The Modern Radio Laboratories®
(MRL®) #39 Crystal
(Please click on the image above
more detailed photo of the radio. It is well worth it!!)
By Mike Tuggle
One evening, while perusing MRL®
Handbook-17, which I hadn't done in some time, the MRL®
#39 circuit hit me between the eyes ! This circuit has some
similarities to, and differences from, the Australian Mystery Set.
The original MRL® #39 had the
to point 3. Consider points 1 through 5 on the schematic: there are 20
possible ways to attach antenna and ground to these points -- 25, if
also consider antenna alone. Using a prototype set, I looked at all 25
and found that point 5 consistently was the best ground, and points 2
4 gave the best Selective and Broad tuning antenna connections. That's
for my particular antenna and ground, anyway -- the point here is,
Very little thought or science, on
into the design: I just roughed out a primary which would match
550 pF variable capacitor and followed the Australian design, making
secondary half that number of turns.
Here are some
start with -- feel free to adjust as needed:
C1 two-gang, 550/500 pF per gang air variable
L1 "Mystery" type basket-wound coil: 36 turns 660/46
in an 'over 2 posts / under 1 post' pattern on a 5-inch diameter,
form. Inductance 184 uH; unloaded Q = 867 at 0.7 MHz.
L2 18 turns 420/46 litz wire, inter-wound and
Details below. The total length of L1-L2 is about 1-7/8 in.
D1 diode: 1N34 type, or galena
Note on winding L2: The
carefully "synchronized" with the L1 winding so as to preclude any
turn segments lying side-by-side. This reduces the L1-L2 coupling
by some small amount. Whether this offers any performance
I don't know, but this is good basket-winding practice. For the
2 / under 1' pattern on a 13-point form, synchronized windings can be
by following exactly the pattern shown:
The elegance of the Mystery Sets,
39, and other like sets in the Telefunken genre (untuned secondary
coupled to tuned primary) is what they accomplish with just single-dial
control. The MRL® # 39 is the
ONE coil (albeit a special one), ONE capacitor (albeit a special one),
ONE diode -- that's it !
added 18 jun 2004 by Mike Tuggle
My original intent was to design an
basket coil with primary and secondary turns lying side-by-side for
capacitive coupling. But, the needed coil length came out way too
I compromised and wound the Over-2/Under-1 coil described above. This
has a near-perfect 2.5-to-1 diameter-to-length ratio, and its Qs show
unloaded Qs are above 600 over all the band and above 800 over most of
Vernier Readout Scale
I added a vernier readout scale to
help with loggings. Vernier scales are best made by
Hand-scribing tick marks is exceedingly difficult, and results are
so-so. While the idea is elegant, I'm finding that eyeballing
of a scale division with a fine hairline pointer is entirely adequate
crystal set work.
Performance is quite impressive, although
-- it doesn't hold a candle to a double-tuned set. Using a trap
a 690 kHz local, I can pick up the 720 kHz 5 kW station on Kauai, 116
away, in the daytime. [I later found it could be pulled in without the
trap.] DX conditions are way down with summer approaching, but in
a few evening sessions I've heard a few of the strongest W.C. stations
including KFI 640 L.A., KPNW 1120 Eugene and KXTA 1150 L.A. The
one is by far the most reliable west-coast station here. I'm
forward to improved conditions in the fall.
I find the BROAD connection has
for DXing, plus the advantage of more signal. While the SELECTIVE
antenna connection might be useful for coping in the urban jungle, I
thinking of omitting it in future versions of the set, until ...
I went through a systematic test of the 20 possible
connections with this set. Qualitatively, behavior was similar to
earlier versions: some connections work fine, others not at
One surprise was, at the upper end of the band, the SELECTIVE
was not only more selective, but it was also more sensitive than the
connection. The BROAD/SELECTIVE connections might be better
LOW BAND/HIGH BAND. The crossover is in the 1300 to 1400 kHz
Of course, all this may be specific to the particular antenna-ground
SW QRM Susceptibility
One potential drawback: the set is
to lower-end short-wave QRM. Fortunately I only have to contend
2.5- and 5 MHz WWVH out here. A small coil in the antenna lead may help
to knock out this QRM.
Telefunken and Other Precedents
It appears that the precedent for
the 'Mystery'-type sets is the classic Telefunken circuit that has been
around since the year one:
This particular circuit appears in
literature as the MRL® #4 set. If you
the dashed line connection you have the "Mystery Plus" set from the
Sunday Mail, April 16, 1933.
My theory has it that 'Proton'
original Mystery circuit when he screwed up the antenna and ground
to his Telefunken set. Then, making further "improvements" to his
newfound Mystery set, lo and behold, he arrived back at what is
the original Telefunken circuit !
Per the MRL®
(MRL® Handbook-17), it appears it was
Mr. R.B. Richardson, who came up with the two-gang capacitor (so-called
"Tuggle") in the tuning circuit.
Dan Petersen did some interesting,
work, starting with the Mystery circuit:
Basically, using a two-gang
Dan 'discovered' the MRL® #39 set ! He
up with Broad/Selective configurations different from mine -- possibly
due to our antenna-ground differences.