MTX Thunder81000D issue

mbates14 · 12th September 2011, 02:33 PM

Ran into one of these that had the fillter burned loose from the board.

replaced filter, and everything is working fine, but the unit just sounds a little odd. its hard to explain.

But then i noticed when the amp is idling not only does it making noises, but if i turn the volume knob up and down, you can watch the speaker push in and out.

paying closer attention to the speaker when music is playing, it seems like the DC offset is just swinging around, almost like a broken RCA ground as seen with a previous amp. But it is not, I am getting a .3xx reading on diode check beween RCA ground and speaker ground. (no RCA connected).

so i assume trouble in op-amp circuit.

any ideas?

Perry Babin · 12th September 2011, 05:55 PM

You shouldn't be reading below 1 ohm between the 'input' RCA ground and the speaker -.

Check D198 and D199.

mbates14 · 13th September 2011, 01:46 AM

I didnt take the measurement in ohms. I took it in Diode Check. had about .4XX or so voltage drop. between spk and rca shield. no RCA was connected.

Perry Babin · 13th September 2011, 02:43 AM

0.4 on diode check isn't a problem.

In some of the MTX amps, when the VCA (THAT 2180/2155) fails, it will cause a surge of DC when you adjust the gain. You can jump them from pin 1-8 with a 1k resistor to eliminate them as a possible fault.

mbates14 · 13th September 2011, 02:46 AM

yea, its definately somthing. Because with or without the RCAs connected, you can vary the gain and the speaker just pushes in or pulls out before it settles.

And it sounds kinda "muddy" or a box full of bass. its hard to explain. it doesnt sound "normal" per say. probably the offset playing a role into the audio. because it shifts around alot when the music is on.

Perry Babin · 13th September 2011, 04:39 AM

Did you try jumping the VCA out?

mbates14 · 14th September 2011, 02:42 AM

not yet. the customer took his amp back and said heck with it.

Anyway, whats the VCA do?

Perry Babin · 14th September 2011, 03:05 AM

The VCA is a voltage controlled amplifier.

In MTX amps, it's used to adjust the signal level in the amp. Its main purpose is to adjust the signal level when the remote level controller is connected. In amps with a compressor, it is supposed to reduce the signal level to prevent hard clipping.

12th September 2011, 02:33 PM	#1
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	MTX Thunder81000D issue Ran into one of these that had the fillter burned loose from the board. replaced filter, and everything is working fine, but the unit just sounds a little odd. its hard to explain. But then i noticed when the amp is idling not only does it making noises, but if i turn the volume knob up and down, you can watch the speaker push in and out. paying closer attention to the speaker when music is playing, it seems like the DC offset is just swinging around, almost like a broken RCA ground as seen with a previous amp. But it is not, I am getting a .3xx reading on diode check beween RCA ground and speaker ground. (no RCA connected). so i assume trouble in op-amp circuit. any ideas?

12th September 2011, 05:55 PM	#2
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	You shouldn't be reading below 1 ohm between the 'input' RCA ground and the speaker -. Check D198 and D199. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

13th September 2011, 02:43 AM	#4
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	0.4 on diode check isn't a problem. In some of the MTX amps, when the VCA (THAT 2180/2155) fails, it will cause a surge of DC when you adjust the gain. You can jump them from pin 1-8 with a 1k resistor to eliminate them as a possible fault. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

13th September 2011, 04:39 AM	#6
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Did you try jumping the VCA out? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

14th September 2011, 03:05 AM	#8
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	The VCA is a voltage controlled amplifier. In MTX amps, it's used to adjust the signal level in the amp. Its main purpose is to adjust the signal level when the remote level controller is connected. In amps with a compressor, it is supposed to reduce the signal level to prevent hard clipping. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links


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13th September 2011, 01:46 AM	#3
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	I didnt take the measurement in ohms. I took it in Diode Check. had about .4XX or so voltage drop. between spk and rca shield. no RCA was connected.

13th September 2011, 02:46 AM	#5
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	yea, its definately somthing. Because with or without the RCAs connected, you can vary the gain and the speaker just pushes in or pulls out before it settles. And it sounds kinda "muddy" or a box full of bass. its hard to explain. it doesnt sound "normal" per say. probably the offset playing a role into the audio. because it shifts around alot when the music is on.

14th September 2011, 02:42 AM	#7
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	not yet. the customer took his amp back and said heck with it. Anyway, whats the VCA do?

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