Studer 089 MkII hum/noise issue

RFSiesta · Jul 8, 2016

Hello,
I try to solve an issue with a Studer 089 MkII broadcast console that's beeing refurbished in the studio. Andy has done great work and effort to bring back the PSU and modules back to spec and restore the sonics of this beautifully sounding board. (Especially the PWM comp/limiters are sweet)
However, we didn't get a hold of some ground problem that results in excessive noise and hum, if - and ONLY - if it's connected via patchbay, even only on one channel. If we use an external cable and cut all connections to the pb, it's almost gone. It also decreases a lot when I ground the cable shield separately.
The (newly installed) pb seems to act as an antenna, since most connections are still unused. There are no connections between channels, though.
So to mee, it looked like a big big pin1 problem, but then we checked grounding resistance: it was ~0,1Ω between shield and modulation gnd when switched off, and increased to 30-100Ω (!!) when the device was powered. Measured the other way around, R gets 0Ω. So I figured there could be DC offset and voilà: 13mV offset between the two grounds, which are PHYSICALLY connected to each other with a bridge on the outside...
Since the manual gives precise details about each module, but not too much about housing and interconnections, we're stuck now.
Next will be to try and implement a copper bar for ground and connect each Lemo connector's ground pin to it. Still have no explanation for a DC offset.
Any ideas?

Thanks!
Richard

zamproject · Jul 8, 2016

Hi Richard

Not so many ideas right now, but maybe you can check the X1 capacitor between frame and -22 at bus board.
Did the noise stay with P48 off ?

Best
Zam

RFSiesta · Jul 8, 2016

zamproject said:
Not so many ideas right now, but maybe you can check the X1 capacitor between frame and -22 at bus board.
Did the noise stay with P48 off ?

The 0.1 WIMA mp3-x2 suppression cap? have to check... good input!
And no, I think we didn't try without 48. Noise comes when I connect ANYTHING to the shield pin of ANY input. Don't have to even pull up the channel, only the master. So most of it seems to go over the 2bus sum, which is what I would expect from an active summing network with noise residing on the ground bus...

zamproject · Jul 8, 2016

Hi
Yes this one.
This metal film tend to degrade after 40 years...
And what happen if you remove the bridge between 0V audio and ground/frame at the back of the desk?
Also is the PB fitted in a rack with some other gear ? What happen with the PB completely "floating" ?
Best
Zam

RFSiesta · Jul 8, 2016

zamproject said:
Hi
Yes this one.
This metal film tend to degrade after 40 years...
And what happen if you remove the bridge between 0V audio and ground/frame at the back of the desk?
Also is the PB fitted in a rack with some other gear ? What happen with the PB completely "floating" ?
Best
Zam

Caps had been changed. So should be OK.
We compared with removed bridge of course, not better.
The PB is not connected to anything but other patchbays

It should be completely floating! except that we had some unexpected ground connections between channels somewhere, I guess mechanical shortcuts.
I felt it's the sum of all screens and shields, the noise is easily influenced even by touching the pins.
br richard

RFSiesta · Aug 1, 2016

Bump !

One more try!

Still no solution, but we have not tried much since then. There's still the strange issue of a relatively clean signal through an (even unconnected) mic cable VS. lots of noise, a little hum and a *peep* when I go through the patchbay. Which is, as stated before, clean and not connected to anything, except test equipment...
The noise gets a lot less when I connect the ground pin to - ground. Which should be, and is, the case already at the back of the mixer.
When the mixer is off, I get near zero resistance between ground pin on the connectors and the ground merge point at the back of the 089.
When switched on, there is a resistance (only in one direction

. It's different, between 30 and 100Ω at different input's pin1 plus I measure >10mV DC between both "grounds".
Well.

Still waiting for ideas. The scope shows something in a couple of HF regions, but it also does when I only measure a 1k resistor. Looks like we have this in the air, our building's mains power is done completely new and should be as clean as available in any civilized country...
And: No, it's not the scope. It's a legendary 4-ch. Tektroniks in perfect working condition, and my crappy Voltcraft cheepo scope shows similar results.

...