Notes on putting my 38 Special on 40 Meters...
For the adventurous amongst us.... I put my other 38S on 40 meters
a couple of days ago, and it works ok, transmit and receive. I don't
have the power mod in yet, but the stock version appears to put out about
300 mw on 7035-7045. Haven't worked anyone yet. I only got it running
late last night and hadn't much time to try it out. Harmonic suppression
appears quite reasonable. 7040 signal is 40 over S9 a few blocks away at
K8IDN's location; second and third harmonics are waaaay down. HIS
harmonics at my place, are the same low levels from his Sierra.
BEWARE: There might be some huge flaw in this somewhere. I make no
claim that this will work for anyone else. I'm just publishing what
I did. I only have very preliminary results. That's all I'll probably
ever have.
The essence of this 'mod', if you will, is to change the LO and IF
frequencies and then scale up the resonant circuit values to get them
in the right band.
Here's what I did:
I used (2) 9.216 mhz crystals (Mouser 520-HCA921-S):
one for the IF filter (X2)
and one for the transmit oscillator/BFO (X3)
I used (1) 16.257 mhz crystal (Mouser 520-HCU1625-S) crystal for the
receive local oscillator (X1).
OTHER PARTS CHANGES (ALL VALUES APPROXIMATE)
--------------------------------------------
APPROXIMATE new inductances Associated APPROXIMATE new cap values
(same T37-2 cores)
T1 22 turns (1.8 uH) C3 -> 250 pf
L1 40 turns (6.4 uH) (assume abt 15 pf varactor diode, no chg)
L2 25 turns (2.6 uH) C22 -> 150 pf
L3 10 turns (.37 uH) C28 -> 1200 pf
L4 14 turns (.83 uH) C29 -> 820 pf
L5 32 turns (4.16 uH) (PROJECTED value; UNTESTED; this is what I'll try)
These new values were calculated simply by scaling them up according to
(10.110 / 7.040), as suggested by Ori. Thanks Ori! (As a check, I
recalculated the resonant freqs, anyway, with the new values. They're
pretty close.)
That's it. All new parts go in the same places as the old parts. Trimmer
caps are left in place and work as usual.
I have no sophisticated test equipment to measure the performance of
this thing. I just winged it and got lucky. The receiver may be a bit wide
for 40m but useable. I haven't changed the receive offset cap, but it's
in the ballpark somewhere, at least for me.
Still has bad key click like my other 38S. Tick chip works fine.
Will do the power mod in a day or so.
Marty Himes WB8FNH Columbus Ohio CQRp #26
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