BBC Mk3 Mixing Desk

[KevinE]

Hello, I've been dropped in at the deep end here. I have a BBC Mk3 mixing desk with intermittent desk O/P 'A' channel dropout and almost permanent low output compared to the B channel. The intermittent faults can sometimes be momentarily be cleared by powering down the PSU and back again.

 

I know this is less BR and more broadcast tech but I'm assuming a few BBC technicians will be lurking on here somewhere!

 

Kevin

[Pete Alcock]

Hi Kevin,

 

By Mk3 desk, do you mean a GP (General Purpose) studio type desk? These were made for the BBC by several suppliers (Neve, Audix and Calrec) and are all different even if the basic layout/facilities are the same. Is it in situ in a studio somewhere or are you trying to get an old one working after it's been in a garage for years ;-)

 

Because the desks have extensive PFL/AFL (I forget what the BBC labelling of this was, "PH" I think), with a logical approach you should be able to isolate the fault to a module or specific area of the desk. Have you checked the insert points on any attached patchbays or jackfields?

 

Pete.

[KevinS]

....or do you mean a BBC local radio mark 3 studio desk? (80's vintage - 17 channels - lots of wood - weighs a ton)

 

K

[KevinE]

Yes, the local radio Mk3. Designed in the late 70's by BBC ...certainly wouldn't like to lift one. The standard built-in facilities are a mile away from a PA desk and include a radio car OB panel, an OS control panel (leased line to remote sites), a station intercom panel and various relics such as the sonifex cart players and multi tape recording controls.

http://www.btinternet.com/~roger.beckwith/bh/locrad/images/mk3_2.jpg

 

For the uninitiated, the main input channels are in 19in card racks underneath the telephones in the pic. (under the presenter's left elbow). Behind that are the main desk ouput, PFL and Aux bus amp cards. The main input patchpanels are behind those. The broadcast limiters are on an individual channel basis and to the rear of the desk along with the cubicle monitor amps and power supplies. The main desk power supplies are right at the end of the L shape underneath the main patch bay rack.

 

I have moth-eaten schematics but to be honest they are double dutch. The desk resides at a local radio station, we're testing it with the 440Hz stereo test tone. Help!

[KevinS]

As you've identified, the input and output amps are in a rack by the operators left knee. These were standard BBC amp modules of the time and could die unexpectedly but were easily swapable - even by non-technical presenters. If one side of the main output is down or intermittent try swapping the relevant output amp module for one of the Aux. ones. If that doesn't work it could be a dodgy connector at the back of the amp rack that the slide-in module connects to.

The only things on the desk itself that usually gave problems were the multi-pole rotary selector switches - of which there were quite a few - and the multi-pin connectors that allowed easy replacement of the various panels you mention.

The horizontal top surface (supporting two LS3/5A monitors in the picture) made an ideal launchpad for coffee cups whose spilt contents ended up in the rather nice dual-rail P&G faders in the recess below. Later desks had a sloping top surface to prevent it being used as a drinks shelf.

Apart from that I think you are going to have to do some signal tracing through the birdsnest of wiring. These desks were a typical example of BBC over-engineering and the drawings you have cause much head-scratching even for BBC engineers familiar with them.

These were one of the last desks to be built in-house by the BBC's own equipment department and were constructed to battleship proportions. Despite dating from the 70's/80s they were quite easy to adapt to digital and there were quite a few in daily use until very recently so there are BBC local radio engineers around who should be able to give more insights. Ask the engineer at the station where the desk currently lives to point you to other "Mk3" stations.

KS

[KevinE]

If one side of the main output is down or intermittent try swapping the relevant output amp module for one of the Aux. ones

 

It's not going to be fixed that easy! The site engineer had done that before I arrived and I made sure it was done again while I was there.

 

This is in cubicle 1A, 1B is therefore on duty around the clock so we can't swap anything that's not already in a pair.

 

I've been inside all the racks and tapped everything from the plugs to the backplanes and routing relays, and despite a minor blip to the desk PPM when I tapped one of the channel limiters, nothing changes.

 

The building is an ex BBC station which is now run by a commercial station. The BBC engineer there retired. As you mention its all over-engineered to almost aerospace standards (or vintage B&O) and is quite impressive (mind boggling) in its complexity! Hand-loomed with cord and totally discrete components.

 

I've made contact with another BBC guy who despite having not worked on a Mk3 since 1988 has agreed to pop down in his lunch hour for a quick poke about. I'll keep you posted!

[KevinS]

What often happened in recent years when these were decommissioned was that engineers from similarly-equipped neighbouring stations would descend like vultures to plunder the useful bits - mostly the removeable panels and amp modules - to keep as spares or replacements for their own ageing desks.

Bearing in mind this is a complex 30 year old piece of gear whose caps are due for their bus pass it might be more realistic to persuade the owners to cannibalise this one to keep the other one going and buy something newer/simpler to replace it. Not that I would want you to miss the fun of spending hours/days/nights with a tone generator and signal probe admiring all that bespoke twisted copper craftmanship .....

 

KS

[KevinE]

:) That's ok, I love this sort of nonsense, but as I charge an hourly site rate, I'd be embarrassed to spend all day there when it might be fixed by some BBC wizard in 10 minutes.

 

The BBC spares stores are all intact at the comprehensive station engineering stores and available as the station was bought lock stock and barrel by the new people in 2002. However most of the racks and modules have things like 'intermittent channel' and 'B channel down -4dB' and 'warning do not use' marked all over them due to their being used by the old engineer to maintain cubicle 1B! He obviously couldnt be bothered to repair the faulty modules if he knew he'd be retiring and the BBC selling up, I suppose, or some other honest reason.

 

The place is a timewarp now and although PC playout screens reside on the console tops, they do have the Technics turntables, spendor cub monitors and H:H cub amps all in place and working.