MA Audio M489i 4 channel class D crackly output

[y8s]

I've got a 4 channel class D MA Audio amp... all but one of the outputs works fine and I'm thinking it's a dead output transistor (D998/B778, one each per channel).

I tried the tests on the basic amp repair site but the transistors are still in situ so I'm uncertain if there are other things affecting my readings.

Off the top of my head (at work now) I did not get any short circuit readings between legs but one combination of diode test probing yielded an open circuit. I *believe* it was red probe base, black probe emitter but I need to verify that.

I compared that result to the rest of the transistors and they all show the same number (not "open" like this one).

I will pull at least that channel's outputs and re-measure tonight and maybe pull a known good channel's transistors and compare.

Any suggestions?

There are no visible burn marks and the questionable crackly channel still makes some noise, albeit faint and very distorted.

Pictures:
Q307 is the device in question.


Q307 is in the upper left.


Thanks,

Matt

[Perry Babin]

If there is a static/crackling but the channel produces audio, it's not likely an output transistor (but it's possible).

Is the noise in any way related to vibration?

Does it make a difference in the noise if you push on various points on the board?

Have you tried moving all potentiometers and switches to see if the noisy channel every played cleanly?

[y8s]

The audio is barely considered audio... it requires a pretty significant input signal peak to get it to make a noise. Like a fuzzy buzzy crack on a loud cymbal crash for example.

I will check vibration and board flex. I can also check the user inputs but would they affect both channels on that pot/switch or not necessarily (ie if it's dual gang/pole)?

I seem to recall the channel used to play low input levels fairly well but once I crossed a certain volume threshold, it would cut out for an extended period. I also seem to recall prior to that that if I really cranked the bejeezus out of the volume that it would cut back in. Those both feel to me like intermittent connections which makes sense given your suggestions.

Thanks Perry.


matt

[Perry Babin]

Dirty connections on the pots and switches could affect only one channel.

Confirm that you have the same rail voltage on the center legs of all of the output transistors with the same part numbers on their face.

[y8s]

all center legs have rail or -rail voltage.

the input at first seemed flakey so I pulled the RCA stack off the board and ran new RCA inputs to the pads. they no longer seem intermittent but... the 3rd channel reliably plays at very low volume and with a serious crackle. if I turn the volume up, the amp "protect" led lights up and the amp remains silent until I turn the volume back down.

There's also a high pitched hum/squeal coming from near the power supply toroid. It changes in intensity slightly when I push on the board and flex it a little. seems to do this with or without a load connected as long as the power is on.

I swear that the original RCA connector block may have been intermittent but did occasionally work when wiggled. But what about an input could possibly cause the amp to go into protect mode at higher volume levels?

[Perry Babin]

What are you referring to as an 'input'?

Do you have a scope?

[luka]

this is not class D, but probably is class AB

[y8s]

I'm not actually sure why I thought it was class D. I think the size/wattage put that in my head.

Perry, "input" meaning the RCA plugs in the RCA jacks of the amp. If I wiggled that the plug where it was connected...

I don't have a scope.