# 1 Channel of 4 Channel Amp Failed



## TEKWRX (May 30, 2017)

After taking a long break from car audio, I slowly started to get back into it after I replaced the surround on my JL 10W6v2 that hasn't worked for many years. Once I got that up and running, I replaced my door speakers with Focal 6A1's and got a new head unit to run active (Pioneer FH-X730BS). I re-checked my wiring and ran new speaker wire for the tweeters to allow active mode, and all seemed good. I have been trying to get the tuning done over the last few days. Anyway, today I notice I have no sound output out of the left door 6.5". I check everything. Swapped RCA cables, swapped drivers, checked to see I have continuity back at the amp (reads 4ohm at the amp, so I know the speaker wire is good). 

The only thing it has to be is that the rear left channel failed on the amp, right? How can I check? What would cause something like that? The rear right channel is working fine. The amp in question is a Polk Carbon MOMO C400.4. I'd like to get it back up and running because I have the matching Polk C500.1 for the sub. I'd hate to have to look for a new setup just as I was getting back into the hobby, especially because I'm out of work right now and don't have much money for new gear unless I can sell a few things first.

If I do have to get a new setup, what are some decent amps to look for these days? Is it better to look for a 5 channel? Or I guess I can look for a 4 channel and run the Polk mono amp for now until I can save up and get a matching mono amp for the new 4 channel (I'm a bit OCD about gear matching LOL)

TLDR: What would cause 1 channel to go out while the other 3 are fine? What can I look for to troubleshoot if i open the amp up Thanks?


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## miniSQ (Aug 4, 2009)

swap the speaker wire leads...i the right channel is playing music and the left is not. Hook the left speaker wire to the right amp and see what happens.

If nothing changes..then flip the RCA's and see what happens.


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## TEKWRX (May 30, 2017)

miniSQ said:


> easiest way to check a channel on an amp is to flip the RCA L&R...if the "same speaker" is not playing when you flip the rca's it is not the amp.



Yeah, I guess I wasn't clear in my above rambling. I did check all the wiring, both RCA and speaker wires. When I swap the speaker wires at the amp, the left speaker started working and the right went out. So that means it's a bad channel in the amp, right?


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## miniSQ (Aug 4, 2009)

TEKWRX said:


> Yeah, I guess I wasn't clear in my above rambling. I did check all the wiring, both RCA and speaker wires. When I swap the speaker wires at the amp, the left speaker started working and the right went out. So that means it's a bad channel in the amp, right?


yup amp channel may be bad....and i deleted my first post because i was wrong in what i wanted you to do swapping RCA's. That would have been to troubleshoot the source device.

Is there any signal processor in the chain that could have muted that channel?


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## TEKWRX (May 30, 2017)

miniSQ said:


> yup amp channel may be bad....and i deleted my first post because i was wrong in what i wanted you to do swapping RCA's. That would have been to troubleshoot the source device.
> 
> Is there any signal processor in the chain that could have muted that channel?


Nope. Not running any DSP yet. Just doing active from the head unit. I guess the next step is to pull the amp out of the car and open it up? It really sucks because I just spent the last 5 days getting everything installed (it's been raining here and I have no garage) and was just getting somewhere with the tuning. It was nice to have some good tunes in the car again.... I do have a really old Sony XM-405EQX 5 Channel I could temporarily hook up while I take the Polk apart. But I haven't powered it up in 20 years so who knows if that still works (plus it's about half the power of the polk, sucks  )


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## miniSQ (Aug 4, 2009)

TEKWRX said:


> Nope. Not running any DSP yet. Just doing active from the head unit. I guess the next step is to pull the amp out of the car and open it up? It really sucks because I just spent the last 5 days getting everything installed (it's been raining here and I have no garage) and was just getting somewhere with the tuning. It was nice to have some good tunes in the car again.... I do have a really old Sony XM-405EQX 5 Channel I could temporarily hook up while I take the Polk apart. But I haven't powered it up in 20 years so who knows if that still works (plus it's about half the power of the polk, sucks  )


did you flip the RCA's to rule out the Head unit? Amps are generally more bullet proof than HU's


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## TEKWRX (May 30, 2017)

miniSQ said:


> did you flip the RCA's to rule out the Head unit? Amps are generally more bullet proof than HU's


Yeah, I did that first. But I guess I can try again after i redid the speaker connections. Should I swap them at the amp or head unit? Doesn't matter right?


