MTX Thunder 250

[S Merlington]

Amp has very distorted output and the subwoofer pops almost like there is voltage on the outtput terminals..

ON U101 (NE5532) here are the voltages i get

Pin 1ulsing
Pin 2ulsing
Pin 3ulsing
Pin 4:-21.70
Pin 5ulsing
Pin 6ulsing
Pin 7: pulsing
Pin 8:21.72

All the pins that are pulsing voltages the voltages are in the 20 volt range..

Any ideas?

 

[Perry Babin]

Is there pulsing on any other op-amp?

Does the pulsing happen with and without RCAs plugged in?

 

[bjc]

I was going to say it could be dodgy RCA's but have only had the one similar problem 15yrs ago!

 

[S Merlington]

There is only 1 opamp in this amp..

Pulsing happens with and without rca's plugged in

 

[Perry Babin]

It's different than the one I thought is was. Post a photo of the board if you can't find the problem.

 

[S Merlington]

Here is a pic of the board

 

[Perry Babin]

I think I'd pull the op-amp and check for strange signals on the pads. If there weren't any that looked suspicious, I'd replace the op-amp.

 

[S Merlington]

Removed the op-amp and heres what i get on the pads..

Are these voltages normal for this amp or are they still to high?

Pad 1:22.05
Pad 2:22.06
Pin 3:22.07
Pin 4:-22.64
Pin 5:22.08
Pin 6:22.10
Pin 7:22.10
Pin 8:22.04

 

[S Merlington]

Replaced the opamp..

The left channel is still very distorted and the sub still pops like the channel has DC voltage on it..

The right channel will play clean audio for 5 secs then become very distorted and after about 10 secs it will play clean again then become distorted its cycling back and fourth

 

[Perry Babin]

I wouldn't expect any significant voltage on the audio pads (if this op-amp is used for audio). Do you have that level of DC on any of the terminals of the RCA jacks?

 

[S Merlington]

Yes on the shields of the RCA jacks i get 20 volts

 

[Perry Babin]

If you ground the shields, do you see any increase in current draw?

 

[S Merlington]

No

 

[Perry Babin]

Is your signal source grounded to the DC power supply for the amp?

 

[S Merlington]

Yes

 

[Perry Babin]

Is the shield open on the head unit?

 

[S Merlington]

No

This amp has done the same thing with 3 different head units..

 

[Perry Babin]

How can you have 20v on the shields if the shields are grounded through the head unit?

 

[S Merlington]

I had the RCA jacks unplugged when I tested the voltage on the shields

 

[Perry Babin]

Does this amp have a 1k resistor parallel with a diode that connect between the RCA and main grounds?