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Pioneer621 11th September 2007 08:50 AM

MTX 6500D - No Output
Hello, I am working on an MTX 6500D car amplifier, I am not exactly sure where to begin on this one, so I thought to ask for help here. I have repaired a bunch of amps already, but I need to learn more.

Here is what the amp does / what I checked: It powers on but produces no sound. The power supply is operating fine as it is generating about +/- 59 Volts on the rails. I checked the output transistors (in circuit) & they seem to be fine, though I should probably remove them for proper checking, but again, I think there ok. I tested the voltage on the op-amps and it is about +/- 17 - 18 volts, one thing I thought was strange, though may not be a problem, is the Positive voltage on the op-amps is about +18 volts & the negative voltage on the op-amps is -16.97 to -17 volts, seems like it should be the same voltage for the +/-, but that may be normal and not far enough off to be an issue. I used the ground terminal on the amp for my reference. Maybe I should be using something else as my ground reference?

There are a couple large surface mount resistors that get hot after the unit is on for about 1 minute, there are also some large surface mount transistors that get hot after about 1 minute, that may be normal, but they get hot enough I can not touch them for very long. The power supply & output transistors stay cool to the touch. I did all the checks with current limiting resistors to help prevent damage while testing. I also did some probing on some large surface mount transistors on the op-amp side of the filter caps & some of the voltages seemed a little high on 2 compared to another one next to them, though when it comes to correct voltages on transistor in those areas I am not exactly sure what I am looking for, so that bit of info may be useless.

At this point I am not really sure what to check, I am still learning, and the surface mount parts are a little different than the through hole components I am use to. I am thinking maybe some transistors in the driver stage are bad, & possibly the protection circuitry is active? There is no DC on the speaker terminals, but if there is DC protection that would be why. Again, I am not really sure what to check or what I am looking for from here, maybe someone can point me in the right direction as I would like to repair this amp, & this amp will be perfect for learning on.

Since at this point I am not sure what to check, what exactly should I be looking for in here, how do I know if the protection circuit is on? If the protection circuit is on, if I new how to disable it, it may be easier to trouble shoot with it temporarily disabled, as that is what I did to a Coustic 600SE to find a bad driver transistor that was causing DC on that channel, it was much easier finding the problem after the protection circuit was disabled, though that amp was a little easier to work on as it uses mostly through hole components. So how do I locate the protection circuit?

MTX told me they will not release schematics, so thats out.

Ok, I thank everyone who took the time to read all of this & anyone who replies back with some info! I hope I covered everything. I look forward to getting some help!


I tried to attach a picture of the inside of the amp, but I guess it is to big of a file, I tried resizing it, but I still could not get it right, but I do have pictures of the inside of the amp, fairly high quality images, I could probably email them if needed. Oh well, I tried!

ppia600 11th September 2007 01:55 PM

Very nice powerful amp, I hope you get it figured out. I had one I had to send to mtx because it was too difficult for me to figure out. Mine had bad ouputs and some preamp problems. Yours sounds like its just in the preamp section. That amp most likely puts out around 750 watts rms. All of the ones we sold put out WAY more than they were rated. Nowadays they barely put out more than the rated power.

Perry Babin 11th September 2007 03:38 PM

Do you have audio on pin 1 of the op-amp near the 4 conductor connector on the corner of the board?

Are any of the op-amps getting too hot to touch?

The NE5532s run warm but should not be too hot to touch.

Also, may sure that ALL of the op-amps have plus/minus supply voltage. There are resistors that open and break the connection to the op-amps.

The difference in the regulated voltage isn't a big concern.

Pioneer621 12th September 2007 12:57 AM

2 Attachment(s)
ppia600: Thank You for the reply! I have not had the opportunity to hear a 6500D yet, though they look like nice amps, & the guy I bought it from told me they were way under rated and do put out a lot of power. I bought this amp and a few others from a guy locally for parts or repair, I repaired the legacy LA1880 (I think that was the model) fine a while back & the old Pyramid seemed fine other than was missing some screws, but the MTX 6500D amp I have not been able to fix yet, it has just been laying around here getting moved around & I figure it is about time I fix it. To bad you had to send one of yours to MTX to fix, I hope I will be able to fix this one as I need the learning experience from this type of amplifier, if I can't fix it I will probably keep it for parts or sell it to someone who can, though I want to try my best at fixing it before I give it up that easy, so I hope for the best! It does seem amps nowadays barely put out more than there rated power, like Rockford Fosgate amps I have seen the birth sheet on, I have seen some of the newer 800 watt 2 channel amps barely put out much more than 800 watts, I have a brand new in the box Rockford 800a2 and it's birth sheet is like 1028 watts or something (it has been a while since I looked at the sheet on mine new one, but it is over 1000), quite a bit different than there newer ratings, my other 800a2 I repaired has do 900+, though I do not have the birth sheet for it. I sure do love repairing and testing amplifiers!

Perry Babin: Thank You for the reply! I think I checked the correct op-amp you are referring to. I will try to attach a picture again, though the quality is not as good since I cropped & resized it and changed the color quality. If I attach the picture ok, the op-amp I believe you are referring to is the farthest one to the right in the picture; I get virtually NO audio output on pin-1, I can see the very slightest bit of audio on my scope, I almost did not notice there was an audio signal there, but nothing like the op-amp by the RCA connectors that I also checked for audio output on.

