[Heathkit] SB-104 VFO question

Fri Jul 20 13:08:25 EDT 2012

Hi,

The output of the VFO (same as the input to the VFO filter) should be 180mVrms. 
The output of the filter should be 50mVrms. Low VFO output can result in 
both erratic display and receiver operation. If the output is too high the 
birdies will be strong, too.

For troubleshooting I'd crank up the VFO output to see if the display still 
jumps. I'd also measure it on a scope. If cranking up the output makes reception 
stable then its probably a VFO level issue and not the tuning cap in the 
VFO. If it doesn't solve the problem then I would look at the tuning cap in the 
VFO. Also make sure that the large ceramic coil form inside the VFO isn't 
broken. 

73, Bill WA6NEQ

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Subject: Heathkit Digest, Vol 102, Issue 11

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Today's Topics:

  1. SB-104 VFO question (Alexander N. Gerli)
  2. Re: SB-104 VFO question (Don Cunningham)
  3. Re: [Heath] SB-104 VFO question (G3OOU at aol.com)

----------------------------------------------------------------------

Message: 1
Date: Thu, 19 Jul 2012 22:03:32 -0400
From: "Alexander N. Gerli" <ac1wye at gmail.com>
Subject: [Heathkit] SB-104 VFO question
To: <heath at puck.nether.net>,    <heathkit at mailman.qth.net>
Message-ID: <000901cd661b$deeebb30$9ccc3190$@com>
Content-Type: text/plain;    charset="us-ascii"

Hi, folks,

Well, finally after a two-month 'break' to fix up our (now) nice retirement
QTH and move in, I finally had a chance to try out my kit-built tennis-ball
antenna launcher. Works great, but you gotta aim right. Even though my
80-metre loop is up, the bottom apex is just a few feet off the lawn. At
least I can tune it and get some sort of signal out HIHI.

About the '104. It runs all right, but the VFO seems to tune a bit funny. I
can go across a signal, but sometimes the digits (frequency) jumps a bit. I
tried to adjust the VFO output before I packed it up and thought I had it
right. Well, no I guess not. The manual gives some information on how to
adjust the trimmer inside the access hole on the side of the VFO can, but I
can't seem to nail the output correctly. Once you get to a frequency, it
doesn't drift. But getting there can be a bit of a wobbly tuning exercise.

Anyone have some similar experience and a possible fix they can share?

73,

Sandy AC1Y

Alexander N. (Sandy) Gerli

'Corgi Cottage'

548 Allred Mill Road

Mount Airy, NC 27030-2202

919-625-7181

A Scotsmen walks about 900 miles each year

Scotsmen drink about 40 gallons of whisky each year

Scotsmen get about 25 miles per gallon

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Message: 2
Date: Thu, 19 Jul 2012 21:19:30 -0500
From: "Don Cunningham" <donc at martineer.net>
Subject: Re: [Heathkit] SB-104 VFO question
To: "Alexander N. Gerli" <ac1wye at gmail.com>,
    <heathkit at mailman.qth.net>
Message-ID: <E9DA83AC397C402C8961E8342DAD0814 at DON>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
    reply-type=original

Sandy,
My 104 does that also.  I suspect that mine is lubricant dried on the shaft 
of the VFO.  Some seem to have felt that if a little bit was nice, a LOT 
would be better, hi.  I think if I clean mine and apply only a little 
synthetic oil on the shaft, mine will settle down.  Should you find it's 
another cause, please share, but it just acts too much like some of my other 
rigs that have been solved by cleaning the gobs of grease off the shafts.
73,
Don, WB5HAK 

------------------------------

Message: 3
Date: Fri, 20 Jul 2012 10:17:12 -0400 (EDT)
From: G3OOU at aol.com
Subject: Re: [Heathkit] [Heath] SB-104 VFO question
To: heath at puck.nether.net, heathkit at mailman.qth.net
Message-ID: <3c235.2ed5dd56.3d3ac268 at aol.com>
Content-Type: text/plain; charset="US-ASCII"

Hi Sandy

If the audible signal jumps in frequency as you tune then it is probably  
the rotary contact(s) on the VFO tuning capacitor that have dried out or you  
have an intermittent contact somewhere in the VFO. The former is a common  
problem on the SB-Line valve rigs and is easy to cure once you can get at 
the  tuning capacitor to re-lubricate the bearings and sliding contacts.

I recall that another suggestion was to solder a length of braiding between 
the rotor and the body of the tuning capacitor although I have not tried 
that on  my rigs.

73

Bob

Bob F Burns
C Eng,  FIET, MSE
Amateur Radio Callsign: G3OOU
Registered RSGB  Instructor
G-QRP Member No: 6907; Zone: 14, QRA: IO91WH; WAB:  TQ25
Secretary of Crystal Palace Radio & Electronics Club: _www.g3oou.co.uk_ 
(http://www.g3oou.co.uk/) 
Technical web site: _www.qsl.net/g3oou_ (http://www.qsl.net/g3oou) 

------------------------------

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End of Heathkit Digest, Vol 102, Issue 11
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