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Old 28th January 2009, 08:02 AM   #21
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Measure the DC voltage on the LM311 near R140/142. Place your black meter probe on pin 4. Measure the voltage to each of the other pins.

Don't let the probes slip and don't touch them. All pins have ~negative rail voltage on them.

'Across' means one probe on each of the terminals of the resistor.
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Old 28th January 2009, 08:25 PM   #22
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Ok,

pin 1 - 0.00v (hard traced to pin 4)
pin 2 - .036v
pin 3 - 1.537v
pin 4 - REF (hard traced to pin 1)
pin 5 - 11.65v (hard traced to pin 6)
pin 6 - 11.65v (hard traced to pin 5)
pin 7 - .247v
pin 8 - 11.75v

All measurments at idle with no signal, DC voltage scale. Thanks!

- Matt
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Old 28th January 2009, 08:34 PM   #23
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Q110 may have failed.

Using pin 4 of the LM311 for the reference again, post the DC voltage on all 3 terminals of Q110 (if it's not obviously defective).
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Old 29th January 2009, 05:57 AM   #24
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MatthewS,

Hello! No, I have not worked on my 6500D in sometime now, a month or two ago I actually reassembled the amp so the board does not get damaged as I have been moving the amp around to make room for other things & I was afraid the board was going to get damaged from something possibly falling inside it (which already almost happened). I thought I lost some of the screws, the logo, the heat sink transistor clamps etc. however, I was able to locate it all, so it is complete.

The last time I tried to check what Perry asked me to check the amp was acting kind of funny; it seemed like when I tried to check, R133, Q110, R112 (I cannot remember which one it was but) every time (or most of the time) I put my multi-meter probes on one (maybe more) of those parts trying to check the DC voltages the amp would start oscillating on its outputs, the oscillations were clean at first, but then they would start to get really distorted on my scope & after a while they would stop again, while oscillating though the amp did have audio output, although I noticed while adjusting one of the controls on the amp (I do not remember which one) the speaker would pop or thump really bad. It seemed like every time I tried to check the voltages on those parts it would start oscillating, I checked the solder connections on & around those parts, I even tried pressing down on them with a plastic tool to try to make them make contact, but nothing, it only did it with my multi meter or scope probe.

I never did get around to posting what was going on & what I did find; I figured sooner or later I would get to it. Here I am a year later & the amp is still sitting. I really want to repair this amp & I have not given up, just need to sit back down with it & go over what Perry asked to check again. I will follow along with yours, as it seems like a similar problem. Good luck with your repair! Take care!
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Old 29th January 2009, 08:39 PM   #25
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Ok, update time.

Perry,

Using pin 4 of LM311 for GND REF, I measured the following on q110:

Pin 1 (base) - 5.85v
Pin 2 (emitter) - .003v
Pin 3 (collector) - 59.3v

Which tells me this transistor is off in the weeds somewhere. I threw the board back under the magnifier, and sure enough the top was tinted. It looks like it thermaled it's life away. I'll have to pay closer attention to this in the future.

For posterity this transistor is labled G1 and I have that as;

BC847
BFS20
BF199
MMBT5551

Can I use the MMBT5551 here? it's got higher ratings..

I'm going to put these on my next order and check back in next week when this thing starts making noise again. Thanks a heap for all your help Perry!

- Matt
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Old 29th January 2009, 08:47 PM   #26
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Yep. It's toast.

In the future, use a colon instead of a dash for the separator. My feeble mind thinks 'negative' when I see the dash.

G1 = MMBT5551LT

http://www.onsemi.com/pub_link/Colla...T5550LT1-D.PDF
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Old 30th January 2009, 07:21 PM   #27
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I forgot I had 9 of these left from a fosgate 200DSM I've been slowly bringing back to life. I popped the replacement 5551 into the amp today and it roared back to life!

I ran it for about an hour @ around 50 watts of output while I cleaned up the rest of the chassis, etc. Clean sine wave, no thermal issues, no other problems detected. It's ready for a home.

It's a very nice feeling to know it will be making music again somewhere. I hate to see an amp go to the dump and thats where this one was headed when I got it for free.

Note to anyone who finds this to be an issue; Remove the two through hole caps that box this part in before removal and replacement. It's not necessary but if your not careful you'll burn them.

Thanks a heap Perry! Your contributions here are invaluable!

- Matt
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Old 8th February 2009, 11:56 PM   #28
fredrow is offline fredrow  United States
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Hi, I have been looking over this post for a little bit now. Im a college student taking classes for electronics Tech. I bought a MTX 6500D amp off of ebay, and the guy said that it powers up but dosent get any audio going to the subs. I plan on working on this amp next week, I just got one little question. Im going to hook the amp up to a DC power supply in our lab at school. What should I do with the remote lead from the amp?? Do I even need to worry about that??
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Old 9th February 2009, 12:02 AM   #29
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You have to apply 12v to the remote lead for the amp to power up.

Before you do any work on the amp, read through the basic repair page (link below).

When you connect the amp to the power supply, you'll need to insert a 10 amp fuse in the B+ line (unless the power supplies have a current limiting function).
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Old 9th February 2009, 03:10 PM   #30
fredrow is offline fredrow  United States
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Hey thanks alot for getting back to me so soon. I have went through and spent alot of time reading up on that web page, I even gave the link to a bunch of my classmates because their is so much great info on that site. I was just a little confused about how to hook the amp up to the power supply. So should I just run the power from the PS to the ground, and the remote of the amp?? Or should it go to the ground, power, and the remote of the amp??
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