[Elecraft] AGC treshold adjustment

[Don Brown]

Hi

This is an email I sent to Andy Wallace. I thought the list may be
interested also as there have several other builders with this problem

Don


Hi

If your pot was marked 50K then that is not the problem. I was guessing
maybe the wrong pot was in your kit. There are 5K pots used in other
Elecraft products that look just like the 50K pot so I thought it could be
mixed up.

Looking at the circuit there are only 4 things I can see that would make the
voltage adjust range low. 1) the wrong value pot 2) the wrong value or type
RP6 3) the wrong voltage on the +8 supply 4) U2B loading down the circuit at
pin 5. The circuit is a simple resistance divider. If the RF gain pot is at
maximum then pin 6 of rp6 is grounded. Doing the math I get a maximum of 3.8
volts if the supply voltage is 8.0 volts just what you are getting.

I went out and measured the circuit on my two control boards and I am
getting 8.5
volts out of the regulator. This is why I can adjust R1 above 3.8 volts. The
tolerance on the regulator may be the problem if your 8 volt supply is on
the low side then you will not be able to get 3.9 volts on the R1
adjustment.

The simplest fix for this is to tack a resistor around 51K across pins 3 and
4 of RP6. This will raise the adjustment range and make up for the lower 8
volt supply.

Don Brown
KD5NDB


----- Original Message -----
From: "Wallace, Andy" <awallace@mc.com>
To: "'Don Brown'" <DolfinDon@msn.com>
Cc: "Wallace, Andy" <awallace@mc.com>
Sent: Thursday, January 23, 2003 7:03 AM
Subject: RE: [Elecraft] Serial 3000+ K2: AGC threshold setting


> Thanks, Don -- will check that. Two other people emailed me (possibly
> cc'ed to the List) saying they had the exact same problem.
>
> What was the value of the wrong-value pot?
>
> When you say across the pot, you mean from wiper to one end, since
> it's about halfway? Then you say 9K full clockwise. I'm confused.
> I'd expect more like 25K in the middle.
>
> I may just pull it and check end to end!
>
> One thing I did not mention was that over the weekend, after I had
> found I could only get to 3.8, I did continue with construction. (My
> risk!) Well the RF board is complete to the point where the 40m rx
> is alive. HOWEVER: now that it is, it seems insensitive. With the filters
> properly adjusted (AC6RM's recent forwarded method works great!) the
> sensitivity with the preamp ON is about the same as my K1. I'd expect
> better. All other alignment went fine, and the 40m coils peaked fine.
> But I'd expect better. With the preamp off, I can barely hear 40m
> local hash. With preamp on, audio has to be at about ten o'clock,
> and SWBC stations I'd expect to be at 40 over are more like S-9.
> For what it's worth, my CAL S LO is 183 and my CAL S HI is 019.
> Does that match one of yours at all? My feeling is the AGC is holding
> signals down.
>
> Thanks very much for the post -- I checked my workmanship several times
> and have been scratching my head on this one. If I remember right,
> the pot WAS marked 50K, too.
>
> Andy
>