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## TEKWRX (May 30, 2017)

OK, I just went out and swapped the RCA's at the HU. It made no difference. The left speaker is always out. I notice know when playing around with the balance slider and other functions on the HU I hear static I never heard before, and even with the RCA's for the tweeter channels disconnected while testing, i still hear a very faint sound coming out of them. Is that just from crosstalk inside the amp? 

Anyway, I'd LOVE for the HU to be the problem, but I'm still within the return window, but I think it might be the amp. Any other ideas?

EDIT: Is it safe for me to connect an AUX cable from my phone directly to the RCA inputs on the amp and see if I get sound out of both speakers? That would rule out the HU. Should I turn the gain way down if I try it?


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## miniSQ (Aug 4, 2009)

TEKWRX said:


> EDIT: Is it safe for me to connect an AUX cable from my phone directly to the RCA inputs on the amp and see if I get sound out of both speakers? That would rule out the HU. Should I turn the gain way down if I try it?


yes that would be my next suggestion...eliminate the HU by playing music from your phone or other source. Just do with the the system turned off. Gains should not matter, but it can't hurt to turn them down.


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## TEKWRX (May 30, 2017)

Well, I think that confirmed it. Playing my phone with an aux cable connected to the rear channel RCA's on the amp gives me sound in just the once speaker, same as the head unit. So it's the amp. I guess the next step is to un-hook everything and pull it from the car so I can open it up. I hope it's something fairly obvious. I can give it a try to repair it myself. I've been learning some about stereo repair, but I'm still a newb. I'm actually in the middle of repairing a vintage Sony TA-N77ES home power amp right now. I have a friend who's helping me and he's much more advanced than I am, but I'm not sure if he has any experience with 12v stuff. I'll post more some pics I get it opened up (figures, it just started raining so I'll have to wait a bit). Uggg, not how i wanted to spend my weekend after taking all week to get everything hooked up.

Could the fact that the amp has seen _very_ little use in the past 10 years have anything to do with why it failed?


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## miniSQ (Aug 4, 2009)

Age could be a factor...could be a contact that needs cleaning. Got any Detoxit or other contact cleaner? You could spray some in the preamp section and more all the switches back and forth a couple dozen times to see if that at least gets you a scratchy noise out of the channel. Looks like that amp has slope adj and HP/LP adjust as well ad gain. Can't hurt...and good luck. Hope you find something obvious when you open it.


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## dsw1204 (Mar 23, 2015)

TEKWRX said:


> Well, I think that confirmed it. Playing my phone with an aux cable connected to the rear channel RCA's on the amp gives me sound in just the once speaker, same as the head unit. So it's the amp. I guess the next step is to un-hook everything and pull it from the car so I can open it up. I hope it's something fairly obvious. I can give it a try to repair it myself. I've been learning some about stereo repair, but I'm still a newb. I'm actually in the middle of repairing a vintage Sony TA-N77ES home power amp right now. I have a friend who's helping me and he's much more advanced than I am, but I'm not sure if he has any experience with 12v stuff. I'll post more some pics I get it opened up (figures, it just started raining so I'll have to wait a bit). Uggg, not how i wanted to spend my weekend after taking all week to get everything hooked up.
> 
> Could the fact that the amp has seen _very_ little use in the past 10 years have anything to do with why it failed?



It is possible that is the reason it failed, but maybe not. It really doesn't matter why. You need to get a new amp or get that one fixed and that may not be easy or inexpensive. What will be your budget for the new amp?


