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G8MNY > TECH 21.07.03 23:16l 134 Lines 6458 Bytes #999 (0) @ WW
BID : 6579_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: A Quality Crystal Set
Path: DB0FHN<DB0RGB<DB0FSG<DB0MAK<DB0MRW<OK0PKL<OK0PPL<DB0RES<ON0AR<IK1ZNW<
LZ0FBB<GB7CRV<GB7CIP
Sent: 030721/1634Z @:GB7CIP.#32.GBR.EU #:6579 [Caterham] $:6579_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
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 S.London it can pick up over
7 MW & 4 LW stations (3 French) on 40M of aerial wire. 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 tuning range.
4/ DUAL BAND, MW & LW, plug in capacitors extend LF range.
5/ HIGH & LOW IMPEDANCE AERIAL, for best matching.
6/ AERIAL NEON for protection against High Aerial Voltages.
7/ LENS to see "CAT's WHISKER on CRYSTAL"
8/ R.F.C. to improve diode linearity & performance.
9/ TAPPED AUDIO MATCHING TRANSFORMER for optimum loudness.
THE CIRCUIT
-----------
GERMANIUM
AUDIO CRYSTAL ADJUSTABLE AERIAL &
MATCHING VIEW __ DETECTOR FERRITE ROD AERIAL EARTH
TRANSFORMER LENS<__> COIL <COUPLING> COIL CONNECTIONS
= = = = = = = = = = = = = =
HEAD- _________((()___!/!____((((()_ ((((()____
PHONES ! ! ==== !\! ! ! ! ! HIGH
_ _)|| ! RFC !---. ! !---------)-----O Z
@ \ _)|| ! 2x --- ! ! 2x --- ! !
! _)|| ! 500pF --- /LW ! 500pF --- /LW ! LOW Z
@_/....._)|| ! variable ! ! ! variable! ! !-----O
! TAP _)|| --- ! --- ! ! --- (:) / Earth for
! _)|| --- ! --- ! ! --- !NEON ! High Z
! _)|| ! 10nF RF ! !1n5 ! ! !1n5 ! !
! ! ! decoupling ! ! ! ! ! ! !
!______!_______!______________!___!____!__________!___!_____!_____!___O 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 wire head band to connect 2x 40ê rocking armature Telephone
earpieces connected in series. These are the most sensitive
transducers made & peak @ 3KHz. Can be connected permanently with
flexible leads or with a plug & socket for comparing with others.
AF TRANSFORMER. for best load matching & volume I used 7 equal windings of an
old telecomms line transformer that had 2x600ê windings in 4
sections each, but the first 2 windings paralleled up 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 baseboard as it is heavy component.
10nF CAPACITOR for blocking RF from the AF load. Theory says yes, but not
really necessary.
R.F.C. Approx 20 turns of enamelled copper wire (0.3mm?) on small polo
mint sized (12mm dia) toroidal 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 seem higher, but it is not really necessary.
Mount with an insulated screw through the middle.
GERMANIUM DIODE. Viewable old large glass bubble type eg an old OA91. I mounted
a small plastic lens (from cardboard slide viewer) in front of the
crystal with paper clip wire, so that the surface of the crystal &
the internal cat's whisker could be seen.
GANGED 500pF VARIABLES. Two needed, air spaced ideally. Large value gives best
tuning range, so the sections are paralleled. Put knobs on & mount
them well to the baseboard as they get a lot of tweeking.
1n5 LW CAP. Two needed. Connected with 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 air space coils!
Mount with plastic cable clip in the middle high enough to allow
coils to move.
COILS. Two standard MW radio type with tags, eg about 30 turns of 0.3mm
enamelled copper wire on a loose fitting paper tube. 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 screw up type for Earth & the 2 aerial options, with a
swinging earth link 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 wires
compared to a 1/4 wave are always high Z. Higher & longer are always better.
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 by adjustment of the 2
variable capacitors, reduce the coupling by moving the coils apart & try
again. Note the affect of tracking both variables, there is a sweet spot for
optimum loudness.
5/ Adjust the transformer tapping for loudest output, note that there is some
treble difference as the match changes.
6/ Try the other aerial impedance option & see if the station is louder.
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 range.
For Demonstrations, connect an AF amp right across the full transformer for a
few 100mV of good quality audio!
/QSL
73 de John G8MNY @ GB7CIP
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