Setting the AGC Threshold on K2

[jrb-3]

Hello All-
       
Once again I'd like to tap this deep font of knowledge.

I've just completed the first alignment and test and everything  
checks out except setting the AGC threshold.

When I put the probe of my DMM on pin 5 of U2 (a hellish place to get  
consistent readings) and adjust R1
I get only .002 DC volts.

I've resoldered the pins on both U2 and R1. I've looked through  
Troubleshooting and the Reflector and
haven't  found anything relevant.

By the way I had no trouble with the S-Meter alignment which is  
dependent on the AGC threshold.

Is there any place else I should look  on the control board for a  
possible source of trouble, or could U2 or
R1 be defective? (I know components are rarely defective).

TIA,

Jeff

KG6ZVT
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[Geoffrey Mackenzie-Kennedy-2]

Hello Jeff,
Have you tried measuring the voltage at the wiper terminal of R1 (Control
Board) or RP6 pins 4/5? These points are all connected together along with
U2 pin 5, so you should get the same voltage +- a couple of millivolts at
each of these points. The voltage at all these points should vary but remain
equal as you tweak R1 and be >>.002V, approx 3.8V downwards.
However your .002 volt reading makes me wonder whether or not the probe is
actually making contact with U2 pin 5 or is sitting on top of a layer of
gunge that has formed on the IC's pin e.g. corrosion.

73,
Geoff
GM4ESD

 "jrb" <[hidden email]> wrote on December 18, 2005 at 1128 PM

> I've just completed the first alignment and test and everything  checks
> out except setting the AGC threshold.
>
> When I put the probe of my DMM on pin 5 of U2 (a hellish place to get
> consistent readings) and adjust R1
> I get only .002 DC volts.
>
> I've resoldered the pins on both U2 and R1. I've looked through
> Troubleshooting and the Reflector and
> haven't  found anything relevant.
>
> By the way I had no trouble with the S-Meter alignment which is  dependent
> on the AGC threshold.
>
> Is there any place else I should look  on the control board for a
> possible source of trouble, or could U2 or
> R1 be defective? (I know components are rarely defective).

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[Don Wilhelm-3]

Jeff,

Try measuring the voltage at U2 pin 5 again making certain your probes both
have good contact.

The voltage at U2 pin 5 is determined only by a resistor network - if it is
still quite low, check the voltage on the 8A line - the output of regulator
U4 (upper leg) - it should be 8 volts.

If you have 8 volts at U4, then check and re-check the soldering of Control
Board RP6, R1 and U2, then remeasure.  Remember that the voltage is
determined only by a resistive network (which includes the RF Gain pot),
there are no active components involved, so if the voltages to the resistors
and the connections are proper, you should achieve the correct voltage for
the AGC Threshold.

Be aware that the voltage for the AGC Threshold may not be optimum for your
K2.  After your K2 is fully built, aligned, and everything is working
properly, you can optimize the AGC Threshold so that the AGC does not become
active with no signal into the K2 antenna -  so don't be concerned if the
3.8 volts is near the upper limit, I find that the optimum setting will vary
from a low of 3.65 volts to a high of 3.78 volts.

73,
Don W3FPR

> -----Original Message-----
>
> I've just completed the first alignment and test and everything
> checks out except setting the AGC threshold.
>
> When I put the probe of my DMM on pin 5 of U2 (a hellish place to get
> consistent readings) and adjust R1
> I get only .002 DC volts.
>
> I've resoldered the pins on both U2 and R1. I've looked through
> Troubleshooting and the Reflector and
> haven't  found anything relevant.
>
> By the way I had no trouble with the S-Meter alignment which is
> dependent on the AGC threshold.
>
> Is there any place else I should look  on the control board for a
> possible source of trouble, or could U2 or
> R1 be defective? (I know components are rarely defective).
>
> TIA,
>
> Jeff
>
> KG6ZVT
>

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[Gregg R. Lengling]

Don I worked with him last night, he has it all going it was a cold solder
joint on RP6 so he didn't have the 8 volt line.  He's got it working
now....we emailed off the list.  (Sometimes I forget to hit the reply all.)

Gregg R. Lengling, W9DHI, Un-Retired
K2/100 SN 3075
http://www.milwaukeehdtv.org

----- Original Message -----
From: "Don Wilhelm" <[hidden email]>
To: "jrb" <[hidden email]>; <[hidden email]>
Sent: Monday, December 19, 2005 7:45 AM
Subject: RE: [Elecraft] Setting the AGC Threshold on K2

> Jeff,
>
> Try measuring the voltage at U2 pin 5 again making certain your probes
> both
> have good contact.
>
> The voltage at U2 pin 5 is determined only by a resistor network - if it
> is
> still quite low, check the voltage on the 8A line - the output of
> regulator
> U4 (upper leg) - it should be 8 volts.
>
> If you have 8 volts at U4, then check and re-check the soldering of
> Control
> Board RP6, R1 and U2, then remeasure.  Remember that the voltage is
> determined only by a resistive network (which includes the RF Gain pot),
> there are no active components involved, so if the voltages to the
> resistors
> and the connections are proper, you should achieve the correct voltage for
> the AGC Threshold.
>
> Be aware that the voltage for the AGC Threshold may not be optimum for
> your
> K2.  After your K2 is fully built, aligned, and everything is working
> properly, you can optimize the AGC Threshold so that the AGC does not
> become
> active with no signal into the K2 antenna -  so don't be concerned if the
> 3.8 volts is near the upper limit, I find that the optimum setting will
> vary
> from a low of 3.65 volts to a high of 3.78 volts.
>
> 73,
> Don W3FPR
>
>> -----Original Message-----
>>
>> I've just completed the first alignment and test and everything
>> checks out except setting the AGC threshold.
>>
>> When I put the probe of my DMM on pin 5 of U2 (a hellish place to get
>> consistent readings) and adjust R1
>> I get only .002 DC volts.
>>
>> I've resoldered the pins on both U2 and R1. I've looked through
>> Troubleshooting and the Reflector and
>> haven't  found anything relevant.
>>
>> By the way I had no trouble with the S-Meter alignment which is
>> dependent on the AGC threshold.
>>
>> Is there any place else I should look  on the control board for a
>> possible source of trouble, or could U2 or
>> R1 be defective? (I know components are rarely defective).
>>
>> TIA,
>>
>> Jeff
>>
>> KG6ZVT
>>
>
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> You must be a subscriber to post to the list.
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>
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