What happens when two "virtual ground" buses short?

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brianroth
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What happens when two "virtual ground" buses short?

Post by brianroth »

The Usual Intro...long time lurker/seldom yacker. Yada yada <g>

Greetings to Wayne and John and others I have yet to meet ! Thanks for having me aboard!!

I am posting this here cuz I do NOT want to get into BS from other "net boards" that I read and reply to. This forum seems to be a more reliable source of smart people.

Long story short....

I'm rebuilding an ancient MCI JH-416 desk...one of the originals.

I always use a 1 kHz square wave and a Tek scope as a test of basic "go/no-go" as I fly though through the signal paths of a "new to me" desk. I have no budget for an Audio Precision sigh. Just old school/survival.

I can quickly "sniff out" things as simple as "broken" signal paths, and bad 'lytics,

Test results from This Desk briefly confused me...swap modules...change...no...another change...arrgh. LOL

I am 99% certain that a motherboard error or a "rogue" module has buses 11 and 12 tied together.

My question.....and I never thought about it

WHAT what happens when two virtual ground summing buses short together?

In an instant, I would think "NO SIGNAL"

In this case, the "answer" was a weird response from each "out"/sum bus.

Still looking...

THANKS...


Bri
Professional audio and video systems design/installation/maintenance.
www.BrianRoth.com
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JR.
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Re: What happens when two "virtual ground" buses short?

Post by JR. »

I would expect the two opamps to "share" proportionately based on their open loop response. If the two opamps are very close to identical It might only express as the routing error (two buses play when only one is assigned). If the two opamps are not identical (and they wont be) their outputs will be deviate from the average hot and cold, so each is wrong but the two together average out to right.

JR
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mediatechnology
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Re: What happens when two "virtual ground" buses short?

Post by mediatechnology »

+1 what JR said.

Thanks for the compliments Brian.

To find where it is I would work down the motherboard traces (if they're on the bottom) with an Ohmmeter and work to the lowest point of resistance.
If you see a falling resistance as you sample the length then you're getting closer; as you move past the fault the resistance will rise.
I've had to do this before and it may narrow the search area and the "relative" function of the meter helped a lot.
Ultimately, once I found it, I couldn't see the short and had to run a scribe between the traces.

If the traces are on the top, dollars-to-donuts there's a lockwasher, strand of wire, corrosion from some old Cocaine, an old razor blade (for either editing or for the aforementioned Cocaine) or something laying on top of the motherboard.
Nothing ever falls into a console right?

Based on the age you could also have a tin whisker.
I recall Grey Ingram of SSL, RIP, once had to use a high current power supply to blow open a small tin whisker on a motherboard.
brianroth
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Re: What happens when two "virtual ground" buses short?

Post by brianroth »

Thanks for the replies, guys! After decades working with studio gear, I just can't ever recall "seeing the results" of two VG buses shorted. The clue was the skewing of the square waveforms, as if some moderate amount of EQ was being applied to the two buses. Each "EQ" was different, which fits in with John's explanation. A frequency response sweep showed a few dB of "bumps"in the response of each bus, but each different from the other.

I laid on my back to get to the motherboard traces, and yep..nearly a dead short. Pulled half the I/O modules, no change. Pulled the remainder, no change. On a whim, I stood behind the desk so I could look directly down into the bus edge connectors with a strong light. Sure enough, down inside connector #9 was a tiny speck of "something shiny". I coaxed it out with an Exacto knife, and all is well!

As far as chasing shorts, I found that my "Blue" ESR meter does double-duty. Several times I've used it to "walk" a power rail to find a shorted bypass cap. But, the "relative" resistance function sounds like another excellent suggestion.

Thanks again...

Bri
Professional audio and video systems design/installation/maintenance.
www.BrianRoth.com
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