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Old 5th March 2007, 05:39 PM   #1
diyAudio Member
Join Date: Jun 2006
Default Trying to undestand MTX 4250D

I have this amp for a long time, it works but have excessive DC in output, leading to overheating outputs and a lot of unwanted things.
And more I am dig into this amp, more question I have. What is the idea behind having two very different gate resistors for 2 outputs? Very asymmetrical design. On speaker terminals one is at -42V, other is like -30V
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Old 5th March 2007, 06:02 PM   #2
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Join Date: Nov 2003
Location: Louisiana
Isn't this a mono amp?

If so, the voltage across the speaker terminals should be the same (not 12v difference).

What do you read if you measure the DC voltage with the meter directly across the + and - speaker terminals?

If you have voltage across the speaker terminals, does it vary with the gain setting?

Is the same basic amp as the 250D?

The signals on the outputs are radically different. The designer may have chosen the resistors to optimize the efficiency for each output.
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Old 7th March 2007, 02:34 PM   #3
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Join Date: Jun 2006
The voltage across speaker terminals is around 8V and is not affected by gain. I think this should be basically the same amp as 250D.
What I did so far:
- found shorted A42 and damaged diode D119 close to RCA jacks
- replaced both A42 drivers, as their P-N resistances were very different, probably one was damaged from overheating
- replaced ALL SOT23's in driver stage
- replaced 0.1 (?) Ohm output resistors as they were 20% different
= Still 8V on outputs . I am having a pretty messy-damage situation here

Thank you!
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Old 7th March 2007, 03:01 PM   #4
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Join Date: Nov 2003
Location: Louisiana
If the diodes and transistors that failed were part of the regulator, you may have a big mess. This was really common in the 250Ds. Sometimes, regulator failure would destroy every op-amp.

In the 250D, there is an op-amp just behind the RCAs. It's U5 in the 250D. To see how similar the circuits are, can you confirm that the first half of the op-amp goes back to the crossover frequency pot (via capacitors and resistors).

If that's the case, the second half of U5 should be part of the feedback loop and you can begin troubleshooting by measuring the voltages on pins 5, 6 and 7 of that op-amp.

Also make sure you have plus and minus regulated voltage on that op-amp.
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