Some useful SB-301 mods

[Stu Lyon]

Here are some useful SB-301 mods that have been incorporated in the many 
SB-301 receivers I've owned over the years. Most (all?) are simple and 
easy to install. At one time I owned six -301s -- but right now my count 
is zero so I can't help with any pictures etc..

1. *_Using the VHF converter switch as a input attenuator._* This is an 
enhancement of Mark's work (WB8JKR) described in his posting of:  
http://listserv.tempe.gov/admin/WA.EXE?A2=ind0307&L=HEATH&P=R5283&I=-3  
My contribution is a more rigorous calculation of the component values. 
I used a noise bridge to measure the input resistance and calculated the 
attenuator components based on these values. I found the Zin on 40 mtrs 
to be about 25 ohms and on the higher bands -- 50 ohms (no measurements 
were made on 80). Using *240* ohms between HF and VHF2 jacks and *1.2K* 
between VHF2 and VHF1 jacks gives -15db / -30 db when Zin is 50 ohms. On 
40 meters where the measured Zin was 25 ohms, the attenuation was -21db 
/ -36db.  I found that Mark's idea was very useful when my neighbor is 
running a swap net at max pwr.

2. _*Fast attack AGC from the SB-101 (etc).*_ This mod. copies the 
SB-101 AGC to improve the attack time. Install a 2.4K resistor in series 
with C228 (0.2uf) at pin4 of the AGC switch. Short out R112 (4.7K). 
Parallel C114 with an additional 10pf. The AGC diodes can be replaced 
with 1N5711 Schottky diodes ($0.13 at Mouser).

3. _*Less audio hiss from product detector.*_ At the plate of V9A there 
is a 47K/ 470pf combination  (R223/ C241) that forms a 7.2KHz 1st order 
LPF on the detected audio. Place a .001uf cap in parallel with C241 to 
lower the LPF cutoff to 2.4 KHz for more pleasant audio.

4. _*A good counter cheap.*_ QST for Sept. 2006 (pg 28) describes the 
FCC1 counter that is ideal for measuring the various BFO and HFO 
oscillators in the SB rigs. It has a resolution of 1 hz, a max frequency 
of greater than 60 MHZ and cost about $40.
See: http://home.earthlink.net/~stu.ebay/images/FCC1/DCP00930.JPG  where 
I am measuring a GDO at 80 MHz.

I am now using the kit of the 21st century -- an Elecraft K2/100. But I 
still have a HW-101 running just to remind myself what high voltage 
feels like.

73, Stu W6CUX


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