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Old 5th August 2009, 06:19 AM   #21
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You need to be careful when you use the terms 'always' or 'exactly'. Virtually nothing is absolute.

Power supply FETs and class A, AB and B outputs are sometimes easy to sub but class D amps are a different animal. In some class D amps, the deadtime is determined by the gate load. Changing the FET changes the load and the amp may have too little deadtime. This can result in the amp failing. It may not fail immediately. A change in the operating temperature may be enough to make the amp fail if the deadtime isn't right. Too much deadtime can also cause problems.

At least a couple of times a month I have to deal with this problem. People either send me an amp or email me about amps and the only problem is that they (or a previous tech) used a substitute part. This is the reason I am so concerned when substitute parts are mentioned with class D amps.

As an example, the early MTX amps like the 81000d used the SSP45N20A. That transistor was discontinued and the new SSP45N20B was the replacement (same manufacturer, VERY close specs). The B version would NOT work as a sub.

In a different amp (one of the many clones that has the fan between the driver board and the two inductors), the IRFB42N20D may seem like an upgrade over the original IRFB31N20D. In some amps, they will work and never have a problem. In other amps, they work for a while but the amp will be unreliable (slight changes in operating conditions will cause the FETs to fail). In other amps, they will not even power up (they go directly into protect. Replacing them with the original parts makes the amps as reliable as they were originally.

In this instance, if the IRF3415 amp uses the same EXACT driver components, the 260N may be an upgrade. That's not always the case when using substitute parts.

The braid 'may' be OK but you won't know until you buy it. Many times, braid sits around, exposed to air and it oxidizes. That makes it very nearly useless. Adding liquid flux to poor quality braid can make it usable. The braid I generally buy is ~$50/100ft. I dread having to buy a new roll but it works and saves time in the long run. The roll will last more than a year for me. I generally only use it to clean up SMD pads. Most of the desoldering I do is with a desoldering pump (DS017 Edsyn).
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Old 5th August 2009, 08:04 AM   #22
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Perry meant to say IRF P 3415 Him and those P's
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Old 5th August 2009, 03:47 PM   #23
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ok,ordered 34 irfp260npbf and 26 irfp064npbf. Unfortunately i forgot to order the drivers mmbta and the resistors. May be this a problem?However, before replacing mosfets, I'll check drivers and resistors by tester.
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Old 5th August 2009, 05:37 PM   #24
Dimon74 is offline Dimon74  United States
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Thanks for explanation Perry,I will keep it in mind.
To rocco4796
Don't even try to power up the amp before you changed drivers transistors and gates resistors.In my T20001BD almost all of them were dead.And also you have to know ,the ground ,I mean negative terminal in this amp going through the body of the amp.All drains of power suplly fets connected together through the case of the amp.Before power it up ,you have to install board inside the case ,and screw it to the body
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Old 5th August 2009, 06:05 PM   #25
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ok, resistors are 1 per mosfet (12), drivers are 4 mmbta06 and 4mmbta56 is it correct?. However, the amp had worked at 0,75ohm, this led to the fault.
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Old 5th August 2009, 06:12 PM   #26
Dimon74 is offline Dimon74  United States
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Yes, transistors are MMBTA56 and MMBTA06
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Old 6th August 2009, 05:54 PM   #27
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All checked. 9 out of 24 gate resistors of 10 ohm (12 each amp) are failed. I think I'll change all 24. Then, I have in the same area also 4 100ohm resistors and 4 1kohm resistors each amp, with a total of 16. 8 of them are failed. I didn't check the mmbta56 and 06, I think it's useless...Nice damage, isn't it?
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Old 11th August 2009, 04:16 PM   #28
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sorry people...almost everything arrived, but I'm getting mad...how to unsolder the mosfets from the "heatsink"???I read that I have to unsolder, but from where?I cannot understand it. Please helppppppp
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Old 11th August 2009, 04:37 PM   #29
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You'll need a small torch (similar to the following).

http://www.radioshack.com/product/in...ductId=2062753

or

http://www.bernzomatic.com/PRODUCTS/...3/Default.aspx

You'll have to...

*clean the compound from the insulators
*clamp one (adjacent) FET to the insulator (binder clip or large forceps - clamp should be lightweight)
*heat the insulator behind the unclamped FET until it releases
*desolder the legs of the FET and remove it
*tin the back of the new FET
*check the FET to confirm that there is absolutely no continuity between leg 1 of the FET and the other legs. sometimes they fail if you heat them too much when tinning them
*place the new FET in place and solder the center leg
*heat the back of the insulator while applying light pressure to the FET
*when the solder melts and the FET drops onto the insulator, hold it on the insulator and remove the heat
*solder all of the legs on the newly replaced FET
*then do the next FET on the insulator

I prefer to do it this way because the insulator remains at the correct height and everything remains aligned so there are no problems when you go to reassemble it.

Be careful not to use too much heat or dislodge any of the other small components on the boards.

If possible, clamp the 3 terminal jumper to the insulator. If you use too much heat, they can become desoldered.
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Old 11th August 2009, 05:09 PM   #30
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But can I put the new mosfets directly on the body?so without the isolator, because I don't have any torch.Or is there an alternative method to get them out?
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