Although I once owned a
block diagram for this, it is long gone. I recall that the basic
building block was a unit called mic/prog amplifier. This was a 2
stage amp with a 30/600 ohm balanced input and a 600 ohm balanced
output ( all transformer ).
It used an EF86 and
an E88CC in a cascode circuit of some sort. There would need to have
been at least
36 of these in this desk.
The mic signal was fed in
to the first unit and the output from this fed the preset gain.
( stud rotary ) control
The preset gain output fed
into a second mic/prog amp which lifted the signal to a fair old
level for passing into the channel fader.
The outputs from the
channel faders were grouped as A B & C , which fed the A B and
C group faders and then somehow merged into a master signal. ( note:
rotary control )
From looking at the
layout, there appear to be 4 mic channels in the A group, 10 in B and
a further 4 in C, making 18 in all.
Each of these 18 channels
appears to have PFL switch, Echo mixture switch (another stud rotary
), preset gain control ( see above ) , PA button ( ?) and foldback
key.
The echo arrangements
appear to have 2 returns via quadrant faders, although there’s
apparently only 1 echo send. Echo 1 appears to be switchable to only
1 group of channels at a time, but likely that the centre group of
channels, B, were tied to echo No. 2 permanently.
Most of the other panels
are taken up with monitoring and switching functions
Being a TV sound desk,
it’s interesting to note that there appears to be some sort of
camera selector switching panel, presumably to facilitate switching
between different mics as the vision was switched between cameras ?
I can also see what
appears to be a locking Kellogg key and two panels with lighted
legends. I am assuming that this was transmit/rehearsal switching
with accompanying indication.
Extensive switchable PPM
monitoring, but note the RH mechanical zero meter movements, being
driven by valve amps which when powered on gave the meter its
electrical zero on the LH side. With the return spring to aid the
meter ballistics, these were extremely fast PPMs. ( and probably
needed to be, as there were few compressors in existence ).
All in all a wondrous
valve desk, but being a TV sound desk quite basic. No channel EQ and
definitely no pan, being mono. I hate to think how many Kw the whole
thing consumed.
I note that there is a
detailed list of Marconi TV equipment here >
www.tvcameramuseum.org/marconi/marconi_list.htm
Which mentions this desk (
BD965 ) briefly
I think that’s about it
for this one. What a shame I didn’t keep what was left of this
amazing piece of TV history !!
