Digital design

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I don't know if the 5v reading was taken before the 4v readings. Something could have drained the battery or something could be pulling it down. Either way, you need to perform whatever tests that you can to determine why the drive signals are different. You want to make absolutely sure that the drive circuit is OK before installing the output transistors.
 
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My mistake! I replaced the four 2.2 ohms one watt resistors in front of the
Drive Board, but only 3 were open, as a result I ended up with 2 resistors
In parallel. So now everything is OK, can I test this amp with only one
transistor Per shoulder, Also the audio output transistor were replaced with
Irfp4004 is that the original audio output transistor for this amp?
 
There isn't really any safe way to install FETs. With 1 per bank, it's likely to blow the FET if there is a problem. This will likely destroy the driver IC. If you install all of the FETs, the protection circuit will have a better chance of protecting the FETs but there's still a chance that 1 or more will fail if there is a problem.

It may be possible to modify the protection circuit to better protect the output transistors but I haven't tried it on these amps so I cannot give you any definitive information on modifying it.

If the drive signal looked OK and the amp doesn't have any shorted inductors, it should power up without any problems.
 
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