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Old 22nd October 2006, 03:25 AM   #1
rader is offline rader  United States
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Join Date: Oct 2006
Default soundstream amp help

Hello, First let me say I am not a ET junkie like most of you guys. I do have a fair understanding of electronics, and have repaired TV' s for a living as a kid. (years ago). That being said, I need help repairing a Soundstream Reference 404. Currently it will not power up. It had 3 visibly damaged transistors. I removed them. I am having a devil of a time finding repalcements. They are Motorola chips marked: MTP50N05EW. If anybody has any advice to offer it will be greatly appreciated.
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Old 22nd October 2006, 06:52 AM   #2
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Join Date: Dec 2005
Location: Northern California
E-bay has them for 2 or 3 bucks each.
You can replace all of them with better parts like RFP70N06's or IRFZ-48N , or STP60n06, or just about any Fet rated at or more than 50 amperes.

You better check the little metal can 2n2222a and the flat pack transistor beside it as they are the Fet drivers and when the Fets blow they also go.

Also check the 1/4 watt resistors tied to the gate lead, they blow like fuses and don't even show signs of damage.

hope this helps
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Old 22nd October 2006, 09:08 AM   #3
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Location: Louisiana
When replacing the power supply FETs, you need to replace all of them. Even if there is no obvious damage to the other bank of power supply FETs, you need to replace them.

It's likely something caused the excessive current draw that destroyed the FETs, you need to check the output transistors (check each one individually) and also check the to-220 case rectifiers connected to the secondary windings of the transformer.

When you initially power up the amp, you need to use a current limiter or a relatively small fuse (10 amps). I'd also recommend that you have the power supply transistors clamped down to the heatsink. This will help prevent damage to them if you have missed something.
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Old 22nd October 2006, 08:31 PM   #4
rader is offline rader  United States
diyAudio Member
Join Date: Oct 2006
Thanks for the help thus far. Sorry to be so simple but I am not sure of a couple of things. How should I check the FET's? I have a good DVM, and know how to use it. ALso best I can remember from electronic school, years ago, seems like the only way to check resistance on many components, like resistors, was to remove one leg. Is this true of the components I need to check? Once again for the "hand holding"
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Old 22nd October 2006, 09:18 PM   #5
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Join Date: Nov 2003
Location: Louisiana
If any of the power supply transistors show any signs of damage, there's no need to check them. They all must be replaced.

If you didn't see any damage but wanted to check them, you can do a quick check by measuring the resistance between the center leg and either outer leg. Neither should read near zero ohms.

For definitive tests, the FETs need to be removed. Actually, for definitive testing, any component should be removed from the board.

For general testing of semiconductors, scroll approximately 1/3 of the way down the following page.

The testing section begins with Checking Field Effect Transistors.
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