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Old 21st October 2008, 05:53 PM   #1
damage is offline damage  United States
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Default MTX 8302 problem

Hi!

I have an 8302 with high DC voltage at the speaker outputs. The outputs are fine. Disconnecting a couple of other components allowed me to trace voltage to some SMD components.

One side shows a short between B and E on Q310.

On the other side, the capacitors get hot.

So I have two needs.

1) What point should I be using as the ground reference to measure voltages.
2) Does anyone have a schematic?

Thanks!
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Old 21st October 2008, 06:59 PM   #2
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That transistor (and a 150 ohm resistor in series with its base) is across the 0.22 ohm resistors. If it's shorted, confirm that the emitter resistors are within tolerance and haven't opened.

There is no isolation between the secondary ground and chassis ground. The most convenient reference points are the chassis ground terminal or the non-bridging speaker terminals.

Which capacitors are getting hot?
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Old 21st October 2008, 08:18 PM   #3
damage is offline damage  United States
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C216 gets especially hot. Probably because it's marked 10V and there is -22V there. Though even with one leg disconnected it gets hot? Is that right? I think it is.

Other one is same value, but no PCB marking that I can find.

Emitter (Source) resistors for the outputs? One pair it's on Source, other pair is Drain. I assume these are RL.

These amps are a foreign design to me.
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Old 21st October 2008, 08:46 PM   #4
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The two 430 ohm resistors are probably producing the heat. They are connected across the output of the amp. If you have DC, they will get hot.

Yes, the 0.22 ohm resistors are on the source of the high-side FETs and on the drain of the low side FETs.

Is the DC on the output positive or negative?
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Old 21st October 2008, 09:35 PM   #5
damage is offline damage  United States
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I believe you are right that the 430ohm resistors are heating the PCB which is heating the board, transferring to the caps.

R- 20.9V
L+ -6.3V
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Old 21st October 2008, 09:40 PM   #6
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Do you have both positive AND negative voltage on the op-amp power supply pins in the amp.

Check ALL of the op-amps (TL074s and NE5532s). Some are isolated by 10 ohm resistors so there's no guarantee that you'll have supply voltage on all op-amps just because you have it on one op-amp.
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Old 22nd October 2008, 01:42 AM   #7
damage is offline damage  United States
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With respect to ground:

TL494C
4 = .837
7 = .3
12 = 12.2 (input voltage)
16 = .4 (should this be 5V?)

LM339M
3 = 12.2
12 = 0.3

TL074C (both)
4 = -19.79
11 = -21.43

NE5532
4 = -21.43
8 = -19.82
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Old 22nd October 2008, 02:00 AM   #8
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With respect to ground:

TL494C
4 = .837
7 = .3
12 = 12.2 (input voltage)
16 = .4 (should this be 5V?) >>>> No.

LM339M
3 = 12.2
12 = 0.3

TL074C (both)
4 = -19.79 >>>> Isn't this positive?
11 = -21.43

NE5532
4 = -21.43
8 = -19.82 >>>> Isn't this positive?
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Old 22nd October 2008, 02:23 AM   #9
damage is offline damage  United States
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Quote:
Originally posted by Perry Babin

TL074C (both)
4 = -19.79 >>>> Isn't this positive?
11 = -21.43

NE5532
4 = -21.43
8 = -19.82 >>>> Isn't this positive? [/B]
They are definitely negative when referenced to ground. Shorted cap?
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Old 22nd October 2008, 02:27 AM   #10
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Probably an open positive 18v regulator. What is the voltage on the 3 terminals of the A06 (PZTA06) transistor near D90?
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