MTX Thunder 8302 issue

mbates14 · 4th January 2011, 05:31 PM

Ran into a weird problem.

the U100 LM833 was rail to rail shorted, and the regulators were blown up.

Replaced the regulators, and the LM833 with an NE5532 which is all i have.

High pass works, full range works, but NO lo pass at all.

I can see the low-signal on the outputs of the TL074 behind the FREQ pot.

However, U100 I have somewhat dirty audio signal on both op amp positive and negative inputs.

the negative inputs pin 2 and 5 are a +0.5v bias.

the positive inputs have no offset.

however BOTh op-amp outputs are -13vdc. hmmm.... strange....

any ideas?

Perry Babin · 4th January 2011, 06:12 PM

It sounds like you have an open trace or possibly an open pot or switch.

The op-amp appears to be working correctly. If the inverting input voltage is greater than the non-inverting input voltage, the op-amp output will swing toward the negative supply rail. The gain (determined by circuit components, not the gain pot) determines how far it swings. Since the VCA (SIP IC, 2155?) is generally connected to the inverting input of the op-amp with no resistance, slight DC offset can cause very large swing.

Remove the VCA and connect a 1k resistor from it's input terminal to it's output terminals (the end pins are input and output). Does that eliminate the DC offset and allow the amp to produce audio when set to low pass?

mbates14 · 4th January 2011, 07:09 PM

the THAT2155 IC as no audio in its input or output if i scope it.

appears to be a single channel IC, but its a 2 channel amplifier.

Perry Babin · 4th January 2011, 07:24 PM

The preamp level is mixed in low pass, if I'm not mistaken.

You probably won't see audio on the input or the output.

Did you try bypassing it with the resistor?

mbates14 · 4th January 2011, 07:47 PM

getting ready to do that now.

mbates14 · 07:52 PM

i removed the IC with the old fashoned bend it until it falls off method. I got replacements kicking around just in case it was bad anyway....

Installed 1K resistor in place, no go. no sound on LPF, offset still present on U100 outputs

Perry Babin · 07:53 PM

Do you see audio on the resistor?

Do you still have DC on the output of the op-amp?

mbates14 · 4th January 2011, 07:56 PM

yes. theres still DC on the output of U100. There is -1.43v on the resistor DC.

no audio on resistor.

Perry Babin · 4th January 2011, 07:58 PM

Check all of the op-amps. Are there any op-amps that the voltage on the inverting input and the non-inverting inputs don't match within ~0.01v DC?

mbates14 · 4th January 2011, 08:06 PM

hard to take accurate measurements down to the milivolt but nothing out of the ordinary. all ok.

except for U100. its the only one thats wacked out.

4th January 2011, 05:31 PM	#1
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	MTX Thunder 8302 issue Ran into a weird problem. the U100 LM833 was rail to rail shorted, and the regulators were blown up. Replaced the regulators, and the LM833 with an NE5532 which is all i have. High pass works, full range works, but NO lo pass at all. I can see the low-signal on the outputs of the TL074 behind the FREQ pot. However, U100 I have somewhat dirty audio signal on both op amp positive and negative inputs. the negative inputs pin 2 and 5 are a +0.5v bias. the positive inputs have no offset. however BOTh op-amp outputs are -13vdc. hmmm.... strange.... any ideas?

4th January 2011, 06:12 PM	#2
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	It sounds like you have an open trace or possibly an open pot or switch. The op-amp appears to be working correctly. If the inverting input voltage is greater than the non-inverting input voltage, the op-amp output will swing toward the negative supply rail. The gain (determined by circuit components, not the gain pot) determines how far it swings. Since the VCA (SIP IC, 2155?) is generally connected to the inverting input of the op-amp with no resistance, slight DC offset can cause very large swing. Remove the VCA and connect a 1k resistor from it's input terminal to it's output terminals (the end pins are input and output). Does that eliminate the DC offset and allow the amp to produce audio when set to low pass? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

4th January 2011, 07:24 PM	#4
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	The preamp level is mixed in low pass, if I'm not mistaken. You probably won't see audio on the input or the output. Did you try bypassing it with the resistor? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

4th January 2011, 07:50 PM	#6
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	i removed the IC with the old fashoned bend it until it falls off method. I got replacements kicking around just in case it was bad anyway.... Installed 1K resistor in place, no go. no sound on LPF, offset still present on U100 outputs Last edited by mbates14; 4th January 2011 at 07:52 PM.

4th January 2011, 07:51 PM	#7
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Do you see audio on the resistor? Do you still have DC on the output of the op-amp? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links Last edited by Perry Babin; 4th January 2011 at 07:53 PM.


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

4th January 2011, 07:09 PM	#3
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	the THAT2155 IC as no audio in its input or output if i scope it. appears to be a single channel IC, but its a 2 channel amplifier.

4th January 2011, 07:47 PM	#5
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	getting ready to do that now.

4th January 2011, 07:56 PM	#8
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	yes. theres still DC on the output of U100. There is -1.43v on the resistor DC. no audio on resistor.

4th January 2011, 07:58 PM	#9
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Check all of the op-amps. Are there any op-amps that the voltage on the inverting input and the non-inverting inputs don't match within ~0.01v DC? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

4th January 2011, 08:06 PM	#10
mbates14 diyAudio Member Join Date: Jun 2002 Location: non	hard to take accurate measurements down to the milivolt but nothing out of the ordinary. all ok. except for U100. its the only one thats wacked out.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
MTX Thunder 1501D	spooney	Car Audio	49	16th July 2010 04:55 AM
MTX Thunder 6152 issue	gsg529	Car Audio	10	7th July 2010 12:45 AM
MTX Thunder 81001	megael9	Car Audio	13	30th May 2010 03:23 AM
same amp, new prob MTX 8302 LP filter	damage	Car Audio	8	8th December 2008 03:30 AM
MTX 8302 problem	damage	Car Audio	46	27th October 2008 04:53 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read