| |
G8MNY > TECH 04.03.05 09:34l 153 Lines 6876 Bytes #999 (0) @ WW
BID : 35036_GB7CIP
Read: DG5YM GUEST
Subj: A Quality Crystal Set
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<DK0WUE<7M3TJZ<HG8LXL<ON4HU<GB7YFS<
GB7CIP
Sent: 050304/0053Z @:GB7CIP.#32.GBR.EU #:35036 [Caterham] $:35036_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Aug 04)
Here is my design using fairly modern parts. With it came 1st in my local radio
club home constructions contest a few years ago, mainly because it was so LOUD.
It gives VERY good sensitivity & selectivity. In South London with 40M of
aerial wire it can pick up 7 MW & 4 LW stations (3 French). The stronger ones
can even just about drive a good small 8ê LS.
FEATURES
--------
1/ DUAL TUNED CIRCUITS for Selectivity.
2/ VARIABLE COUPLING for Sensitivity/Selectivity.
3/ VARIABLE INDUCTANCE also widens the tuning range.
4/ DUAL BAND, MW & LW, plug in capacitors extend range to LW.
5/ HIGH & LOW AERIAL IMPEDANCE option, for best matching.
6/ AERIAL NEON gives visible protection against High Aerial Voltages.
7/ LENS to see "CAT's WHISKER on the CRYSTAL"
8/ R.F.C. to improve diode linearity & sensitivity performance.
9/ TAPPED AUDIO MATCHING TRANSFORMER for optimum loudness.
10/ HIGH EFFICIENCY HEADPHONES home made.
THE CIRCUIT
-----------
GERMANIUM
AUDIO CRYSTAL ADJUSTABLE AERIAL &
MATCHING AUTO VIEW __ DETECTOR FERRITE ROD AERIAL EARTH
TRANSFORMER LENS<__> COIL <COUPLING> COIL CONNECTIONS
= = = = = = = = = = = = = =
SENSITIVE ÚÄÄÄÄÂÄÄ((()ÄÄÄ´<³ÄÄÂÄ((((()Ä¿ ÚÄ((((()Ä¿
HEAD- 7_³ ³ ==== ³ ³ ³ ³ HIGH
PHONES 6_)º ³ RFC _ ÃÄÄÄ¿ ³ _ ÃÄÄÄÂÄÄÄÄ)ÄÄÄÄÄÄO Z
@-\ 5_)º ³ |\ ³ ³ ³ |\ ³ ³ ³
³ 4_)º ³ 2x 300pF === /LW ³ 2x 300pF=== /LW ³ LOW Z
@Ä/.....3_)º ³ variable ³\ ³ ³ variable³\ ³ ÃÄÄÄÄÄÄO
³ TAP 2_)º ³ ³ ³ ³ ³ ³ (:) / Earth for
³ 1_)º === ³ === ³ ³ === ³NEON ³ High Z
³ )_)º ³ 10nF RF ³ ³1n5 ³ ³ ³1n5 ³ ³
³ ³ ³ decoupling ³ ³ ³ ³ ³ ³ ³
ÀÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄO EARTH
PARTS & TIPS
------------
BASE
Use a wooden base board about 25 x 10cm. Fit rubber feet so that protrusions
don't scratch.
CIRCUIT WIRING
Use tin plate strips tacked down, this is easy to solder to.
HEADPHONES
Use a coat hanger wire head band to connect 2x 40ê 1970s telephone inserts
(type 4T) rocking armature type. These are the most sensitive transducers made
& peak @ 3KHz. They are around 10dB louder once matched than any high Z moving
iron diaphragm or moving coil or crystal types.
Make a headband from a single piece of coat hanger wire looped at each end &
flattened as the common connection under 2 of the terminal nuts. Bend the wire
to angle the inserts to sit on your ears comfortably. They can be connected
permanently to the set with flexible leads or with a plug & socket for
comparing with other headphones.
AF TRANSFORMER
For best load matching & volume, I used 8 equal windings of an old telecomms
line transformer that had 2x 600ê windings in 4 sections each. The first 2 windings are paralleled up for low Z
loads to reduce the winding resistance loss for low Z loads.
Adjustable connection is made with a small wander crocodile clip to the linking
straps. Typically 2-3 of the 7 taps gives the best for the 80ê headphones.
Mount well to the base board as it is heavy component.
10nF CAPACITOR
Thus is used for blocking RF from the AF load. Theory says yes, but it is not
really necessary component.
R.F. Choke
Approx 20 turns of enamelled copper wire (0.5mm) on small polo mint sized (12mm
dia) toroidal RF ferrite core. Again theory says yes as it will keep the diode
conduction angle high & hence reduce the diode resistive losses. It will also
make the load on the tuned circuit 2x higher. But it is not really necessary
component. Mount with an insulated screw through the middle.
GERMANIUM DIODE
Viewable old large glass bubble type eg. an old OA91. Mount a small plastic
lens (from cardboard slide viewer) 2cm in front of the crystal with shaped
paper clip wire, so that the surface of the crystal & the internal cat's
whisker can clearly be seen.
GANGED 300pF VARIABLES
Two needed, air spaced ideally. Large value gives best tuning range, so the
sections are paralleled up. Put knobs on the shafts & mount them well to the
baseboard, as they get a lot of tweaking.
1n5 LW CAP
Two needed. Connected one side to ground and the other to a 2mm wander test
plugs/sockets to give Long Wave coverage.
FERRITE ROD
Standard radio type 14cm long, 8mm dia. These give plenty of L adjustment &
coupling variation. With the coils the ferrite gives high Q tuned circuits,
many times higher than lit's wire air space coils! Mount with plastic cable
clip in the middle with high enough packing/washers to allow coils to be moved.
COILS
Two standard MW radio type with tags, eg about 30 turns of 0.5mm enamelled
copper wire on a loose fitting paper tube with connection tags. Connect with
twisted flexible leads as they will need sliding along the ferrite rod to vary
the inductances & coupling.
NEON
Wire ended 80v bulb type. Used to safely & dramatically discharge the aerial in
the low Z mode, when static is around.
CONNECTIONS
4mm socket with screw up type for Earth & the 2 aerial options, with a swinging
earth link plate, to make the high Z aerial configuration.
TUNING IN
---------
1/ Connect an earth, mains earth can be noisy, old Gas & Water pipes are better
earths as long as they are not PLASTIC!
2/ Connect the aerial to either the High Z or Low Z options. Short aerial wires
compared to a 1/8 wave are always high Z. Higher & longer aerial wires are
always better, as they pick up more RF power.
3/ With the coils close together adjust the detector & aerial variable
capacitors for a station of interest.
4/ If the station is loud enough, but can't be separated from another one try
adjustment of the 2 variable capacitors at once, also reduce the coupling by
moving the coils apart & try again. Note the affect of tracking both
variables, there is a best matching sweet spot for optimum loudness.
5/ Adjust the auto-transformer tapping for loudest output, note that there is
some treble difference as the match changes due to the RF decoupling Cap.
6/ Try the other aerial impedance option & see if other stations are heard.
7/ If the station is too HF or LF for the tuning range, try moving the coils
more to the middle for LF, or to the far ends for HF.
8/ With the LW capacitors connected the tuning is less effective & moving the
coils gives more frequency range.
For Demonstrations, connect an AF amplifier right across the full transformer
for a few 100mV of good quality AM audio!
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
