Mackie HR624 MK2 feedback howl with no audio source connected - Page 1
Forum section: electronics / repair

author: Timmox
author_rank: Contributor
timestamp: April 02, 2025, 01:17:37 pm
content: Thank you for taking time to read my post.I have a Mackie HR624 MK2 powered studio monitor speaker that howls at 265Hz as soon as you power it on. This is with nothing connected to any of the inputs. It seems to be doing it a full volume. Turning the trim pot on the back does nothing to alter the volume of the howl.Has anyone come across this type of fault before?Also can anyone recommend a good forum for help with repairing audio equipment such as this?Any assistance will be gratefully received.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 02, 2025, 01:39:57 pm
content: Maybe a ground loop.

author: David_AVD
author_rank: Super Contributor
timestamp: April 02, 2025, 07:35:02 pm
content: Could be one or more electrolytic capacitors gone low value or high ESR.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 02, 2025, 11:01:18 pm
content: Is there a ground switch?Try flipping it (turn down the volume first!).

author: themadhippy
author_rank: Super Contributor
timestamp: April 02, 2025, 11:16:38 pm
content: Quote1)with no inputs connected theres nothing to complete a loop2) earth loop hum tends to be at whatever your mains frequency is

author: Langdon
author_rank: Frequent Contributor
timestamp: April 03, 2025, 12:49:33 pm
content: Maybe it's not grounded then.Is there a ground switch?

author: CaptDon
author_rank: Super Contributor
timestamp: April 03, 2025, 01:17:35 pm
content: Langdon, why with nothing connected would it make any difference if the ground is connected or disconnected?? With a 265Hz frequency why do you suggest ground problems at all? There is an internal problem probably in the negative feedback loop of the power amplifier. We used to call this sound 'motorboating'. If the negative feedback loop is disturbed such as open capacitor, shorted capacitor, open or cracked SMD resistor then the amplifier becomes unstable and breaks into oscillation often at full power which complies with an observation by the original poster. If this unit is a copy of the Mackie/Thump units there is an under rated transistor used in the + and - 15 volt power supplies that fails and the unit will make very load noises and finally go into the protect mode. The series dropping resistors can also go open or shift value causing similar problems like a loud fart sound before going into the protect mode.

author: Timmox
author_rank: Contributor
timestamp: April 03, 2025, 02:47:23 pm
content: Hi CapDon,Your descriptions sounds like what I'm experiencing. I will pop the board out and start with a visual inspection and report back.Thanks for the information it's been really helpful.Cheers

author: Langdon
author_rank: Frequent Contributor
timestamp: April 03, 2025, 09:53:07 pm
content: Without ground, what's to keep tiny EMI from causing self-resonance/ringing.The socket could funtion like a really small, really bad antenna and pick up interference; not alot, but enough.

author: David_AVD
author_rank: Super Contributor
timestamp: April 04, 2025, 08:56:43 am
content: That seems like a stretch. Much more likely to be motorboating due to unstable voltage rails or other capacitor issues.