Studio Story Book

The Mysterious Console Blowup

The music industry must be the only industry that doesn't recognise Christmas or Easter!  In my game, you can get called out to an emergency any time, any place.  I often found myself working all weekend and on public holidays, so when I wanted time off, I grabbed what time I could.  One time we planned to take a mid-weekday off to go on a picnic.  The car was packed and we were just about to leave when a phone call put an abrupt halt to our plans...

"Help - the console has blown up!"  Apparently some people had been in doing an all night session and at 4:00am, they were "just listening to a playback when the console suddenly stopped working" (honest).  Upon arrival at the studio I confirmed that the console was in fact totally dead - not a sign of power anywhere.  OK, so the power supply must have blown up.  These things do happen sometimes.  In this case, the console was an MCI JH-600 with two power supplies - one for the audio and one for the peripherals.  Both of them were dead yet the mains-powered fans were still working.  I took the top off one of them and could not believe what I saw.  The power transformer is fitted with a termination PCB with tracks nearly 1cm wide and made of really thick copper.  Tracks were vaporised everywhere.  Fuses blown everywhere.  Wires burnt out.  I looked inside the console at the sub-regulators and every associated filter capacitor had disembowelled itself with distress.

MCI was one of the few manufacturers who went to a lot of trouble to prevent disasters like this happening.  For a start, every single audio stage is separately decoupled with carbon resistors and decoupling capacitors.  Should an IC or something else short out, the resistors are designed to burn out, acting as fuses and preventing further damage.  Every incoming power rail to a module is isolated by a diode.  In the power supply, there is always a heavy duty bridge rectifier reverse connected between the power rails and ground to prevent reverse polarity due to a fault.  Every power regulator has a "crowbar" circuit connected across its output to prevent over-voltage from damaging the equipment, should a regulator fail.  I remember a case of an MCI two track where somebody accidentally plugged a power connector in (with the power on) incorrectly, resulting in +22V being fed up the -15V rail.  Although I had to replace heaps of burnt out resistors, not one single IC was damaged.  All of this made the current disaster even more puzzling.  I figured something really violent must have occurred but when I tried to ask further questions about what could possibly have happened, no one was prepared to talk!

Two days later, I finally got the console going again.  As it turned out, no modules were damaged and the automation system was relatively unscathed.  Most of the damage appeared to be to the ground wiring in the power supplies and to the voltage regulators.  I left the studio without even a remote explanation for what had happened.

Several months later, I was asked to look at faulty jack on a box which was wired via a multicore cable to the console patch bay and used for plugging in keyboards and other instruments in the control room.  The multicore cable was that dreadful old Australian-made cable with exposed braided shields, 50 miles long so you could have taken the box out to the back of the parking area if you wanted to!  I removed the back of the metal box.  To my surprise, I found that most of the exposed braided shields were vaporised where they would have been touching the back of the box.  Burn marks on the inside of the box confirmed my theory.  The jacks in this box were insulated from the metalwork and there was no intentional connection to the metalwork anywhere else...

The console had a producer's desk attached to one side which also housed some outboard gear.  The rack had a home-made power distribution board down one side of it with exposed mains wiring on the back of it.  The side with the exposed mains wiring was bolted to the console.  I recalled that the tie-line box was often perched between the console and this rack.  (Come to think of it the box doesn't live there any more - it's always on the floor these days.)  There was a gap towards the back just big enough to allow the box to slip between the console and the rack - and touch the exposed mains!

I can see it all now - 4 o'clock in the morning, the room packed with out of it people, the monitors turned up full bore and some turkey trips over the multicore cable, somehow managing to knock the box down into the gap...  Mains fed down the console ground.  "Yep - that would do it" I thought to myself.