Fw: Re: [Drake] Drake 2B AVC

[windy10605 at juno.com]

Hi Garey,

Thanks for the input. I see in the manual that it talks about VTVM
"loading" so the AVC is definitely very sensitive to high impedance
paths. I also found it impossible to adjust the -1.5V AVC level until I
cleaned the 2Meg pot ....so there was some high impedance dust/cleaner
path in there. It was a "hot" receiver with less than half that on the
AVC bus before. I would think the same paths are possible with the 15Meg
RF pot, so I'll take it apart too and clean it. I'll also check the
terminal strip.

Subing the 6EA8 for the 6U8 sounds like a good idea. Also that 6BA6 with
the hand picked screen resistor looks like a likely candidate for
affecting the S-meter dramatically if things are not OK. I may try
several (V5) 6BA6 tubes. Wonder what the criteria was for selecting R25
(the screen resistor) "set in production".  

73 Kees K5BCQ


--------- Forwarded message ----------
From: Garey Barrell <k4oah at mindspring.com>
To: windy10605 at juno.com,  Drake at mailman.qth.net
Date: Sat, 23 Jul 2005 13:23:20 -0400
Subject: Re: [Drake] Drake 2B AVC
Message-ID: <42E27D08.6080206 at mindspring.com>
References: <20050723.121159.2592.4.windy10605 at juno.com>

Hi Kees -

Have you checked the timing caps for the AVC.  The stock AVC circuit 
works just fine as long as there are no faults.  It  _IS_  however a 
VERY high impedance circuit.  There are also terminals on the back to 
allow addition of a supplemental AVC timing cap if you want "really 
slow" AVC.  I have seen cases where there is accumulated crud under that 
terminal strip,  forming a leakage pathThe 2-B is one of the "classic" 
circuits of all time!  :-)

Also, if you haven't already, replace the 6U8 with a 6EA8.   Helps a 
little in the First Mixer slot, and makes a big difference in the 
reliability of the HF Oscillator, especially on 40M.

73, Garey - K4OAH
Atlanta

Drake C-Line Service Manual
<http://www.k4oah.com> 



windy10605 at juno.com wrote:

>I have a Drake 2B and found the "common" 2Meg AVC pot to ground leakage
>path (unable to adjust the AVC "sensitivity" level), cleaned the pot
>after taking it apart, and can now adjust for the required -1.5V bias
>level. The S-meter still barely registers and the sensitivity seems to
be
>lower than it should be. Has anyone come across the same problem ? Know
>of any AVC circuit redesign or things to check ?
>
>73 Kees K5BCQ
>
>  
>