POWER SUPPLY SOLVED

[flintstone mop]

Hello fellow OP's

I'm working on a large audio console and there's a strange voltage problem.
There are several power supplies involved. A plus and minus supply and a Phantom power supply for condenser mics.

Should the electronic ground, the power supplies, touch the safety ground??

All of the modules get their ground through the main buss.

IF I connect a ground wire to the stud mounted on the module in the console that attaches to the PS, the 48 volt supply is shorted out.

No circuit damage, as this is current limited supply, and can sit there all day long shorted out.

[W1DAN]

Hi:

I think your question needs a little more clarification.

Is the power supply external or internal?

For the +48v supply (and all the supplies), the ground should tie with the supply (and eventually the signal) ground. The supply and signal grounds should meet at one point in the console (i.e. a star topology).

The primary mains input should be grounded, but if you have a hum loop, you may need to violate NEC code and lift the mains ground at the wall (I had to do this with two analog Neve desks I installed years ago).

In the end it depends on how well the manufacturer managed their grounding system. The star topology is most immune to ground loop hum. The buss topology can be troublesome depending on where the currents are conducted and where the modules are.

Hope this helps.

73,
Dan
W1DAN

[flintstone mop]

Thanks Dan,
The power supply is external to the console.
I get a little dizzy looking at blueprints and following circuits.

The negative side of the + and - supply do not touch the safety ground (the case of the power supply )

The negative side of the 48 volt supply does.

It gets interesting as we move out of the PS to the first module ("control module") what I can see is that it just distibutes the +/- and 48V to the buss. And uses the +/- for its electronic needs. It gets interesting here: The minus wire from the 48V is connected to the power supply minus wire for the +/- volts.

There are about 20 modules that connect to the buss to get their operating volts and put their audio into the mix. There are 5 modules that can use that 48V to provide phantom power.

The problem shows itself when I connect something external to the control module. The 2TR Record out and control room monitors and many other ins/outs are on this module.

Anything external connected to those jacks kills the 48 volts. The recording computer and control room monitors (powered) are temporarily all plugged into the same grounded outlet. There was never any weird stuff like this (difference of potential) with these external devices, until I connected to this console. Wheatstone SP5

My discovery started troubleshooting a loss of 48 volts and noticed that someone cut the 48 volt wires coming from the PS to this module. Apparently they did not want to find out what the real problem is.

I could work around it by isolating the jacks. I could do it by using 1:1 transformers, and the problem is covered over once again.
I hope this is more useful.
Fred

[WD5JKO]

Fred,

With something like what your describing, I think it is possible that this setup was NEVER correct. It could take a while to unravel this onion, and cry as you reveal each layer.

Do you have any schematics, or pictures you can pass along?

Good Luck,
Jim
WD5JKO

[flintstone mop]

The schematics are huge blueprints.
I'll try a little experimentation of grounding the ground lug of this module to the safety ground and see what happens.
That would essentially take the frame of the console to the safety ground level.
Safety ground being the green wire ground of the electrical system.

Fred

[flintstone mop]

OK Hi Brian
It is one transformer with many secondary taps for 48V and 15 Volts. Your usual stuff and regulators to derive +/- volts and the 48V

Fred

[flintstone mop]

I'm almost on my knees. Praise the Lawd!!
This was haunting me and my beeping ohm meter. The CASE of the regulator transistor was touching the chassis ground. It was flipping the polarity of the 48V.
It is large old style output type transistor and nothing to prevent the screws going through the heatsink to touch the chassis. The transistor case has to be insulated from the chassis.
I enlarged the holes into the circuit board and now I can get the electrical connection to the case AND it is not touching the chassis anymore. So, I'm golden.

Fred

[flintstone mop]

Last entry:
For late 80's technology I'm impressed!

I did a noise check and freq response of 32 channels and the noise was around -75dB and freq response did not vary more than a 1dB 20hz-19kc.
The noise figure might not impress an "audio file" but I do not play classical music or produce a commercially available product.
For my purpose, it's Nirvanna

Thanks for reading. This post might get flipped to QSO
Fred...cheers