[Kenwood] SM-220 or 230

[Bryan Swadener]

Hi John,

It's actually quite simple.  Refer to the schematic I
found online:
http://www.tech-systems-labs.com/schematic/SB-620_sch.gif

The Heathkit SB-620's input amplifier "toob" (V1,
6EW6) can be switched to operate as a mixer instead. 
According to the manual, you would toggle slide switch
S2 on the rear from "Ham Scan" to "Spectrum Analyzer",
feed your IF signal to the "RF Input" jack (J2), and
your LO signal to the "IF-SIG Gen" jack (J1).

V1 has a bandpass filter, usually set-up for either
455 KHz or 3395 KHz (mine came to me set-up for 3395
KHz) in its plate circuit, so you'll see only the
desired signal.  To see 8830 KHz, we would need a LO
of either 8830 + 3395 (12225 KHz), or 8830 - 3395
(5435 KHz).  As it happens, crystals for some old 2M
rigs are in the 12 MHz range.  I found one for an old
Icom marked "146.76", which had a fundamental
frequency of 146.76 / 12 = 12.230 MHz.  In order to
sweep your receiver IF, the SB-620 employs a sweep
oscillator that uses a slug-tuned inductor (L3) and a
varicap diode (D1).  It took only a minor tweak of the
inductor to put the 'Pip Center' pot in the middle of
its range.  I fashioned my oscillator "dead bug" style
on a scrap of PCB material, with an RCA plug on the
edge, and it plugs into J2.  Power comes from the
station power supply.

Using the input amplifier as a mixer results in some
decrease in gain.  Since my TS120S (and later TS130S)
didn't have provision for a panadapter, I would need
to build a buffer amplifier anyway, and I threw one
together using a common 2N3904.  As tested on a
network analyzer, it shows about 18dB gain. It has a
high-enough input Z to prevent loading of the mixer
output, and a low-enough output Z to drive the coaxial
cable and cathode of V1.  See
http://www.geocities.com/bswadener/panadapter/ for a
schematic, parts list, and a couple of photos.

I noted something interesting between using the SB-620
with my old SB-102 and later with the Kenwood rigs. 
The SB-102 has some bandwidth limiting via a
"preselector" in the frontend.  While the SB-620 can
sweep much wider than +/- 50 KHz, the SB-102 wouldn't
respond that wide.  The Kenwood frontend uses a
bandpass filter, and you get the entire band at the
input of the SB-620.  Now, the limiting factor is the
physical size of the SB-620's CRT.  I can sweep the
entire band but... the CRT is VERY crowded! hi!

vy 73,
Bryan WA7PRC


> Thanks for the info Bryan.  
> 
> So, I didn't know that I could use the SB-620.
> I sold that last hear!! Dang!!!
> 
> I would be interested in hearing more about how
> you did the interface.  I'm saving my pennies
> and selling some things over the next couple of
> months to purchase a TS-850 from a good friend,
> almost like an internet brother!  We have so
> much in common, it's really scary!! 
> 
> 73,
> 
> John KD7KFT