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Old 12th September 2007, 09:12 PM   #11
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Do you have an oscilloscope?
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Old 12th September 2007, 09:52 PM   #12
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Perry Babin:

Yes, I do have an oscilloscope. I was just wondering where exactly to connect it to the output fets. As I stated before, I am still learning, so I just want to make sure I am doing proper tests how it is suppose to be done and connecting my equipment to the propper test locations. I have probed the output fets with my scope when I was doing checks last time, if I had the scope connected properly, I did not see any oscillation when I had the scope on the output fets. I beleive I probed the center pin on the output Fets. I will check again later today or tomarrow when I get a chance, again I just want to make sure I am doing this correctly. Thanks again for your help!
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Old 12th September 2007, 10:17 PM   #13
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There are two pairs of FETs. One pair will have it's middle leg connected to the other pair's third leg. Those are connected to the input of the filter inductor. All of these should have rail to rail oscillation.

The following was taken from a similar amp (MTX 250D). This is the type of oscillation you should see. The timebase is set to 10us. The Vertical amp is set to 10v/div. You may have to go a bit higher (20v/div) for the 6500D.
Attached Images
File Type: jpg model250dinputtoinductorb.jpg (80.9 KB, 509 views)
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Old 16th September 2007, 08:56 AM   #14
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Ok, I finally got a chance to do the checks on the amp.

Here is what I checked: I checked the audio on pin-1 of the op-amp to see if it changed after remote was applied, it appeared to be heavily clipping and did not change at all as I monitored it. I checked pin-7 and the waveform looked clean but also did not change any after the remote was applied.

I checked the output transistors for oscillation as you asked and it is not oscillating, I probed around the area to see if I could find anything that was oscillating that is connected to the output transistors & nothing else seemed to oscillate either. As I probed around the outputs again it looked like there was some oscillation starting so I checked the outputs again and they were oscillating for a moment, but then stopped and I have not seen oscillation since, not sure what that means.

When you mentioned the MTX 250D I remembered I also have an MTX RT251D, which is probably similar to the 250D. I repaired the RT251D a while ago and that amp only had 1 bad output FET, which of course was an easy repair replacing both FETS after I found the problem. That little amp worked fantastic in my sister’s car for months when she used it, and it still works great! Though it has been sitting in a box for a while.

Anyway, I opened the RT251D up so I could see the oscillations on the output FETS that you showed me in the attachment. The amp oscillates just like the one in the picture. I am glad I got to see the oscillation first hand on the RT251D, as it makes it easier knowing what I am looking for. The RT251D is very similar inside compared to the 6500D, though much, much smaller.

But again, the 6500D did not oscillate on the outputs. Maybe the amp is muted as it has continuous clipped audio on pin-1 of the op-amp.

Perry Babin: Thank You for all of your help!!! It is Greatly appreciated!
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Old 16th September 2007, 06:52 PM   #15
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This is a self-oscillating class D amp so it won't have any oscillation unless there is oscillation on the outputs.

Push the output transistors to the side (gently) to see if any of the legs on them are broken. They sometimes break where the leg enters the body of the transistor.

While the amp is on, move/twist the inductor near the outputs. If it's shorted, it could cause the amp to go into protection. I don't know if the overcurrent protection is latching so you should try disconnecting and reconnecting the remote input to see if it begins to oscillate after moving the inductor.

There are two 475 ohm resistors (R140/R142 -- marked 4750) near the large surface mount resistors. What is the DC voltage across each one? Check it with and without signal.
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Old 17th September 2007, 05:02 AM   #16
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I checked the legs on all of the output transistors and none of the legs appeared to be loose where they go into the body as I gently moved them.

I tried to move/twist the inductor by the output transistors, though it did not really move as it looks to have light coating of glue on it. I disconnected and reconnected the remote to see if it made any difference but it did not start oscillating. One thing I noticed is when I first apply remote to the amp I see what looks like light oscillations starting, but fades back out a little over 1 second later, it does this every time I connect the remote lead.

I checked the DC Voltage on R140/R142, if I did it right. I connected my volt meter probes on each side of the resistor under test, so basically I had the probes across the resistor. Maybe I checked them wrong. (NOTE: the large surface mount resistors by R140/R142 get extremely hot, may be normal for them, but it seems a little to hot for there size)

Here is what I got:

R140) – 0.00 VDC with & without signal)

R142) – 0.00 VDC with & without signal)
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Old 17th September 2007, 06:38 AM   #17
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Check the following components with the amp off then measure the DC voltage across them with the amp on (no signal). For the transistor, measure the voltage from base to emitter and from collector to emitter.


These are part of a constant current source. If there is no voltage across those resistors, the CCS probably isn't doing its job.
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Old 23rd September 2007, 08:45 PM   #18
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Hi all

I have the same amp in my pickup, and some times is muted, no have audio but the amp still on after it silent completely,

but I corrected the problem when solder the "bad joints" pads of the sot-223 A56 A06 and sot-23 2G 1G transistors

sot-223 trans produce to much heat, and transfer it trough pcb and some opamps, making hard to troubleshoot.
Hi Speed
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Old 1st October 2007, 05:32 AM   #19
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Hello everybody! Just want to note: I have been very busy working and have not had time to post new info on the 6500D, have not really had time to work on it. I am hoping within the next few days or within a week I will have some time, but for now it will have to wait. I have not given up on the amp, & I will post back all of the new info on it when I get the chance. Thanks again for everyone’s input & help, it is greatly appreciated!
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Old 28th January 2009, 06:44 AM   #20
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Hi everyone!

Just kicking a dead horse here as I have one now that does the SAME thing (no output) Did the original poster get anywhere with his?

Following along this post I confirmed and did the following:

1) PS good and all rail voltages present at outputs.
2) All voltages present at *every* op-amp +/-17 volts.
3) Reflowed solder on large SOT parts
4) Checked for self-oscillation via perrys diagram - Nothing!
5) Checked for clipped audio @ pin 1 of op amp near high level input - Yes!
6) Measured *across* R140 & R142 and the potential was .07 volts
7) Measured each end of R140 & R142 with PS ground as reference and got over 80 volts on all ends. I assume this is what Perry meant by 'across' actually.

I inspected for physical damage to the output filter and found nothing remarkable. Also found no physical damage anywhere else. I pulled and tested the outputs and they are fine. If I put a scope on the output and run signal through it a modulated version shows up at the output that varies in intensity with the gain. Trouble is you need a pair of headphones just to hear it! The clipped audio @ pin 1 of the mentioned op-amp will also vary it's width with gain adjustment. From what I am reading this means the amplifier is being muted for some reason. Where do I go from here? Thanks!

- Matt
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