Help with problem post-repair Mixer Rodec MX180

[Nhouse1]

Hello everybody. First post in this group! Thans for accepting me

I have a repair issue that i wanted to share.. i bet some of you would recognise this issue very easily. If some info are missing, please let me know and i will do my best to provide them.

Recently a friend brought me an old Mixer Rodec MX180 to fix. I had to replace a lot of potentiometers because they were old and the shafts were spinning freely (on this models, the shaft is attached to the knob instead of the potentiometer itself. Quite weird).




Instead of those potentiometers i had to put other ones and adapt their shape (bend the legs, file the pots.... i got crazy, really) but the repair initially went well and the the mixer was working well at my place. Just a bit humming that i couldn't understand if was too much or not, nor if was there before.


After connecting the mixer at my friend's house, the Hum was clearly too much.. so i brought it at home thinking that was a sort of Grounding issue.
Then i discover that there was a capacitor in parallel with a resistor connecting the chassis to the GND on the PCB.. If i short those, the Hum disappear completely. So i did it.

We connect it at my friend's house and initially is working perfectly.. but after a while he tells me that the noise is back but only sometimes.
I will show you a quick video that my friend sent me so that maybe something comes in your mind.
1)
2)
Do you have any idea what can this be? Maybe something comes in your mind.
Thanks a lot

 

[Whoops]

Then i discover that there was a capacitor in parallel with a resistor connecting the chassis to the GND on the PCB..

The "capacitor in parallel with a resistor" between GND and Chassis is there to prevent ground loops.
That shouldn't be the source of your problem and you should keep it, because without it you might have ground loops when connecting other audio equipment to the mixer

By the videos I would loosely say that the problem seems to be an intermittent ground connection.
Something is not connected to ground or chassis when it should be, or something is making an intermittent connection

 

[Nhouse1]

Hey!
Then , since the mixer has many PCBs inside, I think i will start trying to isolate the source of the problem.i will check continuity of GND between all the PCBs, same between PCB GND and chassis, then I will check if all legs of the OPAMP are properly connected in their socket.
It even might be that an inverting or not inverting pin doesn't make good contact.
Hopefully this will solve!

 

[Whoops]

You can always retouch all the PCB solder joints anyway. Just to be sure there’s no cold solder joints or broken ones

 

[cyrano]

I think you can get original spare parts for these, even front panels and boards. It's expensive, but it avoids problems like this one.

https://www.bekafun.com/en/news/n/302/Official-Rodec-Parts-distributor/
I'd check all solder joints on the pots first. If you need to bend a pot to make it fit, chances are that will cause a bad solder joint.

I've owned a few of these and while these are nice DJ mixers (especially in view of their age), I really don't understand the hype. Every time I need to check for parts, it seems to be another company that suspects a small goldmine.

Mind you, other Belgian brands, like Vetelec are even pricier. In that case, no parts available at all.