Video Mixing in MCR21

Mixer top, BBC type MX1/501

The Mixer control panel with the A & B cut rows, between them is a paper strip to write notes on, to the right are the twin faders. to the extreme right is a set of preview buttons.
Above left is a function switch Insert/Fade/Wipe and top centre the wipe pattern selector.

Mixer rear, BBC type MX1/501

The rear of the mixer with the 6 plug in "crates" Left:- The Stabilising Amplifiers, main & reserve.
Top right:- Split Screen Unit.
Top centre:- Power supply & logic unit.
Bottom Right:- The A & B Cut Fade Amplifiers.
The connections are in the centre below the three black cables that join the mixer top to the electronics crate.

The official BBC handbook(s) for the mixer run to many pages but here is a very simplified over view:-

Mixer Operation (simple)

Mixer Operation (Detailed)

MX1/501(A)    Mixer module list

The 1962 monochrome MX1/501 mixer was later converted to colour operation and given the suffix A and some individual modules were also converted and had the suffix A added. This colour version of the mixer was used in the next series of colour OB CMCRs Numbers 1 to 12. The mixer was also used in studios with appropriate modifications. 

A BBC Designs Department Technical Memorandum NO. 8.240(67) was published in 1967 detailing the modifications required.

PA6/507     Producers Control Panel.

CT3/4A       Cabinet for electronic units.

UN3/502     Control Unit

UN4/501      Split screen effects unit  1 used.    Modules:-
GE1/506      Field sawtooth generator
GE1/507      Line sawtooth generator
GE2/505A   Clamp pulse generator
GE2/506      Switch pulse generator
GE2/507      Line pulse clipper
GE2/505      Blanking pulse generator
UN9/514A  Two channel video switch
PS2/503A   Stabilised power supply

AM1/504A    Cut Fade amplifier unit   2 used.    Modules:- 
AM5/506A   Two Channel amplifier
AT3/501       Remotely controlled fader
UN9/509       Single channel switch unit
UN9/510       Three channel switch unit   3 used
PS2/503A     Stabilised power supply

AM18/503    Sync pulse stabilising amplifier unit   2 used.    Modules:-
AM18/507    Stabilising amplifier
AM3/501      Error amplifier
UN3/503       Auto time constant controller
GE2/503        Gating pulse generator
GE2/504        Sync pulse generator
UN1/501       Sync pulse separator
UN9/511       Sync source switch
UN9/512       Video source switch
PS2/10           Stabilised power supply

AM18/503(A)  Sync pulse stabilising amplifier unit   2 used.    Modules:-
AM18/520        Stabilising amplifier
AM1/558          Burst error amplifier
AM3/501          Error amplifier
UN3/503           Auto time constant controller
GE2/503B        Gating pulse generator
GE2/504B        Sync pulse generator
UN1/510A       Sync pulse separator
UN9/511A       Sync source switch
UN9/512A       Video source switch
PS2/57            Stabilised power supply
NE1/514         Relay Network