Hi
To add a little more and possibly make detection quicker.
First with power off, use a multimeter on resistance range to measure if
the solo audio bus is actually low resistance to ground. If this appears
so, then try again with power on and note if it is the same reading (or
similar).
If it is shorted then I would initially suspect something phyiscally
near (or even in) the module where solo mix amp lived. You reported HISS
rather than HUM so it must be pretty close physically to the mix amp
otherwise the 'loop' would attract any hum in the studio and you would
hear it.
If you get the possibility of 'breaking up' the busses easily,
disconnect as many of the channels as possible to eliminate them. This
is determined by the way the frame is bussed.
I believe that some of the switching is 'series + shunt' using pairs of
FETS. (one off, the other on). It is then possible that a 'series' FET
has failed 'short' (permanently on) so dragging the bus impedance (or
input impedance it it is actually one of the FETs in the monitor
switching path) down.
In extremes a 'bus short' can actually leave the mix working even if the
short is down to 1 Ohm or less as the mix amp 'strains' to locate some
audio (poetic licence here!).
Happy hunting
Matt S
Monte McGuire wrote:
On Jul 10, 2009, at 10:19 AM, Matt Syson wrote:
Hi
I would suspect that the left mix bus is shorted to ground or that a
channel is 'contributing' to it all the time which,without the DC bus
being engaged you won't hear in the monitors until you solo something
else.The FETs are used in a 'switching' mode rather than in a linear
amplifier fashion so I would not really be looking at these until
other things were checked.
I was at a loss to figure this out initially, but Matt's analysis
makes a ton of sense. To elaborate, if something severely reduced the
summing bus impedance, the noise gain of the summing amp would
increase drastically, causing lots and lots of hiss, yet the
transmission gain of any channel might not be affected.
I'd look for some sort of "soft" short, something that would
drastically reduce the summing bus impedance, but not completely short
out the summing bus. Maybe there's a whisker of stranded copper that's
sorta shorting the bus, or possibly some switching FET that is somehow
clobbering the bus, but not completely. Many switching FETs have
channel resistances of around 20-70 ohms when fully on, and this would
be about right - enough to make the summing amp run at a really high
noise gain, but not enough to clobber the signal currents sent from
the channels.
Best of luck tracking this down! One possible procedure would be to
pull all but one of the input strips and see what happens. If that
"fixed" it, cycle each strip in individually until you find the
culprit. (BTW, if it's not obvious, you need to power the console down
before you pull or add a module, then power it up to see what the
console does after you've reconfigured it ;-)
If the problem remains with most of the modules pulled, pull every
module that sums to the 2-mix bus (e.g. all inputs, effect returns,
etc.) and see what happens. If it's still noisy, then some sort of
connection in the frame is messed up and you can look there, and avoid
messing with the modules.
Regards,
Monte McGuire
[email protected]
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