Studer 269 ground issues

[Jonsson]

I’m having some trouble localizing a ground related problem with my Studer 269.
I would really appreciate some help since I’m starting to second guess here. I’m a bass player who’s basically self taught at electronics. I know just enough to be dangerous, I guess.

When nothing else is connected except my speakers there’s no hum.
When I connect my converters to Studer Line Inputs there’s 50 Hz and multiple overtones, mostly 350 Hz.
I also have a Studer 169, same desk but 8 inputs instead of 14. When I connect the 169 to my converters there’s no hum.
Since the Studers are modular I have swapped modules around and concluded that the problem must be in the power supply.

I’ve already replaced the electrolytics in the PSU.
I get correct voltages, +15, -15 and 48V.

So my question is if the problem could be the other capacitors, de-coupling film caps or ceramic caps?
Could it be something else?

Fredrik

 

[zamproject]

Hello Fredrik

There is a ground lif jumper at PSU secondary, how it is yet ?

Best
Zam

 

[Jonsson]

Hi Zam.

Thanks for your reply.

What jumper is that?
Is it on the psu pcb?
I can't find it. I don't see any info about it in the manual either.

Fredrik

 

[zamproject]

Hi Zam.

Thanks for your reply.

What jumper is that?
Is it on the psu pcb?
I can't find it. I don't see any info about it in the manual either.

Fredrik

In the schemo it's the one at center right close to the 0V point (between +/-15)
At pcb you can't miss it... 3x M3 screw with large metal plate...
Best
Zam

 

[Jonsson]

Aah, thank you.

I was looking for a small jumper. This is more lika a big metal bridge 

It's connected to earth right now.
I'm pretty sure it's the same as on my 169 which doesn't have any hum.
That suggests there is something else causing the hum.

But I'll try to lift it and see what happens next time I'm at the studio.

Thanks

 

[JohnRoberts]

I am not enthusiastic about lifting safety grounds.

Perhaps google  "pin 1 problem"...  Improper handling of shield noise currents can corrupt audio signal.

JR

 

[Jonsson]

Hi John.

Thanks for your reply.

I have read a lot about grounding problems lately 
Not least the great Rane articles.

I'm pretty sure it's not a "pin 1 problem".
Mainly because there's no hum when I connect my converters to my other Studer desk. They are identical except the number of inputs.

I have swapped input, master and monitor modules between the two Studer mixers and it's not related to the modules.
That leaves the power supply.

Would a faulty de-coupling cap on the psu result in this behavior?

Thanks
Fredrik

 

[Bo Deadly]

Ground lift is a hack. Fix it for real.

Divide and conquer.

How is the "converter" connected to the console? XLR cables? If multiple, does the hum change as you disconnect one or the other but not both?

If you disconnect all cables at the console but use a DMM to measure DC between pin 1 of the XLR cable coming from the converter and pin 1 of the XLR sockets of your console. Do you see any voltage? How much? Do the same measuring current - how much current? Do the same using continuity - how much resistance?

You should see no voltage difference (maybe a few mV is ok) and absolutely zero current and nearly zero resistance (a few ohms is ok). If not, something isn't quite grounded as it should be.

What are your converters and console plugged into? The same wall socket? What else is plugged into that wall socket? Get a power strip and plug just the converters and console into the same power strip. Use an extension cord to try plugging that into different wall sockets. Any change in hum?

And so on and so forth ...

 

[zamproject]

I am not enthusiastic about lifting safety grounds.

I totally agree, I just ask the status of the jumper, it can be a temporary test (with all care due)
beside that this one is for all regulated out 0V ref to earth, the primary and chassis still linked anyway (if no faulty wire at primary...)

Best
Zam

 

[Jonsson]

Ok, something else is causing this ground hum. I'll have to investigate further into that.

But what's bothering me is that my other mixer is dead quiet. An identical mixing console. Everything else is the same, same cables, same converters, connected to the same outlet and so forth.
Surely this means one console is suppressing the hum more effectively?
Wouldn't that imply that something is broken in the other console?

Am I right in assuming this could be related to the de-coupling caps in the power supply?

I really appreciate your help.
Trying to learn as much as I can.

Thanks.
Fredrik

 

[Bo Deadly]

Am I right in assuming this could be related to the de-coupling caps in the power supply?

No. It could be lot's of things. Just to give you some random examples, it could be some corrosion on a screw or a bad solder joint. That introduces a little resistance and then you get a voltage (noise) across it. Some tests with a DMM as described previously might confirm that (if you see a voltage between ground of the "converters" unit and ground of console, that is a bright red flag). Another possibility would be bad caps on some important part of the signal chain (eg filter cap on input of high gain circuit).