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## TEKWRX (May 30, 2017)

dsw1204 said:


> It is possible that is the reason it failed, but maybe not. It really doesn't matter why. You need to get a new amp or get that one fixed and that may not be easy or inexpensive. What will be your budget for the new amp?


I still need to try and figure out why it failed, so I know if it was something that can be avoided (problem with the install, wrong gains/settings etc...) or if it was just age. Anyway, I will remove it from the car when I get a chance if it stops raining tonight.

As far as budget for a replacement, has to be as low as possible (being out of work sucks). I'm not sure of what brands and models to avoid or look for these days, I haven't paid attention to the scene in almost 15 years. I did see a guy selling a JL 300/4 for $100 shipped on another forum. That's seems like a good deal?

In the meantime, I guess I'll hook up one of the amps I just found in the closet so I can at least have some music while I figure out what to do about the Polk amp. Which of these models would you guys think would be the best for the Focal Access 6.5 & tweeters for the time being?

Sony XM-405EQX (40wpc-I'm leaning towards this because I used to run it, but it may not fit in the space and I don't need the sub channel)
Sony XM-4301 (60wpc)
Alpine MRP-F306 (30wpc)


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## miniSQ (Aug 4, 2009)

Jl 300/4 is one of my favorite amps...but i would be skeptical of such a low price. IN good condition those amps will usually get twice that much.


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## TEKWRX (May 30, 2017)

OK, I opened it up. Unfortunately nothing is obviously wrong. No burnt traces, all the resistors and caps look good. I guess one of the output transistors could have failed, but don't they usually take out other parts when they go? I guess I can hit all the pots and switches with DeOxit and see if that helps anything, but I'm not hopeful. Many compared to the vintage stereo receivers/amps I've been working on there sure are a lot of smd parts in this thing. I have zero experience soldering smd stuff, LOL. Anyway, I guess I should hunt online for the service manual. Here are pics of the inside if anybody has any ideas. The rear channels (left rear is the bad channel) are on their own board. Also, the RCA connectors are firmly attached to the board


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## TEKWRX (May 30, 2017)

Also, since I haven't really worked on 12v stuff before, how do I go about testing and troubleshooting out of the car? I have an old DC power supply thing, will that work? Or should I use one of these little 12v batteries I picked up from Radio Shack a few weeks ago when they were closing?


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## TEKWRX (May 30, 2017)

I guess I have good news? After cleaning all the pots and switches with DeOxit, I hooked the amp up to the DC power supply and plugged in some 4ohm desktop speakers I have....*and I have sound out of both rear outputs!* 

I really hope that was all there was to it, but I'm doubtful. I will put the amp back in the car tomorrow and see what happens. Fingers crossed. 

I'm not sure what the issue would be if it doesn't work again. Like i said I don't think it's the speaker wires because when I check them in the truck with my DMM I get a reading of ~4ohms, that that should tell me the wire is making good connection to the driver and isn't cut or shorted somewhere along the way to the door, right? Swapping around the RCA's at the HU didn't change anything, I even ran a different temp pair of RCA draped through the cabin into the truck and that didn't help. So if I still have the same issue in the car tomorrow, what else to do? Get a new HU? I'm still under the 30 return window...

EDIT: Here's a picture of my ghetto test setup on the living room floor. My wife will be thrilled when she gets home from work . I'll leave music playing for a while to make sure everything's good.


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## miniSQ (Aug 4, 2009)

That is good news, but the question is did you have sound from both outputs using the power supply before you used the DeOxit?


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## TEKWRX (May 30, 2017)

miniSQ said:


> That is good news, but the question is did you have sound from both outputs using the power supply before you used the DeOxit?



You're right, I should have thought to test it before the DeOxit, but by the time I thought about using the power supply I already had the amp all pulled apart. Anyway, I put it back in the car this morning and everything is working and sounding great, knock on wood.


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