Rockford T3002 Shorted Channel - diyAudio
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Old 15th September 2011, 09:48 PM   #1
yatser is offline yatser  Canada
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Default Rockford T3002 Shorted Channel

Hello Everyone!

New to the forum, so a little background information. I've have a few amplifiers lying around for a few years, one of which (the T3002) I would really like to not only use, but fix for experience. I have two Xtant amplifiers and an Alpine MDR to fix as well, but those were given to me, I'd like to start with my own. So this thread is dedicated to fixing the T3002.

First, the symptoms. Wired it all up in the second vehicle it's been in, and had previously worked fine. As soon as the remote lead came on, the smoke came out of one of the two amps, and my component set was done for as well. I looked over the install time and time again and could find no external source to blame. I have a diploma in Electronics Technology, and I have 7 years of mostly part-time but some full-time professional installation experience.

I have the schematics. The right channel negative has ~-39V on it. There is another thread that hinted this was a common problem, but that thread didn't seem to be going anywhere, thus the new thread.

Q218 and/or Q222, both 28N15's, appear to be either shorted, or forced into saturation. Bear with me on this, because I went into software development haha, so I'll be re-learning as we go.

Is anybody familiar with this problem, with the next troubleshooting steps?
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Old 15th September 2011, 09:55 PM   #2
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Of those two transistors, find the one with the lowest reading from leg 1 to either of the other two legs and remove it from the circuit (cut two legs free).

Does the short disappear?
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Old 15th September 2011, 10:06 PM   #3
yatser is offline yatser  Canada
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Yup, the short is now gone... which would indicate that the Q222 side was the problem...

Next step, replace? Or must we verify that the gate voltage was not the issue with this circuit...

PS, Perry, your posts and your website is awesome and I was hoping for some help from you! I intend on buying your DVD at some point to show my support and maybe fix more than just these first four amplifiers!
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Old 15th September 2011, 11:28 PM   #4
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With a 15 amp fuse in the B+ line and the transistors clamped to the heatsink, power up the amp. If the fuse doesn't blow and there is no more than 0.1v of DC across the speaker terminals of the blown channel, play the amp to see if it produces clean audio. If it does, Confirm that the 0.1 ohm source resistor and the gate resistor are within tolerance. If they are, you will need to replace both of the 28Ns in that channel. For the best reliability, you may want to replace all 4 outputs in that channel.

If this is all that's needed to repair this amp, it's definitely going to be the easiest of the amps you listed. Start new threads for each new repair.

If you're not familiar with the MEHSA insulators, read this:
t30001bd schematics
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Old 16th September 2011, 01:29 AM   #5
yatser is offline yatser  Canada
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That is a great way to do the the insulators. I already desoldered the one using a different method... But i like the cleanliness of method described. I cant stand subpar soldering.. But especially when its from me lacking a better method.

The channel is less than 0.1v, tomorrow I will try audio, and measure the resistances. I will replace all four likely regardless. Can you describe the logic behind needing to replace them both if the resistors are within tolerance?
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Old 16th September 2011, 04:28 AM   #6
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If they're closely matched in all parameters, they're more likely to share current evenly. Replacing them at the same time, likely means they'll be from the same production batch which means they'll likely be closely matched.

You also have to realize that they were in parallel and something pushed one beyond what it could withstand. The other one may have been injured (even though there are no apparent signs of damage).
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Old 16th September 2011, 04:31 PM   #7
yatser is offline yatser  Canada
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Ok, right, I guess I was mislead by the need for the resistors to be in tolerance. Since that transistor is blown anyway, even if the other resistors weren't in tolerance, I would still have to replace both to get a match, as well as a bunch of other components.

The resistors are in tolerance. I have not played any sound yet.

I have much easier access to an FQA32N20C, which is a cross referenced part at close specs. Unless there are any objections I will order 4 of them and replace the entire channel, and put this thread on hold until I have some results.
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Old 16th September 2011, 09:28 PM   #8
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Some Rockford amps are VERY sensitive to changes. One example is the IRF540 and the IRF540N. The N version can cause all sorts of strange problems even though the specs are very similar. I'd suggest ordering the correct parts if they're available. Do NOT order them from eBay.
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Old 21st September 2011, 02:17 AM   #9
yatser is offline yatser  Canada
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I ended up trying the 32N20C's since I received them anyway, and they seem to be working great. The amplifier is back in the car and working. Thanks a ton for your help Perry.

I'm not sure when I will tackle any of the remaining amps, I am way too busy at the moment. They might even have to wait until the spring. But either way, I know where to come for help
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