MTX RT2200x changing smps output voltag - diyAudio
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Old 6th March 2007, 10:42 PM   #1
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Default MTX RT2200x changing smps output voltag

This amp had shorted outputs on one channel, but I removed those. The other channel still works. What I want to do now is put in some chip amps and use the existing smps. The problem is that the ps puts out +- 30v. I need to bring it down to about +- 20v. Unfortunately, I do not have a schematic for this amp and I don't know where to start. The smps uses a tl194 chip. If I decrease the voltage, would the working channel (will not be used) be ok with this? Nothing expect the smps will be used from this amp. I would appreciate if someone helped me out or posted a schematic for this amp.
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Old 7th March 2007, 03:45 AM   #2
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If this amp is like some of the other MTX amps, they use pins 1 and 2 of the 494 for regulation.

Pin 1 is connected to a voltage divider. The top resistor of the voltage divider is connnected to the positive rail. The bottom resistor of the divider is connected to ground.

Pin 2 is also connected to a voltage divider. The top resistor is connected to the 5v reg of the 494. The bottom resistor of the voltage divider is connected to the negative rail.

Changing the resistors that are connected to the rails will lower the rail voltage.

I have an amp here that uses a 130K resistor and a 10k resistor for the dividers. That gives a rail voltage of ~32v. Changing the 130K resistors to ~82K sets the rail voltage to about 20v.

Look for these voltage dividers in your amp.
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Old 8th March 2007, 05:53 PM   #3
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I found the voltage divider on pin 1, 130k to positive rail and 10k to ground. On pin 2, there is a 100k/10k divider, but it's not connected to the negative rail. The 10k is connected to +5 while the 100k is connected to a cap which looks like it's connected to pin 3. Could this be a single rail feedback?
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Old 8th March 2007, 06:07 PM   #4
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The cap between pins 2 and 3 is used to stabilize the regulator. The 100K is likely there to provide a bit of feedback.

There is likely a second 130k ohm resistor somewhere that connects to the negative rail. Of course, the circuit could be different but it's unlikely since there's a 10k resistor from the 5v reg to pin 2.

You can adjust the regulation using only one of the 130k resistors. Lower it's value and the regulated voltage will drop. Use a potentiometer to make it variable.
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Old 8th March 2007, 06:11 PM   #5
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You're right, I just found the other resistor connected to the negative rail. So changing only one of them should work, right? (just in case I can't find 2 matching resistors)
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Old 8th March 2007, 06:17 PM   #6
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If you're going to set the desired voltage with a pot or by trial and error, you can change only one.

If you wanted to 'calculate' the value based on the desired voltage, it would be easier if they were the same value.
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Old 8th May 2007, 07:46 PM   #7
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I found 2 matching resistors, and installed those. The rail voltage decreased to what I wanted. Then I proceeded to solder in some LM3886s, and now I'm having problems with the PS turning on. It turns on for a split second, the voltage jumps up to about +/- 15, and slowly fades away (and it repeats). (I had the same symptoms with a blown output stage, but the repetition frequency was much higher [the caps discharged quicker]). I desoldered the LMs with no luck. I cannot find any shorts in the original output stage. Any ideas on where to look would be appreciated.
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Old 9th May 2007, 02:41 AM   #8
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Pins 3 and 4 should be near ground for normal operation. Which of those two pins is going high when the amp shuts down?
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Old 9th May 2007, 02:49 AM   #9
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Pin 4 is sitting at 5v, but when the amp turns on, it drops to about 1.7 (momentarily).
Pin 3 is at .06, but jumps up to about 1.2 when the amp turns on (momentarily).

I hope that makes sense. When the amp shuts down, pin 3 is near gnd while pin 4 is at 5v.
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Old 9th May 2007, 02:58 AM   #10
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You need to get pin 4 to ground. You can not ground it without disconnecting the PNP transistor that's driving it high. The PNP transistor will have its emitter connected to pin 14. Its collector will be connected to pin 4. Its base will be connected to pin 1 of the 339 via a resistor. With the transistor out of the circuit, you can ground pin 4. Connect it to pin 7 of the 494 instead of to the chassis ground terminal.

This will defeat the protection circuits so you will have to be VERY careful. All of the transistors should be clamped to the heatsink and you should have a 10 amp fuse or current limiting resistor in series with the B+ supply.

If the amp powers up with the transistor removed and there is no excessive current draw, then the protection circuit may be damaged. If there is excessive current draw, there was a good reason for the amp to go into protection and you need to find the problem so you don't damage any more components.
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