If it were bad power supply filter caps the console would always be noisy. But you said it was only noisey when you plugged in a particular piece of gear which would suggest a ground issue. Modern gear often use switching power supplies and floating grounds. So it could be bleeding in from that gear. In fact this very common of the gear are plugged into different ground points. For example if you have the console plugged into one ground socket and the "converters" unit plugged into a different ground plug, the console ground probably has a lot of current on it and thus the ground potential could be a little different from the "converters" unit. That will create DC between the units which can cause noise. The solution could be as simple as plugging them into the same power strip so that the ground potentials are closer and thus less DC between units.

If you remove the shell of the XLR and attach an allegator clip to pin 1 and then ground that separately to earth ground or test various grounds, does the noise change? Try it with pins 2 and 3. When debugging noise in circuits, it is common to ground different points in the circuit (being careful not to touch anything with DC!) and see if the noise stops. If it does, then you know the problem is upstream. But I don't recommend that you do that in this case because it's dangerous if you really don't know the circuits really well. You could just break something else. But you can try grounding pins of the XLR. That would confirm that it is bleed from this other piece of gear.

 

[Jonsson]

Ok, I got some time off to do some testing today.

I measured between pin 1 of Studer line in and out of converter.
No voltage, no current and about 170 Ohms.
I have a Metric Halo 2882 which have an external power supply. I guess that’s the reason the ground potential isn’t exactly the same.

I discovered one thing, though.
The Studer 269 have some ground noise even when nothing else is connected. And it gets worse when connecting the Metric Halo.

Compared with my Studer 169:
Quiet on it’s own and quiet connected to the Metric Halo.

I tried lifting 0V from earth ground on the Studer psu, as Zam suggested. The Studer was then quiet on it’s own, no noise. When I connected the Metric Halo I got noise again.

Somethings wrong with the Studer power supply, for sure.

I’m not sure what to do next.

 

[zamproject]

ok...

Check "power pack" (1.169.113) earth IEC socket conductivity across chassis (no main powered !)
Tel us what do you read.

If you don't have ultra low resistance, remove and open this "power pack" (it's just IEC, V selector and main trafo)
and check wire, especially the yel/grn one which should be IEC earth to chassis.

Best
Zam

 

[Bo Deadly]

I measured between pin 1 of Studer line in and out of converter.
No voltage, no current and about 170 Ohms.
I have a Metric Halo 2882 which have an external power supply. I guess that’s the reason the ground potential isn’t exactly the same.

Actually there is more too this than I communicated in my previous post.

From Googling Metric Halo 2882 images, it is clear the external power supply is the "floating" variety. Meaning if the MH power supply is not grounded then comparing it's ground (by itself) to the ground of the Studer is not going to give you a meaningful result. By connecting the Studer and MH, you are setting the ground of the MH to follow the Studer which is fine.

However, is there anything ELSE connected to the MH? Is it plugged into a computer? If yes, then realize that you are connecting the ground of the computer to the ground of your Studer. Is the computer ground floating? If you unplug this stuff like the USB cable do you hear a change? Is there anything else plugged into the MH?

I discovered one thing, though.
The Studer 269 have some ground noise even when nothing else is connected. And it gets worse when connecting the Metric Halo.

Ok, so you swapping modules, changed power supply filter caps and the other Studer does not have noise. So what circuitry is left? I'm not familiar with these Studer units but there is a backplane right? Does that have circuits on it? Are there electrolytics on it? Or is it just the PS? What filter caps did you replace with? Where are the summing amps? Maybe you have a bad buss connection. Maybe there's a corroded spade connecting the ground plane of the backplane to the chassis? Use the DMM to santity check ground points everywhere.

Compared with my Studer 169:
Quiet on it’s own and quiet connected to the Metric Halo.

I tried lifting 0V from earth ground on the Studer psu, as Zam suggested. The Studer was then quiet on it’s own, no noise. When I connected the Metric Halo I got noise again.

You could also try ground lifting at the XLR just as a test. Meaning disconnect pin 1 (unsolder it temporarily in a non-destructive way). That would not even be a "hack" actually. Ground lifting the PS is not a good idea. But disconnecting grounds between the MH and Studer is a completely reasonable thing to do (although apparently unnecessary if the other Studer works flawlessly in place).

Somethings wrong with the Studer power supply, for sure.

Transformers are very good at rejecting noise so I still think it smells more like a grounding issue. But if the other console works flawlessly in it's place, then it would have to be something between pin 1 and the earth ground (check resistance with a meter) like a corroded wire.

Either that or your test is not really what you claim in that the other Studer is not really using the same peripherals. Meaning a cable or wall socket or whatever is actually different. Not all wall sockets are identical.

 

[Jonsson]

Thank you once again for your help.

Since there is hum in the console with nothing else connected I'm just going to focus on that for the moment.

The power supply is mounted on the chassi and connected to a backplane pcb. No other circuits.
There are five polyester caps at different points between 0V bus and earth ground on the backplane.

Nothing connected to the console except a speaker on master output.

I have measured resistance between IEC earth and different parts of the chassi.
It reads about 1 Ohm everywhere.

