MTX 5601 weird noise

[ray9043]

Hi!

I have a MTX 5601 amp, its working but doing weird noise when i hook up the subwofers. It has +- 60 v, +- 15 v, everything looks great. But subwofers move very weird, they move towards like dc voltage on subwofers then hit the bass. More weird is the fan, it turns on every time the subwofer hits.

 

[Perry Babin]

Does it work correctly with a normal head unit that's grounded to the same power supply as the amp?

 

[ray9043]

same ground in the car, i test more amps the same day with the same subwofers, only mtx do that. The same problem with 12v power supply and a 4ohm subwofer on the bench.

 

[Perry Babin]

What is the resistance from the RCA shields on the head unit to the case of the head unit?

What is the resistance from the RCA input shields on the amp to the ground terminal of the amp?

No RCAs plugged into either.

 

[ray9043]

No resistance between gnd amp and rca shield. I cant measure rca wires, i test the amp on my friend car. I test anothers amps witout issues only the mtx.

 

[Perry Babin]

What do you mean by 'no resistance'? If you read zero ohms between the RCA shields on the amp and the amp's ground terminal, you have a shorted diode in the amp.

 

[ray9043]

Sorry! 38k ohm betwen gnd and rca shield!

 

[Perry Babin]

This amp probably uses the newer input circuit (no diode) but I don't see a problem here.

Noise issues are difficult to find under the best circumstances. This type of troubleshooting makes it more difficult.

Do any of the settings on the amp make the noise better or worse?

 

[ray9043]

Move the frequency, gain, etc. No change. I put out all the outputs looking for lekeages or shorts, all are fine.
Gates voltages are ok, -51v and 0 v without input signal
Lm311 vcc is only 4.4v, coming from a 1gm transistor, 12v on collector, 5v on the base and 4.4v on the emiter, replaced and same voltages

 

[Perry Babin]

The LM311 is likely for over-current protection and not contributing to the noise.

 

[ray9043]

Ok, i will meassure the ne5532 voltages and the input tl072. I test with the preamp board disconected, with jumpers in the socket, the same problem so the board are OK.

 

[ray9043]

The output filter caps, these can be the problem? I replace 4 of the 8 and see a diference on the wofer movement. I will replace the 8 and test it.

 

[Perry Babin]

If the output filter caps are in series, only one defective one could allow a lot of high frequency noise to pass and, through a woofer, could produce all sorts of strange sounds. The noise would be easily visible on the scope.

If the capacitors failed, especially if they were swollen, the output filter inductor may have shorted and the short may be intermittent.

 

[ray9043]

No swollen caps. Yes in series because they arent bipolar. I put only 4, im looking for other 4 for test

 

[Perry Babin]

Are all in one series string? If so, any one being defective can cause problems.

 

[ray9043]

8 caps replaced and the same noise. with the gain at half and playing x music i can read about 10 vdc when the bass hit hard. maybe a leaking output mosfet? i will test one by one

 

[Perry Babin]

I think I'd need to see the distorted waveform with a sine wave driven into the amp.

 

[ray9043]

I test with 60hz tone. The ps is +-60v without load. With load and the gain 3/4 the negative voltage goes to -88-90 volts but the positive drops to 35 volts¡¡ i double check the rectifier. The 6 ps caps are ok i replaced all.

I will buy an oscilloscope next month, i really need it¡¡

 

[Perry Babin]

Don't adjust the output level with the gain pot. Adjust it with the signal source.

Where are you placing the black meter probe when measuring the rail voltage?

 

[ray9043]

ok. battery negative side is the common in all the amp, output negative, and center of the rail caps, all points have the same voltage.