Good evening, I have an older mtx 81000d that came in with pretty severe input RCA jack damage. Also inspected all output and power supply mosfets, nothing blown. Powered the amp up, power supply powers up without issue, builds rails voltagr but zero output.
I checked for output switching and there is none. So I disabled the output stage by jumping the capacitor pictured and began to trace the input signal. It looks like the audio input enters into pin 3 of the NE5532. Upon powering the amp up, the audio signal vanishes on pin 3 and is replaced with 2.39v DC which appears to being fed into the TL072 next door.
Is this normal or is the NE5532 damaged? Pin 8 on both the NE5532 and TL072 show 15v.
Thank you in advance
I checked for output switching and there is none. So I disabled the output stage by jumping the capacitor pictured and began to trace the input signal. It looks like the audio input enters into pin 3 of the NE5532. Upon powering the amp up, the audio signal vanishes on pin 3 and is replaced with 2.39v DC which appears to being fed into the TL072 next door.
Is this normal or is the NE5532 damaged? Pin 8 on both the NE5532 and TL072 show 15v.
Thank you in advance
Attachments
1.86v and 1.78v
(Referenced to amplifier main ground.)
Should the voltage be -15v?
Yes but it's also responsible for producing the +15v. Follow the circuit back. You could have a bad connection on a diode or an open inductor.
Thank you!! 🙂
I traced the -15v from the transformer back to L472 which appears to be a smd inductor marked "221K". It was open with infinite resistance. I pirated the corresponding inductor from another board (same make and model) and this board came to life and appears to be working fantastically.
Now I need to replace this inductor on the other board.
Would this be an appropriate replacement?
https://www.digikey.com/en/products/detail/te-connectivity-passive-product/3613C221K/2363412
I don't have any specs on the inductor, other than the marked value. I'd order the one with the highest current rating available for the value and size.
The voltage drop across the resistors R177 and R178 may indicate the current through the inductor if all op-amps lose negative supply voltage when R178 is removed.
Measuring the resistance across the good inductor may also be helpful in matching the replacement.
The voltage drop across the resistors R177 and R178 may indicate the current through the inductor if all op-amps lose negative supply voltage when R178 is removed.
Measuring the resistance across the good inductor may also be helpful in matching the replacement.