Blown Mosfet CDX20A

[volonau 10+ year member]

I have unfortunately blown one Fairchild 9640 mosfet and have decided to replace all 20 with the newer Fairchild 512-SFP9640.

19 of the 20 old mosfets have a second number on them 236, the one that blew has a 105.

Does anyone know whether the 105 is a lower value unit that may have contributed to its demise?

Any ideas why I may have blown a mosfet period?

Any replies would be helpful:up2somet:

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[volonau 10+ year member]

Anybody?

 

[Blazin_Jason 10+ year member]

What exactly do the FET's read on them? Just 9640 on the top, then the 236 or 105 below?

 

[TRU Tech]

The Fairchild 9640 is a switching Mosfet

The sole purpose of the switching FET in the power supply is to make AC power to drive the output devices.

So, based on the parts & quantity, one can easily find out how much and big the power supply is.

In terms of the part#105? It represent the production date code. Those numbers change all the time.

You should buy them more then you need because those parts are considered to be obsolete by Fairchild and once they are out, they'll gone.

 

[snoopdan 5,000+ posts]

the newer fairchilds run a bit hotter than the true IRF replacement 9640's....hmm, ive got about 20 on my desk now!

that 105 number is a newer part, must have been recenly replaced. the 236's are from around 2 years ago....i just replaced 18 of them on my amp //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif

 

[volonau 10+ year member]

The 105 and 236 are above the IRF9640 on the FET.

It looks to me like the amp had been opened and worked on so the 105 being newer

doesn't suprise me a bit.

Now why it would keep blowing is another matter. I have also tested the resistors connected to said FET and found at least one of them to be off spec.

I have installed two 1" fans on the side panels to help with the extra heat.

There were a few mica boards missing when I removed the board but I'm not sure if they were missing from the blown 105 or not.

I guess I will continue with the install as planned and hope for the best.

Is there any other product I can use in place of the mica boards that are missing?

I can't find any were I'm from.

 

[thylantyr 10+ year member]

Make sure the transistors are insulated from the heatsink chassis using

the mica insulator. Apply thermal joint compound on both sides of the mica,

some on the metal transistor tab, thin coat only. You can also use the fancy CPU

goo called Arctic Silver.

You can use this mica stuff, cut the material with scissors.

http://www.mcmaster.com

part # 85955K21

This is 4 mil thick, they make thicker. But I don't think it's a good

idea if it's too thick. You can probably use the 8 mil stuff too if

this one is too thin and breaks during clamping.

After transistor install, use a DMM to verify there is no shorting of the metal

transistor tab to aluminum chassis.

 

[volonau 10+ year member]

Thanks for all the help guys!

I have another question for you.

What exactly am I to order here.

metal film resistors with a valuse of?????????????????????????????

red,red,black,gold,brown.

 

[thch 10+ year member]

wait, irf 9640? wouldn't have expected to find a high voltage pfet like irf9640 in a car amp. are you sure there aren't a mix of 9640 and 640?

2 2 . 0 * 10^-1, 1%

2.2ohm, 1/4watt

if you want, look at =irf9640 on digikey, which would be the closeout model that contains lead.

 

[volonau 10+ year member]

Nope twenty 9640's and thanks for the resistor rating.

 

[TRU Tech]

Forgot about the insulator. Thanks thylantyr

Another alternative if you don't like the messy grease

We use this on our inverters. It does require some clamping power

http://www.bergquistcompany.com/tm_sil_pad_detail.cfm?oid=104252

 

[volonau 10+ year member]

I have all the parts now including the five resistors. I'm also adding a Optima red top in the back just in case voltage drops caused this.

Should get it together Monday.

In order to set the gain properly on my modded CDX20A thats running 1 ohm I need to know what the maximum output voltage should be ? (69.5 now)

 

[volonau 10+ year member]

Anyone?

 

[snoopdan 5,000+ posts]

ohhhh why didnt you just say it was an HK board? lol.....ive got them laying around the house, haha..

The biggest problem with this amp is that the fet's dont have any adjustable bias on them, they use the natural switching characteristics of the devices to keep timing right and to reduce distortion.....Ive found if you replace the original fairchilds with IRF chips it runs a bit cooler, even if it IS out of spec for the bias, it apparently still runs fine.

 

[volonau 10+ year member]

Installed FETS and new resistors. Stuck her on a 60AMP fuse to start and it blew in about five seconds.

Any suggestions before I take it in?