HamCall (October 1991)/HamCall (Whitehall Publishing)(1991).bin/bcast/audvid/consol.txt

Text File | 1990-10-14 | 14KB | 303 lines

                  Audio Console Troubleshooting
                               by
                           Greg Hanks

          copyright 1985 -- New York Technical Support

Console troubleshooting and upkeep will be discussed from the
viewpoints of:

1- The practicing engineer who must quickly isolate and by-pass a
problem ( on a module level )

2- The practicing engineer who must determine that the problem is
on a system level.

3- The studio owner that is to perform routine preventative
maintenance.

4- The less experienced studio person who must fix the beast, and
the type of troubleshooting mentality that encompasses successful
analysis.

To that end we will discuss:

A: Consoles in general

1- Outlining general block diagrams and their use in
understanding signal flow.

     2- Listing the most common problems that are encountered.

3- Illustrating the process of differentiating between a system
and a module problem.

B: The thought process used in troubleshooting

     1- Front, end, middle then middle to problem method.

     2- Power supply voltages!

     3- Intuitive method,(familiarity required)

     4- Most common cause method

C: Session politics, or when to ask the client to leave the room.

D: Maintenance, preventative and corrective

1- Routine stuff to be done

2- Disciplines to adhere to in performing work

3- Modifications, evaluation and implementation.




The centerpiece of any working recording studio is the console.
This piece of equipment is the one element of capital equipment
that does not change with rentals, track formats, or equipment
substitution because of failure.  Even the multi track can be
replaced for a quick fix by renting another, but it is very
difficult to wheel in another board and re-wire it in short order
to satisfy the session needs.  Therefore it becomes of paramount
importance that the console function reliably and consistently.
To this end we dedicate this piece to the operating engineer, the
less experienced maintenance person and the studio owner who must
care for his own equipment.

Consoles are very much like hamburgers in that they come in a
variety of sizes, shapes, complexity and colors but all in all,
they have the same basic functions.  Many different styles of
working exist and the specific console configuration will often
determine your preference.  Many people like to take the channel
directly to the multi track to avoid the mix bus and it's
associated circuitry, whereas others always monitor the tape
returns, with the recorder output switched to 'input' to
accommodate bus monitor.  Manufacturers attempt to follow the
trends of studio work styles and therefore design the console
operation around those ergonometric demands.  The console must
process mic inputs, line inputs, multi track returns, echo
returns, outboard equipment returns, multi track sends, echo
sends, cue sends, two track inputs and outputs and provide
monitoring of the various signal sources present in a control
room.  'Most every 'desk' manufactured today will accommodate
these requirements.  Common to every board is the necessity of
operating in a number of different modes.  The following
definitions will explain what we mean.

Modes of Operation:

Recording
For the process of recording tracks, the console is most often
configured as follows.  The microphone pre-amp is normal'ed to
the equalizer, channel fader, send controls and the output assign
matrix.  The matrix accesses the multi track buses and feeds the
multi track recorder.  The buses also feed a monitor mixer that
is then fed to the control room monitor selector, through a level
control  and then to the monitor amplifier(s) and speakers.
Almost every console does this, and only the specific order of
eq./fader, normal to direct output, placement of monitor mixer
controls and such illustrate the differences between consoles of
different manufacture.  The idea to grasp here is that they all
perform the above functions.  (Two possible exceptions are one
manufacturer who does not provide console bus monitor but always
looks at the multi track return, assuming that the multi track
will provide input/repro/sync monitor selection itself, and
another that doesn't provide bus monitor, but rather feeds the
monitor mixer with the program material that appears in the
channel, rather than the bus signal.)  The basic process of
putting signal on tape however, remains unchanged.

Overdubbing

The process of overdubbing is accomplished with a limited number
of inputs functioning as listed above, with the multi track being
monitored for the majority of the tracks, with the new signal
either being monitored through the machine, or off of the bus.
This requires that the channels (being defined below) be
configured in the 'mix' (see below) status with the exception of
the channels being used in the 'recording' status.

Mixdown

When the console is being used to mix down a multi track tape,
the 'channels' are arranged so that the channel fader and
equalizer are fed by the multi track or line input.  The output
of the fader/ equalizer feed the sends,( echo, cue and aux) and
the stereo bus through the pan network.  Some of the older
'split' consoles required the channels to be individually
switched to line input, and the channel output was then assigned
to two (or four for quad) multi track buses, which then were sent
to the two track often through the monitor pots of the multi
track monitor mixer.

All of the above can be accomplished with almost any console on
the market today, including many consoles that were intended for
the P.A.  or broadcast market, even though the latter would
require serious technical creativity to be feasible.  It can be
seen that today's recording console must incorporate considerable
switching circuitry so as to accommodate the various modes of
operation with any degree of operator speed.

Functions common to the desk are those of input, processing and
output.  These operations can be described in terms that will cut
across 'Brand Name' boundaries and help to make 'console' a
generic concept.

Input

Mic inputs are processed by the microphone pre-amp, and usually
have either a gain trim or a pad on the front end.

Line inputs are usually the multi track tape return input port.
Accessible at the patch bay, this input most often also appears
as a feed to the 'monitor' selector (to be discussed soon).  The
line input is also fed to the channel input for mixdown.  The
mic input is usually routed to the 'channel' input, where it is
equalized, level set and sent to the multi track recorder,
usually through a bus.  Most all consoles incorporate a mic/line
selector to the channel so as to accommodate track bounces, line
level feeds from synthesizers, outboard equipment and echo
devices.

