Heath SB-300 Receiver

[Stewart Lyon]

Hi Eddy (and gang),

Welcome to the world of the suprised over the SB300/1. I agree is a very
good receiver. Over the years I have made several mods to my SB-301 that
may especially interest a CW operator.

1) The easiest is also the most effective. Replace the 6BZ6 RFamp (V1)
with a 6GM6, the 6AU6 1st mixer (V2) with a 6AH6 and the 6BA6 1st IFamp
(V6) with a 6HR6. You can go too far with this and end up with a big
oscillator.  The 6GM6 is especially effective.  (Referance: QST, Feb '66,
pg 22)

2) I changed the time constants of the AVC for faster attack and slower
decay by
A) Change C114 from 5 pf to 10 pf;  B) Change C228 from 0.2 uf  to a
series combination of 1 uf and 2.2 K resistor, all of this paralleled
with a 5 Meg resistor.

3) I added a provision for monitoring the on-the-air signal. This is a
little more complicated to describe.  A) Allow the audio amp to operate
on standby. This involves R234 -- the right side is now connected to the
MUTE jack.  That end is disconnected and grounded.  B) Isolate the RFGain
from the AGC by inserting a diode in the AGC line at (2) of the AGC
switch -- the "Bar" of the diode goes to pin 2 of the switch.  C) Add an
external KeyDownGain control by paralleling the MUTE line to the
transmitter with a 20K (or so) pot. This can be done with one of those
"Y" RCA adapters.
 (Referance: QST - July '62, pg 33.)

4) The frequencies of the USB crystal and the CW filter are such that the
CW beat note is 1 KHz at the filter peak. I like something lower -- like
600 Hz. I worked out a way to diode-switch a parallel trimmer across the
USB crystal when the MODE switch is in CW.  But this is too hard to
describe in words. If there is interest, I'll figure some way to
selectively send it out.

5) I like to use my SB-401in the transceive mode with the SB-301 on CW.
As built, the 3395.4 crystal in the -401 runs all the time when in CW,
causing a beat note in the receiver. I worked out a way to grid-block key
the V2B carrier gen. so that in only runs when transmitting. Required 2
resistors, 2 capacitors and a diode.  But again too hard to describe in
words. (A picture is worth 1K words)

I've got a million of 'em.  Ways of modifing Heathkits, that is.

73, Stu - W6CUX
Winnetka, CA
ARRL Life Member

On Wed, 9 Dec 1998 19:47:10 -0500 "Edward Swynar (VE3 CUI)"
<gswynar at DURHAM.NET> writes:
>If any of you fellas has an effective modification, hint, or kink,
>specifically for or about the SB-300 receiver, I sure would love to
>learn of it!
>
>I acquired mine as a "...junker" some weeks ago, & must admit (much to
>the chagrin of my xyl), that I've absolutely fallen in love with the
>doggone thing, & adamently refuse to part with it.
>
>I'm in the process now of fixing up a thing or two that are "broken",
>but really became bonded to it during the most recent "AWA 1929 QSO
>Party", when it did yeoman's service as my mainstay receiver...
>
>Any & all info gratefully appreciated, & will be acknowledged.
>Thank-you.
>
>73, de Eddy VE3CUI - VE3XZ
>

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