Equalizer hum when engaged, silent when disengaged

[C Hart]

Hello solid state gurus. Thanks for checking out this thread and for any help you can provide. I have very little experience in repair or rehab on solid state components, and I hope someone can help me with suggestions for an equalizer that recently started misbehaving.

My Alesis M EQ-230 30 band graphic equalizer until recently worked silently and effectively. At some point it developed a small but noticable 60hz hum when the "EQ In/Out" switch on the unit is engaged. Here is the behavior:

- The hum is present when the EQ is "In".
- There is no hum when the EQ switch is "Out" nor in any other setting or component.
- The hum appears on both channels when both are "In"
- The hum disappears from each channel when the switch for that channel is "Out", i.e. if one channel is "In" that channel hums, if "Out" it does not.
- The EQ slider settings do not affect the hum.
- The preamp volume does not affect the hum
- The amp gain affects the hum

I do not think there is a ground problem outside the EQ. But in the interest of completeness line power is through an automated power strip with the following connections:

Main sensor switch: Adcom GTP-500 preamp
Unswitched: Technics SL1600 turntable, Sony CD/DVD player
Switched: Yamaha P3500S amp, Realistic APM-300 power meter, Alesis MEQ-230 equalizer

Component connections:
Preamp in: CD, Phono, Bluetooth adapter, Mac Mini/DAC
Preamp out: EQ
EQ out: Amp
Speaker connections: (parallel) (2) JBL L166, power meter

Can anyone provide some troubleshooting advice on this? I am not sure where to begin. Many thanks!

 

[C Hart]

Bump. Is there no tutorial on troubleshooting somewhere? Thanks.

 

[DesertRatt]

I would begin by looking at the EQ's power supply. Sounds like a blown part and a radical increase in RIPPLE in the supply. Pretty common problem. Check all the diodes, resistors and caps in the PSU. I bet you find a blown part.

 

[Oerets]

On reading this and not familiar with you model of EQ.

Usually a hum like described I would be looking at a capacitor in the EQ's power supply. This would be after checking for a grounding issue external in the chain.

A power supply capacitor affects the supply. So will not be so effected by the volume or sources. Both channels if shared will be affected also.

First ground all the units common at one point. Then if still persists look under the hood of the EQ.

Sorry had my eyes dilated and having a troubled time with the keyboard...

 

[bowtie427ss]

230 uses a wall wart.

 

[C Hart]

Thanks everyone for your advice.

Yes it has a wall wart. I can swap it out for a new one I have in storage. Will try it later tonight.

Much appreciated!

 

[r_brumett61]

Could be the p/s but also could be a cable(rca connector).

 

[tom3]

Have you moved the EQ lately, maybe set it on top of another piece of equipment? Rare - but sometimes pick up a hum from outside the unit and the EQ circuitry isn't shielded enough to block it.

 

[C Hart]

Probably not a cable because the electrons flow through the same cables with the eq in or out but is something to check. Nearby gear s a possibility which I will look into. About to hunt down my spare PS and get diagnosis underway now.