Bad hum in mixer

[Estacy]

So I sold my Ecler and since I'm now mixerless I dug out this bad boy again, needless to say the problem is still apparent.

but: I have found out some new things. the power supply is good. I sort of suspected it, it looks like its from Soviet Russia (massive piece of brown plastic).

I took an old wire (the old earth cables from my technics) and hooked them up on one end to the ground post on the mixer. Then I touched the rec outs (RCA) with the other end of the (stripped) cable. The hum instantly dissapeared, sound wise and also from both the master level meter and the PFL level meter (which was set to a mono output, sort of like the master).

Now If I took that wire and touched an rca connection with it near the master section ( main mixing channels are left and right of the master section) the hum also disappears. But If I touch an rca connection further from the master section, like channel 6 or something, the hum silences somewhat, but doesnt disappear.

Now I could rig an RCA cable so its permanently connected to the ground post and rec output, but there must be a way to fix it for good.

Another question: the capacitors look quite shoddy, with some brown goo on them. Could it be that theyre blown and they cause the problem?

[djpenguin]

So I sold my Ecler and since I'm now mixerless I dug out this bad boy again, needless to say the problem is still apparent.

but: I have found out some new things. the power supply is good. I sort of suspected it, it looks like its from Soviet Russia (massive piece of brown plastic).

I took an old wire (the old earth cables from my technics) and hooked them up on one end to the ground post on the mixer. Then I touched the rec outs (RCA) with the other end of the (stripped) cable. The hum instantly dissapeared, sound wise and also from both the master level meter and the PFL level meter (which was set to a mono output, sort of like the master).

Now If I took that wire and touched an rca connection with it near the master section ( main mixing channels are left and right of the master section) the hum also disappears. But If I touch an rca connection further from the master section, like channel 6 or something, the hum silences somewhat, but doesnt disappear.

Now I could rig an RCA cable so its permanently connected to the ground post and rec output, but there must be a way to fix it for good.

Another question: the capacitors look quite shoddy, with some brown goo on them. Could it be that theyre blown and they cause the problem?

That looks an awful lot like the electrolytic paste that's inside of capacitors. If I'm interpreting your picture correctly, we're viewing the mixer from the bottom, and the pots in the background of the picture are the mixer's faceplate controls. If that's true, it looks like the caps have leaked, and for long enough that the paste has dribbled to the bottom side of the capacitor case and collected, perhaps even dropping onto the case itself. Examine the case near where those caps sit, and see if you can find the same brownish paste. Also, do you see the brownish paste on the top (metal) part of the capacitors?

[Estacy]

indeed the bottom of the mixer. the pots form the connection to the faceplate with the circuitbords.

brownish paste is pretty much everywhere... a lot of shit on the back of the faceplate. I have no idea how this mixer was stored before I got it. But it looks a lot like dirt to me.
the metal top part of the caps look quite normal, a bit of dirt and stuff, but nothing like the body of them.

To give an idea of how much abuse this thing has went trough: the ALPS K-faders are literally filled with dirt. I am suprised they still run as smooth as they do

but apart from the hum it works beautifully (the hum is not alway present)

[djpenguin]

Do any of the capacitors look like this?

[Estacy]

they dont

[djpenguin]

That's encouraging. I think we can probably rule out leaky capacitors.

I suggest giving the mixer a serious deep cleaning. Disassemble the whole thing, then use isopropyl alcohol (90% or 99%) and cotton balls/swabs to clean all the circuit boards and components. Clean the faders with alcohol until the dirt is gone, then use DeOxit D5 spray to finish the job. Clean the connections between the channel PCBs and the main PCB especially well. Start with the isopropyl alcohol and a cotton swab, then use a a DeOxit D100 pen on the contacts. Insert and remove the channel PCBs several times to loosen as much dirt and corrosion as possible from the connection surfaces, then repeat the isopropyl and D100.

My suspicion, based on your reports about the RCA grounding, is that the individual channel PCBs ground through the main PCB, not through the chassis (where the ground lug is attached.) Dirty connections between the channel PCBs and main PCB will definitely interfere with grounding, and based on what you said about the condition of the mixer, dirty connections seem to be the most likely culprit.

[Estacy]

thanks! Now I have something to do in the weekend :p