To All from PA0SE
Peter, G3LDO wrote:
Older LF Receivers.
While at Amberley museum, a few weeks ago, I came across a Marconi
receiver, the CR200. It covers from about 15 to 500kHz in 4 bands so
I asked if I could borrow it to see how it shaped up to modern receivers.
The main difference between it and other receivers, apart from being
an LF receiver, is that it appears to be a TRF receiver, not a
superhet (no IF stages or cans).
It worked first time in spite of probably not having been switched on
for umpteen years. It is very sensitive with a wide range RF gain
control and a preset! LF gain control. I could hear the stronger
stations on 136kHz without any trouble.
While in QSO with M0BMU on 73kHz a couple of nights ago (Jim's signal
was 559) I asked if he would stand by while I tested this receiver
with his signal. When I switched from the TS850 to the CR200 I lost
him competely - the problem was from the Decca signals down the band.
So it would seem that the skirt selectivity of the filter was nowhere
near as good as the TS850.
Perhaps the CR200 was not designed for communications as such; it has
a multi-way socket on the front panel labelled 'DF'.
I will be returning the CR200 back to the museum on Friday and hope
that my hernia is better by the time I go on vacation.
In "Radio Bygones" of October/November 1994, Issue No. 31 there is an
article by
G.L. Grisdale PhD G5GZ called "The Marconi CR Series of Receivers".
From this I quote:
" The CR200
The CR200 (Navy B29) is a 5-valve TRF receiver (2-v-2) covering 15 to 550
kHz in four bands. Two audio stages include a narrow bandwidth filter tuned
to 1kHz for CW reception. Aerial arrangements include two 'loop' inputs,
intended for use in submarine installations where a single-turn loop or
frame aerial is fitted for submerged reception, on the lower two frequency
bands (15 to 90kHz).
For operation with such very insensitive aerials, the CR200 was given very
good sensitivity; however, with any normal aerial more than a few feet long,
the atmospheric (route) noise would exceed the inernal receiver noise. For
long-wave DX enthusiasts the good receiver sensitivity would not matter
much."
The following specification is included:
CR200 (Navy B29)
FREQUENCY RANGE: 15 - 150kHz in four bands
DC SUPPLIES: Batteries 200V22mA max, 6V 2.1A
MAINS SUPPLY: 100/110 or 200/250V AC 33W
INPUTS: Dipole or unipole via 80 Ohms feeder.
High or low-impedance loop aerial (15 - 90kHz only)
SENSITIVITY: See Note 1
OUTPUTS: Jacks for Pattern W.621 head telephones.
5000 Ohms line for Auto High-Speed reception using DC
Amplifier M31
DIMENSIONS: 19 x 13.5 x 16in. Weight 82lb
VALVES:
Position Preferred American type
V1,2 VR100 (KTW62) 6K7G
V3 VR99 (X66) 6K8G
V4 NR68 (DH63) 6Q7G
V5 6J5G (L63) 6J5G
Power unit VU71A 5U4G
Note 1: The handbook AP S.S. 104 gives no sensitivity specifications.
However, a Test Certificate for a typical B29 receiver shows a 10dB S/N
ratio was achieved across the frequency range with signals of 0.1 - 0.15µV
to the 80 Ohms input."
73, Dick, PA0SE
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