine. 2nd October 2009 05:18 PM

According to mouser they don't stock that part#. The "123" show it is a
.012uF and the 100 is the voltage. MMK is the series with the next digits representing the lead spacing.

look around for any metallized polyester cap in that value and voltage.


spooney 2nd March 2010 05:11 AM

I finally got everything in I thought I'd need and soldered it in. I replaced all the outputs,small transistors, and source resistors in the bad channel as well as replacing both rectifiers, the ps fets,gate resistors in the ps, and the power supply driver transistors. I powered the amp up and it starts up fine and the newly repaired channel seems to be working well. The amp is not drawing excessive current or doing anything else out of the ordinary. Rail voltage is good as well. The channel that i did not work on however is distorting terribly and the output is considerably lower than the new channel. When i run a sine wave through the amp its plain to see that the severely distorted channel is completely missing the bottom half of the waveform. When I initially checked this amp out i found no shorted outputs in this channel and the current draw is good so i think the outputs are ok. I am thinking the problem lies either in the op amps or driver transistors but i am really just scratching my head as to where to start.

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