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Mackie SRM450v1 - Loud pop at turn on, followed by loud 120Hz Hum and no sound.

Bossonovo03

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Jan 23, 2025
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The speaker worked before working on it.

Initial problem was that the volume was cutting in and out when fiddling with the volume knob.
okay, I started by also changing the filter caps as the old ones were bulging outwards and were ready to leak anytime soon. Nothing too complicated, for now.
After that, I changed the volume pot for a log taper one made by alps alpine. I later realized that i actually need a reverse log taper pot,
I will find something to replace it with, that's not really the issue though, it would not cause the following issues.
When i reassembled everything and turned it on, there was nothing except that all the indicator lights were glowing faintly.
After some research, I found that it would have to do with the +15V / -15V rails coming from Q35 and Q34. Changed those.
Now when i turn it on, it starts with a loud POP, followed by loud constant 120Hz humming and no sound at all coming through.
the indicator lights are now bright and only the green ones stay on after startup (signal detect light and timed turnoff light).
I am from Canada, so 120Hz would most likely be a result of the bridge rectifiers doubling the mains frequency of 60Hz.
So I checked both bridge rectifiers (D55, D56) and they are good. The polarity for the filter caps is good too and they are within spec, they are brand new quality ones.
My test points voltage all seem good according to service manual and schematic included below (+45 -45V, +95 -95V, +15 -15V).
I also checked for shorts in the transistors and FETS in the low frequency and high frequency amp rails and did not find any shorts.


There is a forum post on Badcaps from 2014 with someone having the same exact problem but they never came back with a solution.



What would be my next move? I am at a loss now
Thanks a Lot


 

Attachments

  • Mackie SRM450 Schematic.pdf
    1.3 MB · Views: 27
  • mackie_srm450.pdf
    1.5 MB · Views: 26
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