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MA3600VZ odd distortion on one channel


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#1 Wrighty

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Posted 06 February 2013 - 12:17 PM

I've got a 3600VZ on the bench here, with an odd fault on one channel. Basically, it looks like something's going unstable and oscillating in the positive half of the cycle, resulting in this output waveform (photos were taken a while ago):

With no load attached:
Posted Image

With 4 ohm dummy load attached:
Posted Image

I happen to be using a 200Hz sine wave input, but some kind of distortion happens at all frequencies. The trace is shifted down, so the oscillation can be seen on the positive cycle.

Here are some more photos from today, with no load, at different frequencies:

100Hz:
Posted Image

1500Hz:
Posted Image

10000Hz:
Posted Image

Any ideas?? Note the huge negative spike at the same positive voltage level on every trace... Biases are all in spec and I've swapped everything socketed on the main board to no avail.

Cheers,

Richard

#2 kevvywevvywoo

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Posted 07 February 2013 - 07:05 AM

can you swap the VZ modules over to prove they're ok?

#3 Wrighty

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Posted 07 February 2013 - 08:25 AM

Yeah, could do I guess. Project for this afternoon...

#4 Wrighty

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Posted 07 February 2013 - 08:43 AM

Ok, just done that and the problem is still there with Ch2's VZ module connected to Ch1. So... looks like main or output module fault.

#5 joust

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Posted 08 February 2013 - 02:00 PM

I've had a similar problem a while ago and had to send the unit back to Crown. (Hey, it happens!)
Apparently, it was a (or perhaps more!) output transistor! Since one the sides appear good, swap output transistors (6 at a time) and determine which set (if discovered!) was faulty. In my problem, the error side didn't generate any distortion. As far as your images demonstrates and if this solution is conclusive, could we have determined the fault with this information?

Alain

#6 SAJ

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Posted 14 June 2013 - 10:12 PM

I have repaired hundreds of macrotech. Never seen waveforms like those. Possibly the 100hz.
Did you find the fault?

#7 joust

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Posted 17 June 2013 - 03:32 PM

(Still waiting for a response also! :) )

Alain