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mbates14 4th January 2011 05:31 PM

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?

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 4th January 2011 07:50 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 4th January 2011 07:51 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.

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