38 Special Moved to 40m

by Marty Himes, WB8FNH

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"38 Special" Moved to 40m

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):


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.

 

 
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Page last updated:  April 15, 2004