Channel
The channel is the grouping of control elements that are used to
get the signal from the source, be it microphone or line input,
to the multi track recorder.  The channel, in the recording mode,
is usually comprised of the pre-amp, large fader, equalizer,
sends and output assignment matrix.  When mixing or overdubbing,
the channel is most often the large fader, equalizer and send
controls.  It is fed by the multi track, and returns to the two
track mix bus.

Track

The track location of the signal on the master tape recorder.
Track 19 would be either the return from the master tape track
#19 or the signal on the bus #19 going to the master tape
recorder.

Bus

This is the mixing structure used to group different 'channels'
to go to a common track.  The bus is accessed by the output
assign matrix at each channel.

Monitor

This tricky little devil has two meanings; first it is the feed
to the monitor speakers, but more importantly, it is the facility
to observe the signal going to and/or coming from the master
recorder.  The monitor, and the location of the bus controls are
what differentiate between an 'in-line' console and a 'discrete'
one.  The in-line system integrates the monitor function and bus
system in the same area as the channel controls.  On the other
hand the discrete type systems group the monitor system in a
separate place, the buses in a separate place and the channels in
yet another.  The only real difference between the two systems in
operation is that when monitoring program material, the in-line
system uses the stereo mix bus as the monitor mix, and the
discrete system often treats the monitor mix and stereo mix as
two discrete functions.

Troubleshooting - General Thoughts

Far and away, the greatest cause of system malfunction in the
studio is the culprit of 'cockpit error'.  When you can't get any
monitor- it's probably a dead track in solo, or the track
sheets/ashtray/roll of scratch tape or a telephone on a talk-back
or slate button causing the malady.  Something so simple as the
phantom power supply being switched off can often hang up a
tracking session.  Because today's consoles offer such vast
control and versatility, that same sea of operating controls may
have one switch inadvertently engaged rendering your attempted
action impossible.

Operator error aside, the most common gremlins present in most
well designed systems are the moving parts.  Most specifically
relays, switches, patch bay normals and pots will prove to
require more attention than their passive cousins.  Contacts are
generally designed to be self cleaning through the action of
'wiping' and many a seemingly dead connection will come to life
with but a bit of switch operation.  The preceding statement is
particularly true in the case of seldom used controls.  The
second most prolific cause of session woe is the active building
blocks, such as op-amps and transistors.  Many a time the
failures will be of a non-repetitive type, and in these cases the
mere replacement of the defective device will serve to put the
system back in operation.  Afterwards we are left to scratching
our heads as to what caused the failure.  (Most often such
failures can be attributed to a random device failure caused by
minute impurities in the silicon die).  Sometimes however the
chip will fail shortly after replacement, indicating that there
is something causing the device to self-destruct.  In these cases
it is imperative to locate and repair the offending component.
It seems that at this point we are starting to get a bit ahead of
my intended sequence so let us return to a session in progress-
with trouble come-a-calling!

Whenever a client is in attendance at a session it behooves those
in control to always keep the session moving forward in a smooth
manner.  It sometimes comes to pass that the system will decide
that it no longer wishes to respond to the engineer's caress.  At
times like this a bit of quick system evaluation is required:

Was this I/O working just a second ago?

Is there a misplaced patch?

Is the mute engaged?

These are meaningful and required questions, in that if this type
of quick analysis serves up the answer that the system is at
fault, then an unused channel is hopefully there to be accessed
via the patch bay.  If none is available then the expedient
troubleshooting of the session engineer is in order.  With an
independent engineer who is unfamiliar with the studio, the
second engineer should be there to lend a hand.  It is at this
point that familiarity with the operating habits and vagaries of
this particular console is necessary.  Do the eq's sometimes
'crap out'? To check, remove them from the circuit! Does the re-
mix relay get stubborn? Exercise it and see if the signal comes
up, if only for a moment, to try and pinpoint the problem.  The
first order of business in these situations is to get back to
work as quickly as possible with a minimum of creative
compromise.  At this point, if by using the patch bay, by-pass
switches and whatever else is available on the console for self
troubleshooting has brought no answers, a little deeper level of
troubleshooting is in order.  Let us deviate from the control
room environment a bit and discuss the philosophy of
troubleshooting.

Troubleshooting is the art/science of finding the cause of a
problem.  The science of troubleshooting involves breaking a
system down into it's component subsystems and determining which
has failed.  An understanding of the system and the intricacies
of the interaction of the subsystems is necessary for this
scientific approach.  To implement this method efficiently, a
test signal is injected to the input, and the output(s) are then
measured for proper behavior.  Proper behavior is usually
acknowledged when the signal is present at it's proper amplitude.
If the signal is incorrect, then the logical midpoint of the
subsystem chain is examined.  If the signal is found, then the
point that is logically half way between the middle and the
output is checked.  In this way it is possible to make a minimum
number of tests to determine which area the fault has occurred
in.  Branching systems, such as an input module feeding the 2
mix, aux sends, and multi track assign busses are an example of
such, and are handled in much the same manner.  To facilitate
such an examination a signal flow diagram is very useful and
should be considered as an integral part of every tool
collection.  The drawbacks of such an approach are only those of
time expenditure, which may be at a premium.  A large number of
the technical personnel in the industry today forego a number of
steps outlined above, and will often make a few cursory tests and
then go to the heart of the problem.  Intuition and experience
with the system at hand are the two tools that are used in
exchange for the rigor of the scientific method.  As stated
earlier, familiarity with the system under test is the most
proficient ally of the technician.  When the heart of the matter
is not readily noted then the rigor is needed.