MTX 6500D - No Output

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I do not see any solder bridges & there are no direct shorts.

Pad 2 to pad 3 shows 100 ohms

Pad 2 to pad 4 shows 51 ohms

But no direct shorts anywhere.

And of course pin 2 & pin 6 are connected together. Those are the only somewhat low resistance readings I get on any of the pins & are possibly normal.
 
Thought I would get this amp back out & bump the thread. I figured I would wait a little while to post & let it sit to see how it was acting & see if it would still oscillate after sitting a while.

I still had the 1.2K resistor lifted (the one I put in place of the VCA) but I forgot, & it had no output. I opened it up & seen that it was lifted so I soldered it back in & it has output, but I know there are still some problems somewhere.

It has not stopped working / oscillating yet, I have had it playing music at a low level for over a half an hour. So it has been consistently playing & the audio sounds clean.

As posted back a couple pages, when I adjust the gain the speaker flutters / moves / pops real bad still.

When I adjust the FREQ adjustment it sort of flutters still too, but not as bad as the gain & less at the lower settings.

I still have yet to place an order for any parts, just have not been able to.

Anything else to check?

Thanks again for all the help.:)
 
I pulled some of this from post #84, but re-measured the outputs of each op-amp again & noted it here (very close to what it was when last checked, most are definitely higher then 0.05v; op-amp “F” is the only one close to 0vDC, the rest are right around half a volt, not sure if that would be considered excessive, but a good possibility).

The op-amp I have labeled “E” still has -15vDC on both of its outputs. Pin 1 looks like it connects to D81 the case is marked D4. Pin 7 connects to D80 which appears to be marked D4 with a 2 right after that with a line above it, looks like D42, just the 2 has the line above it. It appears D80 & D81 are connected at the common leg of each package. The -15vDC is on both sides of D80 & D81, it is just slightly higher on the side not directly connected to the op-amps outputs.

For reference, below is the DC voltage on the output pins (pin-1 & pin-7) of the op-amps, using the ground terminal of the amp as my ground reference (gain at maximum, no RCA’s or speakers connected):

Op-amp (A):
Pin 1: 0.585vDC
Pin 7: 0.716vDC

Op-amp (B):
Pin 1: 0.470vDC
Pin 7: 0.567vDC

Op-amp (C):
Pin 1: 0.469vDC
Pin 7: 0.467vDC

Op-amp (D):
Pin 1: 0.462vDC
Pin 7: 0.461vDC

Op-amp (E):
Pin 1: -15.61vDC
Pin 7: -15.61vDC

Op-amp (F):
Pin 1: 0.001vDC
Pin 7: 0.012vDC

mtx-6500d-inside-opamps-edited-1.jpg
 
Here is the DC voltage on all of the pins of op-amp E.

Just to note again since its been a while, most of my voltage readings slowly climb as the amp sits on & some lower back down, so they may not be exact every time I check them, but it is always very close. Example: Pin 2 measured 0.389vDC when first checked, a couple minutes later measured 0.402vDC, then again a couple minute later it started to drop to around 0.36vDC, so not a huge change. I let it warm up for a minute then took the following readings.

Still very close to what it was in post #100 after op-amp E was changed.

Op-amp (E):
Pin 1: -15.7vDC
Pin 2: 0.402vDC
Pin 3: -5.07vDC
Pin 4: -16.91vDC
Pin 5: 0.401vDC
Pin 6: 5.96vDC
Pin 7: -15.7vDC
Pin 8: 18.56vDC
 
Well a quick update on this; (Alright not so quick.:D)

The output has stayed consistent (meaning every time I hook it up it produces audio) since I replaced Q110, so the “No Output” problem is fixed.

The only other problem was the speaker “fluttering” or jumping around real bad when adjusting the GAIN & crossover potentiometers. The output amplitude was not steady either.

Perry mentioned it could be the potentiometer, & after recently getting an old Rockford amp and the speaker fluttered when adjusting it & did basically the same thing, I figured that’s what it was in the 6500D.

I cleaned the crossover pot, but I made the mistake of unsoldering the GAIN pot, which Perry mentioned too much heat may cause damage (it did), lesson learned there, but thought I would give de-soldering it a shot, should have been more careful. Two of the pins lost there tension & contact to the resistive elements.

To try & get it to work again, I managed to heat the pins up pretty hot to where the plastic got soft again & then I pushed down the plastic with a small screwdriver right where the legs connect underneath, I then held the plastic down until it cooled, while I don’t think this will be reliable, it did get the contacts back making connection again.

The GAIN pot was filthy & likely the main problem with the pots, I cleaned it as good I could. Then reassembled it & installed it back into the amp being careful not to heat the pins too much.

The amp now seems to work fine, no speaker fluttering or popping when adjusting the GAIN or crossover pots, & the audio output amplitude stays steady.

Now all I need is to find a replacement potentiometer for the GAIN & it should be done. I have had this amp for a long time now so I plan on keeping it, hopefully the GAIN pot holds up until I get a replacement; this also means I am not too stressed about getting a replacement.

So I guess I am pretty much done with this now. I want to thank Perry Babin for all of the help as I could not have got this amp working without him;

So Perry Babin, Thank You!

I hope this thread helps others down the road! Looks like it already helped a few people.:)
 
I tried to be careful & as quick as possible, but had a hard time with a couple of the pins while de-soldering. I probably would not have tried it on a customer’s amp, would’ve been a last resort, but thought I would practice on mine, maybe not the smartest idea! At least I got a feel for how sensitive they are.

I did read about the low temperature & using the low temp solder, but since I did not have any of the solder, I thought I would go for it anyway. I will be more careful next time; at least I know for sure it was the pots causing the problem.

I used the other method for cleaning the crossover pot & that worked fine, would have probably worked on the GAIN well too.

Thanks again!:)
 
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