Most of the op-amps seem to get pretty warm, though the one just to the left of the one I tested for audio on gets pretty darn hot, I can hold my finger on it most of the time, but it is pretty hot, I would say pretty much to hot to touch, I also checked that one for audio with my oscilloscope on the outputs and it has (-) 15.5 vDC on the outputs pin-1 & pin-7, as I then checked it with my DC Volt meter to see why it was not an audio reading on my scope. So maybe that op-amp is faulty or something before it in the path? Are there other op-amps I should check for audio input / output on?

All of the NE5532's have Plus / Minus Supply Voltage on there V+ & V- inputs.

Thanks again ppia600 & Perry Babin for the replies, I am learning more!

Perry Babin 12th September 2007 01:09 AM

If the op-amp is too hot to hold your hand on and is significantly hotter than the others, it's either defective or being loaded heavily. If you have a replacement and can remove it without damaging the board, replace it. I've seen a lot of them fail in this way and although using the temperature to find the defective one isn't 100% foolproof, it works most of the time.

Pioneer621 12th September 2007 01:53 AM

Thanks for the fast reply Perry Babin! I do not have any NE5532's on hand, so I will have to order some. By any chance do you know a supplier that carries them that does not require really large quantity orders? I checked digikey & mouser, they seem to carry them, are there any others?

I am not sure when I will be able to order any parts so it may be a while, though when I order them for this amp, I will buy parts for one of my personal amps that need new output transistors.

Are there other things I should check on the 6500D that could also be bad that I may need to order as well, or should I wiat until I order the NE5532's to see if that is the only problem?

I think I will be able to remove and replace the NE5532 ok, though it is very small. I did some work on a keyless entry remote for a car, and had to remove the surface mount I.C to repair some traces after the battery jammed into the I.C and partially ripped it off the board, The remote worked for a while after the repair, but the traces were damaged badly, and I guess my repair was not the greatest, I repaired it twice and did not attempt it again after it went out, though I could probably get it working again if I had to. If the traces were not so badly damaged, I think it would of went smoothly just replacing the I.C.

Thanks Again for the reply / info!

Perry Babin 12th September 2007 02:24 AM

Check the solder connections on all of the large surface mount transistors.

The VCA (Voltage Controlled Amplifier) sometimes fails and produces a DC offset but I think this amp may have a DC blocking capacitor in series with the VCA's output. This was a revision they made to some of their other amplifiers. If there is no capacitor in series with it, it could cause DC offset (if it's defective).

Mouser has the NE5532D for $0.80each. Mouser and Digi-Key are your best choices (in my opinion) unless you want to buy large quantities.

If you're not sure you can replace the IC without damaging the board, buy the smallest quantity of ChipQuik available (or order a sample from the company if you can wait a week or so). It's a low temp alloy that will allow you to remove the chip with less heat. You solder the IC back in with normal solder.

Pioneer621 12th September 2007 08:08 AM

Perry Babin,
I made a mistake when I checked for audio on pin-1 of the op-amp by the 4 conductor connector you asked me about. The reason I was not getting any or very little audio on pin-1 was because I had the amplifiers gain potentiometer all the way down from the last time I did tests on the amp! I feel like an idiot! As soon as I turned up the gain, the I.C was putting out audio. So the op-amp you asked me about does put out audio on pin-1.

The other op-amp to the left of that one is of course still getting hot, but it looks as if its inputs are connected to one of the large surface mount transistors by it, maybe thats why it puts ( - ) 15.5 VDC on its outputs? Possibly suppose to do this? Looks like it feeds some very small transistors at its outputs.

I checked the solder connections on the large surface mount transistors and they all look good so far.

I beleive the VCA (Voltage Controlled Amplifier) you are refering to is the THAT 2155 I.C? I viewed the datasheet for it, but how should I check it to see if it is not operating properly, should I look for DC on the input / output pins?

Ok, just wanted to give an update to my previous error! with the amps gain control.

Also, I looked at the ChipQuik you mentioned. I read about it and it looks like some nice stuff, I might have to get some when I need it, as it seems it may make removing the surface mount I.C's really easy and clean. Thanks for the tip on that stuff!

Perry Babin 12th September 2007 02:11 PM

If I'm not mistaken, when the amp is muted, the audio on pin 1 (maybe pin 7) of that op-amp has rail-rail clipped audio and as the muting is released, the signal no longer is driven to clipping. Does the signal change on yours a few seconds after you apply remote (after the mute delay)? Check both pin 1 and pin 7.

If you're getting audio to the op-amp mentioned above, the VCA is OK (or, at least working well enough to pass audio).

Is there any oscillation on the output FETs?

The op-amp that's overheating could be the output buffer for the preamp output jacks just to the left of the IC. The outputs are connected to diodes (circuit board designation D80 and D81 -- the D tells you it's a diode).

Pioneer621 12th September 2007 09:08 PM

I have not checked pin-1 or pin-7 yet to see if the signal changes after I apply remote to the amp. I will try to check that later today or tomarrow.

One more question, how do I check for oscillation on the output FETS?

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