Mackie SRM1801 Powered Sub Fault

[Mike Bullock]

Wondering if anyone can help, or has had similar issues with this unit:

Mackie SRM 1801 powered sub - when I power it up, it emits a loud rumbling sound (like thunder!), even with nothing connected to the input.

Anyone had or heard of this issue and, if so, how did you resolve it?.

 

Many Thanks 🙂

[Bryson]

Sounds like it's maybe getting spurious input.  I'd take a look at the solder joints on the input connectors in the first case.

 

 

[DrV]

First thing to do is disconnect the input board from the power amp to find out which board is introducing the noise. Pull off J1 which is the 7 way ribbon connector. If it goes quiet the issue is with the preamp. If not it's with the power amp.

[KevinE]

Quote

how did you resolve it

The obvious answer is to fault-find the amp module and replace the faulty parts. Is that what you're looking to do? Are you ok with fixing pcbs?

[Mike Bullock]

Apologies, I have been ill since creating this post - I will look at the responses soon

[Mike Bullock]

On 2/9/2023 at 8:39 PM, DrV said:

First thing to do is disconnect the input board from the power amp to find out which board is introducing the noise. Pull off J1 which is the 7 way ribbon connector. If it goes quiet the issue is with the preamp. If not it's with the power amp.

Hi DrV - thanks for this: I disconnected J1 as you suggested and the rumbling stopped.

Bryson suggested looking at the input solder joints, which I will now do - any other ideas spring to mind?.

Many Thanks,

Mike

[Mike Bullock]

On 2/10/2023 at 10:46 AM, KevinE said:

The obvious answer is to fault-find the amp module and replace the faulty parts. Is that what you're looking to do? Are you ok with fixing pcbs?

Hi Kevin,

I have established that the issue is with the preamp, as the rumbling stops when I disconnect it (see message above from DrV).

I wouldn't know how to fault find this - I am competent at de-soldering and re-soldering if I know which component is at fault, but that's about it.

Having disconnected and reconnected the preamp, I notice the rumbling is considerably quieter, but still there.

On 2/9/2023 at 3:21 PM, Bryson said:

Sounds like it's maybe getting spurious input.  I'd take a look at the solder joints on the input connectors in the first case.

 

 

Hi Bryson,

I checked and re-soldered these - problem still there.

Thanks,

Mike

[DrV]

It's worth thoroughly washing the components on the preamp board. I had a unit once (actually it was an srm450 but same applies) which took me ages to fix. Turned out it was just contamination under a cap near the input.

[Mike Bullock]

22 hours ago, DrV said:

It's worth thoroughly washing the components on the preamp board. I had a unit once (actually it was an srm450 but same applies) which took me ages to fix. Turned out it was just contamination under a cap near the input.

Thanks again - when you say 'wash', what did you use and what method?.

Cheers,

Mike

[DrV]

18 minutes ago, Mike Bullock said:

Thanks again - when you say 'wash', what did you use and what method?.

Cheers,

Mike

Can't remember but either isopropyl alcohol or a solvent cleaner like "flux-off". The other clue to where the fault is might be gently and precisely tapping the ceramic caps in that area with an insulated probe (eg. plastic knitting needle). If neither of those finds the fault then you're down to shorting the input to the various stages to see how far back the noise is being introduced. Or just replacing things until it goes away. If you pm me your email address I'll send you the schematic.

[Mike Bullock]

24 minutes ago, DrV said:

Can't remember but either isopropyl alcohol or a solvent cleaner like "flux-off". The other clue to where the fault is might be gently and precisely tapping the ceramic caps in that area with an insulated probe (eg. plastic knitting needle). If neither of those finds the fault then you're down to shorting the input to the various stages to see how far back the noise is being introduced. Or just replacing things until it goes away. If you pm me your email address I'll send you the schematic.

Cheers, I have sent my email address to you.

Many Thanks,

Mike