Mtx 311 d update ^HELP^

[onthethreshold]

The board says th601,it is a two wire lead with a small blob on the end encased in foam.I think it is a thermsistor.The one side of the board has two fet's marked IRFP240 and dar308L 635d8.The other side has two fets marked 75339p,n304 and at the bottom FAT.There is a small circle that says MALAY in the middle of it.the difference between it and the one beside it is it is marked identical except for FBD at the bottom.There is another TH in between them and two REC's.One is marked Cq89,v1020r,co238,KAK.The other says China on it in yellow writing and is marked s77,u19304,BYW51,200. I will try to send a picture.I used to mod Dynacos but know little about solid state.I looked up the numbers at Mousers site and they came up but I just want to be sure what to order.It said some had changed and from what I've read they say not to vary on the values on a class d amp.Any help will be greatly appreciated and thank you in advance.If you could tell me what parts to order from what site I would be forever indebted.I know this is alot to ask but I am at best a good solderer.

[Perry Babin]

Which of those parts is defective?

[onthethreshold]

REC 601,REC 602,FET 1602 ,FET 1603 all appear flashed (burnt ).You can see the flash on the board and the termination points are burnt discolored.Also the TH 602 seems to have one discolored wire,they are both white but one shows some black flash.I do not have a camera to send a pic but I saw a MTX d class board on here that looks similar but searched again and cannot find it.From what I have read they say to replace all the outboard componets ( the ones in the heat sinks but I'm sure you are a good tech and can steer me straight.Thank you for your time and may God bless you.Also these sit in a white paste,dielectic grease i assume?

[Perry Babin]

There is likely heatsink compound under the transistors but there's also likely an insulator of some sort.

Did you find any shorted output transistors? They would be the ones marked IRFP240.

[onthethreshold]

They appear to be okay,I did not want to remove any components from the board until I had the new ones in hand so i wouldn't get them confused,I'm truly not much of a tech,but because they are so cheap (price wise ) I thought I would replace them as well.Thank you for answering.I wish there was a repair facility near where I could take the board and say ok,here is what i wish to replace.

[Perry Babin]

The thing is... some class D amps (MTX is one of them) do not work properly if there is ANY change in the output transistor. The change can be something that won't affect 99%+ of they circuits they're used in and may not even cause the manufacturer to change the part number (adding/changing the suffix of the part number).

If they don't appear to be defective, leave them alone for now. If you replace the power supply FETs and get the amp working properly and then want to replace the outputs, then you can do so and if they don't work properly, you know it's the outputs and you don't have to try to troubleshoot the problem.

I don't know if this amp burns the gate resistors or the gate driver transistors but you should check them before ordering.

[onthethreshold]

ok,one more question.How do i determine what thermistor I need ?It is a two wire white lead with just a blob encased in foam rubber.Do these usually go bad or just get the parts and hope fpr the best? Thank you sincerely for all your help.

[Perry Babin]

I've seen less than 5 defective thermistors in ~25 years of doing repair work. Unless it's physically damaged (crushed, cracked, missing leads...), it's almost certainly OK.