Fosgate Punch 40x2

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#2
I have repaired many of these DSM amplifiers and most times it is a problem in the AF drive circuits associated with the distorted channel, which are all surface mount devices and not the easiest to troubleshoot. Most times it is a small SMT transistor that is shorted and causing one half of the outputs to bias into cutoff and thus output is half wave and while intelligible, it is severely distorted. Hope this helps.....V
 
#3
It's not likely to be a serious problem. As long as the board is in good condition, it's likely to be an easy repair. If you have an oscilloscope, post a photo of the distorted waveform. If there's no significant DC, have a speaker or dummy load connected when you photograph the waveform.
 
#4
I worked on a 4 channel of that same era that had a broken drain leg on one of the outputs. Also check for open or melted out .1 ohm 2 watt resistor. Either of the 2 above problems will result in distorted output. I agree with Perry in that it is probably not a serious problem.
 
#5
Okay so I got that in the mail today a d tried it out and the right channel sounds clean but the left one sounds terribly distorted and I tried bridging but it still sounded dirty. Do you think the outputs are shorted in that channel? I haven't opened it yet but when I got it the sink was all correded like as if it was in a area where there's moister. Any help would be great. Thanks
 
#7
Perry Babin said:
The first thing you need to do is visually inspect it. If the board is badly corroded, there's not much you can do with it.
Click to expand...
Okay I looked inside and saw nothing bad but alot of dust and spider webs inside. Hey I also have another question? What if I remove the power supply transistors and put them in my punch 40.2? Do you think the are the same type of mosfets? Or would it still smoke up?
 
#8
They should work. What are the part numbers of the power supply FETs in both amps?

After you remove the blown FETs from the 40.2 and before you install new transistors in it, you need to check the drive circuit and the gate resistors. If you need help doing that, let us know.
 
#9
Yes indeed I would need help with that procedure. I have a VOM and solder iron and some solder, etc I don't know what the part number is for the MOSFET in the 40.2 because I removed them from the amp and misplaced them somewhere but I remember something like 36N0N06??? Not sure but I'm sure it's them on the 40x2?
 
#10
Are the transistors held down with clamps in both amps or are they soldered to an insulator strip in the 40.2?

With no FETs in the 40.2, connect the amp to a power supply and power it up. Measure the DC voltage on the first leg of each of the power supply FETs. It should be ~5v DC. Place the black meter probe on the chassis ground terminal of the amp when measuring the voltage.

There is a gate resistor connected to the first leg of each power supply FET. What are the markings on them?

What resistance do you measure across them (no power applied).
 
#12
Professor_V said:
I have repaired many of these DSM amplifiers and most times it is a problem in the AF drive circuits associated with the distorted channel, which are all surface mount devices and not the easiest to troubleshoot. Most times it is a small SMT transistor that is shorted and causing one half of the outputs to bias into cutoff and thus output is half wave and while intelligible, it is severely distorted. Hope this helps.....V
Click to expand...

Yes! That's exactly what I'm experienceing with the amp. And would you replace those? I honestly thought the outputs were bad on that side.
 
#13
Hi all, I'm refreshing this old post because I'm having the same problem.. how do you have solved the channel distortion? My board seams to be in good condition, I cleaned and restored the varistors. Were the power supply FET's?

This is the scheme of the board. Hope you can help me, I'm so affectionate to my old school amplifiers! 🙂
 

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