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LazySteve 15th June 2011 03:17 PM

Rockford Fosgate 60ix FETs blown
Hi I'm new here, can I say first of all how much help I have already got just from searching around this forum which is excellent, and I think I have got this mostly sussed out already, I just want to check a couple of things if anyone is kind enough to be able to help.

So, I have a Punch 60ix with power supply failure. It draws 100mA from a 12v source but that's it. There are two gate FETs which are IRFZ40s and which test as shorted (and there is some blackening around them). Also I can see what I believe are the two gate drive resistors associated with these FETs and they have gone open circuit (and they look like they got a bit hot).

The rest of the board looks nice and clean (sorry no pictures at the moment, unfortunately I am typing this from work without the board in front of me).

I can get replacements for these four blown components but I would like to order all the parts I am likely to need in one go. I have seen on here to check the driver transistors for the power supply FETs and the large Zener diodes near the FETs. I think I can identify the zener diodes as the two little bead shaped fellas near the transformer, but I'm not sure which are the drive transistors. I assume they will be assigned a "Q#" number on the board.

I am wondering whether I should test anything else or just change any of the above which tests bad and connect the power (I have noted that I will need to connect with a suitable fuse in circuit).

Also, are there any tricks for removing/replacing the FETs vis a vis the heatsink?

Thanks in advance


Perry Babin 15th June 2011 06:11 PM

What's the PC-# on the circuit board?

LazySteve 16th June 2011 11:15 AM

Hi Perry, for some reason I didn't get a thread reply notification?

Anyway, the number is PC-0764-F

The burnt resistor numbers are R31 and ??2, I think R32 but hard to read due to the blackening.

I'm a bit confused because I downloaded a circuit diagram which says it is for a 60ix but there are four power supply FETs shown on the diagram (Q6, Q7, Q10 & Q11) and my amp only seems to have two (Q6 & Q7) ??

Another odd thing I noticed is one of the little allen screws which hold the board in has got its head missing. I can only think it was over tightened in the factory because both of the warranty stickers were intact before I opened up the amp.

Perry Babin 16th June 2011 04:11 PM

This ^ should be close to what you have. If it's not, you can get the correct diagram from Rockford. The drivers are Q2, 3, 4 and Q5 on the diagram.

LazySteve 16th June 2011 09:00 PM

Thanks Perry, unlike the one I found on the net that diagram actually looks like my amp. !

Re: my original post, should I test anything else before trying power up - or just change any of the components I already listed and which test bad?

Also can I just share this - I didn't know this amp was bad when I bought it but in a funny sort of way I'm now actually pleased that it was broke because it means I can maybe save it. I know that doesn't make sense but somehow it just seems right.

Perry Babin 16th June 2011 09:20 PM

You should check the output transistors to confirm that none are shorted.

After replacing all of the defective parts, clamp all transistors tightly to the heatsink and insert a 10 amp fuse in the B+ line. This will help protect the newly replaced components if there is another problem.

While the power supply FETs are out of the circuit, confirm that the resistance between the B+ and ground terminals is well above 0 ohms. The meter should increase in resistance as the filter caps charge.

LazySteve 16th June 2011 10:22 PM

Message received and understood.

It will probably take me a few days to get the parts and work through this and then I'll report back.

I can't thank you enough.

LazySteve 22nd June 2011 09:16 PM

Well, it's working :cheers:

But only on one channel :(

Hey it does sound pretty good though !!

So now I just need to investigate the bad channel - I guess it helps the diagnosis with one good channel for comparison.

Thanks again.

LazySteve 22nd June 2011 10:26 PM

Hmmmm...... I can't find anything wrong. I already checked the output transistors before, but I checked them again and they seem OK. The resistors associated with them all seem OK (though my meter's lowest range is 200 ohms so not too accurate at reading around 1 ohm level).

A couple of things that I'm sure are clues to what is wrong are these:

1) if this channel is connected and I turn on the HU I get a bump from the speaker, so something is happening;

2) if I have everything else except this channel connected (I'm only testing with a low signal input) then plugging in the RCA feed to this channel shuts everything off; like it's making a short on the output, though I can't measure any difference between the channels input or output.

Anyway it's half past eleven here so I think I'll give it a rest for tonight.

Perry Babin 23rd June 2011 12:39 AM

With no speaker connected to the dead channel, what is the DC voltage measured directly across the terminals for the dead channel?

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