I Have measured resistance between earth ground and pin 1 (and other 0V points).
It reads about 1 Ohm.

I removed all input modules in the console.
No noise!

The hum increase with every input module i put in. It doesn't matter in what order or what position I put them back.

When the console is turned on I get about 2 Ohm between the 0V bus and earth ground.

I compared with the other Studer console:
Turned off I get about 1 Ohm between 0V and earth ground.
Turned on I get about 22 Ohm between 0V and earth ground.
I don't know if that's relevant but I thought it was strange that I get a different reading.

Thanks
Fredrik

 

[Bo Deadly]

Ok, well if it hums with only a monitor speaker connected then that is important to know. That wasn't what you were saying before though. If you only get hum when plugging in something connected through a transformer, then I would immediately suspect ground issues. But now you're not saying that so ...

Just to minimize external forces as much as possible, plug in the speaker monitor power cable into a socket adjacent to the Studer power cable.

Measuring 1 ohm between grounds of the same unit is actually a little high. But it could be your ohm meter. What does your ohm meter read when you simply connect the two test points together? Mine reads 0.5 ohms.

The 22 ohms sounds like it could be error. Make sure you stab the test points into metal that is not anodized. So maybe a machined edge or a bushing is better.

Is the 22 ohms between bus ground and earth ground from the good unit or the noisey unit? I don't know why it would be different. If it's really not just a bad test, maybe the noise is throwing off the meter.

My "console" is just some 1U channel strips with a shared external power supply. If I measure resistance between test point on my meter I get 0.4 ohms. When I measure resistance between earth ground of the external supply plug and pin 1 of an XLR input of one of the channels, I get 0.5 ohms. With the power on, I get 3.3 ohms. So it seems a change is not completely abnormal (although I make all of my gear completely from scratch so I don't know for sure what is "normal").

Polyester caps are probably not the problem. They don't dry out like electrolytics. When you replaced the power supply filter caps, did you replace all of them or just a few? There are probably all sorts of other smaller electrolytics in the PS. They can be just as important if not more important than the rectifier filter caps. What did you replace them with? What size? What voltage rating? Do you have a part or series number? It could also be that the caps in the channels need replacing too.

 

[Jonsson]

Sorry about the confusion.
At first I thought it was a grounding problem when connecting other equipment.
Now I need to find out what is causing this noise. This noise is there even when nothing else is connected to the console.
It goes away if I remove all input modules.

My meter reads 0.7 Ohms when connecting the probes.

22 Ohms was the other good unit.

I have replaced all electrolytic caps according to the specs in the manual.
Vishay and Panasonic FC.

I have also replaced all electrolytics in the modules.

Would you still think it’s a grounding issue?

Since the noise is increasing with the number of input modules I put in, could it be related to the increase of power consumption?

Thanks
Fredrik

 

[chops]

Have  you loaded the "noisy" desk with modules from the "clean" desk?  Same noise? 

 

[Bo Deadly]

Sorry about the confusion.
At first I thought it was a grounding problem when connecting other equipment.
Now I need to find out what is causing this noise. This noise is there even when nothing else is connected to the console.
It goes away if I remove all input modules.

My meter reads 0.7 Ohms when connecting the probes.

22 Ohms was the other good unit.

I have replaced all electrolytic caps according to the specs in the manual.
Vishay and Panasonic FC.

I have also replaced all electrolytics in the modules.

Would you still think it’s a grounding issue?

Since the noise is increasing with the number of input modules I put in, could it be related to the increase of power consumption?

I don't know. It's less clear now. Are we still talking about hum or is it hiss now?

Now we're talking about problems that cannot be solved without opening it up and poking around (which is slightly dangerous because a meter probe tip could slip and short something or a cap that is still charged could zap something, ... etc). But I suppose you don't have any other choice really.

Do you understand how the circuit is laid out on the boards? Do you have a scope? Do you have a power supply schematic (post it)? You can ground an AC signal points and listen. Just make sure you first check that there is no DC at that point. Specifically, install one channel and listen to the noise. Then, with no signal input, carefully ground an AC signal point near the output of the channel. Does the noise change? If it is less, then you know the noise source is upstream and in the channel. You might need to pick different ground points. Another thing to try might be to add a load to the supply (like a 1K resistor across 24V momentarily) and see if the noise changes. Etc. Use deductive reasoning. Divide and conquer.

It is strange that there is 22 ohms of resistance between analog 0V and earth ground when the power is on in the *good* unit. That seems a little high but maybe it's just because it's drawing a lot more power and noise is throwing off the meter. But that is a new phenomenon for me (increased resistance between grounds when measured with power on as opposed to off).

 

[radardoug]

You can only reliably measure grounds when the power is off. To measure resistance, your meter runs a small current through the circuit. If you turn the device on, then other currents can be flowing through the circuit, which will alter the ohmmeters reading. Measuring low ohms is also subject to the meters own resistance. First short the meter probes together and see what it reads. You can subtract this reading from your final reading.