[Heathkit] SB-101 AVC problem

[Greg Mijal]

Hi Chris:
That's a nice rig you have there.  I run a SB 303/401. I also have a 
Marauder transmitter. We love those Heathkits!
The SB series has decent AVC action and I suspect you have a problem with 
yours.
Don't have a 101 schematic to look at  but have some ideas.
First, recommend you use an oscilloscope and view the AVC voltage as you 
tune the band. The circuit is very high impedance so don't use anything that 
will load the line.  Should be only a couple of volts and we are looking to 
verify that the voltage is present. The DC will bounce along with the 
signal. The avc line is hooked to IF tube grids and lots of times the 1st RF 
stage too.  Thru a large value (1meg or so) resistor.  The resistor is 
bypassed with a .01 or so ceramic cap. Suggest you take a highliter pen and 
highlight all the avc circuitry. Start disconnecting each tubes avc line 
until the problem growls at you.
Another place to look in the AVC is the attack and decay avc voltage set - 
up.  One or two capacitors.  Another problem area is the IF and RF tubes 
themselves. A real tube tester may not catch a subtle tube problem like avc 
service. On a side note if the S meter drifts around, increase the wattage 
ratings for the meter voltage divider. From 1/4 w to 1 w will cure that 
completely.
Here's an interesting test: hook the rig to  50 ohm 1/4 w resistor and 
switch thru each band.  The S meter should sit at the bottom of the scale 
for each band.  If it doesn't something is a-miss and a tune-up or repair is 
needed.
Hope this helps and good hunting
Greg
WA7LYO
in sunny Feenix
----- Original Message ----- 
From: "Chris Codella, W2PA" <w2pa at arrl.net>
To: "Heathkit Reflector" <heathkit at mailman.qth.net>
Sent: Wednesday, November 15, 2006 9:56 AM
Subject: [Heathkit] SB-101 AVC problem


>I recently finished restoring an SB-101 that I originally bought as a 
>junker (for parts), by completely disassembling it, cleaning everything, 
>replacing bad parts, and reassembling (except I didn't remove components I 
>suspected were good from the PC boards).  It worked out well and was fun - 
>almost like building a new Heathkit.  Everything seems to be working fine 
>except for some funniness with the AVC.
>
> Strong signals seem abnormally loud in the audio stage.  For example, a 
> carrier at S9+20dB is much louder in the headphones than one that is, say, 
> S5.  This should not be the case if the AVC is functioning properly.  I 
> also notice some abnormal behavior in the S-meter.  While strong stations 
> register at S-meter levels similar to what I get for the same signals on 
> my other rigs, weaker signals and background noise that would normally be 
> around S-4 to S-7 don't register at all on the SB-101.  It's also been my 
> experience with Heathkit transceivers that when you turn the RF gain down 
> all the way, the S-meter pins high.  This one doesn't - it stops near full 
> scale.  No one of these things would raise a flag with me, but taken 
> together they point to (probably) a problem in the AVC function.
>
> I plan to do some measurements to try to figure this out - for example I 
> would imagine that measuring the input signal to the product detector 
> should show nearly constant level for various input signal strengths, if 
> the AVC was operating properly.  But thought I'd pick the collective brain 
> on the Heathkit reflector first.  Any thoughts?
>
> 73,
> Chris, W2PA
>
>
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