Memphis 16-mc600d issues

[mbates14]

I ran into one of these things, it is a 4 channel class AB amplifier plus class D subwoofer output.

All of the power supply transistors had blown. along with all of its resistors. Also 2 out of the 4 KTA1266 driver transistors were smoked and blown as well.

I replaced all the power supply fets, all resistors, removed the PWM riser card and replaced the 4 1266 transistors, AND the TL494. also replaced the 330ohm resistor that goes into the TL494 as it was smoked as well.

the rear right channel IRF540/9540 pair was shorted. Replaced those as well, and tested the associated resistors and drivers. all appear to be fine.

But i tried to fire up the amplifier, the SMPS is emitting an insane noise off/on and the protect light is lit.

Scope shows a noise waveform popping in and out. I tried to measure the waveform off of pin5/6 of the TL494, and its the same noise waveform as well. popping in and out. Appears the TL494 isnt used as the oscillator, something else on the PWM board does, unless the oscillator frequency is generated elsewhere?

any ideas?

[Perry Babin]

Make sure that the crossover 12/18db switch is solidly in either position or set the crossover to full range. Does that make a difference?

[mbates14]

Ya, i checked all that stuff.

[Perry Babin]

Try grounding the scope directly to the ground terminal on the amp. Does that help you get a more stable view of the signals?

If not, can you post a photo of the inside the amp?

[mbates14]

dont have a camera. the battery is toast. one of those sony DSC-V1s and batt wont hold a charge anymore.

Yea i grounded the scope probe. same exact result.

You can see what appears to be an extremely distored squarewave driving the power supply FETS, but its only there for a second or so. then it goes away. then slowly creeps back.

its not a bang-bang switch on-off type waveform.

its a noisy distorted square wave that slowly gains in amplitude within a few seconds, and then drops in amplitude to nothing. then it comes back.

And you can hear the noise/distortion in the toroids. Pluse the PROTECT LED is lit as well.

Remote power, and VCC power is clean. But the VCC power going to the TL494 exibits the same kind of noise.

[Perry Babin]

Is the noise present on the ground pin of the 494?

[mbates14]

I found the problem. the ground lug on the transistor mounts was shorted to one of the cases of the IRF640 output on the subwoofer amplifier.

oops. I removed the short, and now the amplifier is stable. all channels work, except for the subwoofer ouput, and the protection light is still lit.

the subwoofer speaker relay will not engage. if i connect the speaker to the output filter section, bypassing the relay, the subwoofer amplifier works perfect.

Scope monitoring the drains of the outputs, nice clean PWM square wave driving the outputs. nothing wrong in the classD section at all. but something in the protection circuit is damaged. as amp is stable, but still in protect. Rails are 27V

[Perry Babin]

I can't find an internal photo of this amp and have never worked on one so I'm lost at this point.

[mbates14]

ya i figured.

[mbates14]

i fixed it. the VCC of the TL494 was hanging low at 5.8v and wavering around.

hmmm... so i thought. Well, it comes from that 330ohm resistor i put in there, as the old one was smoked. turns out it wasnt 330ohms. it was 33ohms. LOL...

put in a 33ohm in place of the 330ohm, works fine. green light all 5 channels working. Rails now stable.