Low voltage on AGC threshold

[Mike, W8ENG]

Greetings All,  I'm building K2 SN 4245.  

When setting the AGC threshold I was not able to get 3.8 volts on pin 5 of U2 as described in the manual.   The voltage varied from 2.60 to 3.78 when adjusting R1 to the extreme settings.  Is this close enough or should I have more room for adjustment?  

I have a faily good DMM, and it compares very closely to the K2 internal meter, and when checking values on 1% tolerance resisters.  All other checks to date have been fine.  

Thanks and 73,

Mike, W8ENG
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[Don Wilhelm-3]

Mike,

Since your adjustment range is on the 'edge of the cliff', it would be a
good idea to identify what is causing it before it bites you later on.
Check all the components for proper values and check solder connections very
carefully to determine why you are running out of adjustment range.  Look at
the schematic to determine which components are connected to the AGC
threshold pot circuits and check them all with a careful eye, then check
again and when you have finished, check it once more - these things are
difficult to spot even with careful inspection (I know, I've been there,
done that many many times).

3.78 volts may be fine for the AGC Threshold adjustment - I often (usually)
find that the proper AGC threshold is between 3.6 and 3.8 volts.

Just as an afterthought - you do have the RF gain control full clockwise do
you not?  It will alter the reading on the AGC threshold measurement point.

73,
Don W3FPR

----- Original Message -----

When setting the AGC threshold I was not able to get 3.8 volts on pin 5 of
U2 as described in the manual.   The voltage varied from 2.60 to 3.78 when
adjusting R1 to the extreme settings.  Is this close enough or should I have
more room for adjustment?

I have a faily good DMM, and it compares very closely to the K2 internal
meter, and when checking values on 1% tolerance resisters.  All other checks
to date have been fine.



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[Mike, W8ENG]

Don,  Thanks for the fast reply.

I did have the FR gain fully CW during the test.  Anyway, I just checked
(for the third time) the solder joints on the control board. Pin 4 on RP6
may have had a cold joint.  I touched it up, and will re-assemble and
recheck the votltage.

73

Mike, W8EMG

----- Original Message -----
From: "W3FPR - Don Wilhelm" <[hidden email]>
To: "Mike, W8ENG" <[hidden email]>; <[hidden email]>
Sent: Monday, November 15, 2004 7:34 PM
Subject: Re: [Elecraft] Low voltage on AGC threshold


> Mike,
>
> Since your adjustment range is on the 'edge of the cliff', it would be a
> good idea to identify what is causing it before it bites you later on.
> Check all the components for proper values and check solder connections
very
> carefully to determine why you are running out of adjustment range.  Look
at
> the schematic to determine which components are connected to the AGC
> threshold pot circuits and check them all with a careful eye, then check
> again and when you have finished, check it once more - these things are
> difficult to spot even with careful inspection (I know, I've been there,
> done that many many times).
>
> 3.78 volts may be fine for the AGC Threshold adjustment - I often
(usually)
> find that the proper AGC threshold is between 3.6 and 3.8 volts.
>
> Just as an afterthought - you do have the RF gain control full clockwise
do
> you not?  It will alter the reading on the AGC threshold measurement
point.
>
> 73,
> Don W3FPR
>
> ----- Original Message -----
>
> When setting the AGC threshold I was not able to get 3.8 volts on pin 5 of
> U2 as described in the manual.   The voltage varied from 2.60 to 3.78 when
> adjusting R1 to the extreme settings.  Is this close enough or should I
have
> more room for adjustment?
>
> I have a faily good DMM, and it compares very closely to the K2 internal
> meter, and when checking values on 1% tolerance resisters.  All other
checks
> to date have been fine.
>
>
>
>

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[Mike, W8ENG]

This is an update to my earlier post.  I touched up a solder joint and
reassembled the radio.  The voltage on pin 5 of U2 was still 3.78 volts,
with the RF gain set fully clockwise.  The input voltage from my power
supply was 12.4 volts, both on my DMM and on the K2's internal voltmeter.
The voltage out on the 8v regulator was 7.8,a nd on the 5v regulator was
4.9.

I'm thinking that the problem may be low voltage all around.  I don't have a
better power supply to check with at a higher voltage.

Any addational help or comments would be appreciated.

73,  Mike, W8ENG
----- Original Message -----
From: "Mike, W8ENG" <[hidden email]>
To: "W3FPR - Don Wilhelm" <[hidden email]>; <[hidden email]>
Sent: Monday, November 15, 2004 8:23 PM
Subject: Re: [Elecraft] Low voltage on AGC threshold

> Don,  Thanks for the fast reply.
>
> I did have the FR gain fully CW during the test.  Anyway, I just checked
> (for the third time) the solder joints on the control board. Pin 4 on RP6
> may have had a cold joint.  I touched it up, and will re-assemble and
> recheck the votltage.
>
> 73
>
> Mike, W8EMG
>
> ----- Original Message -----
> From: "W3FPR - Don Wilhelm" <[hidden email]>
> To: "Mike, W8ENG" <[hidden email]>; <[hidden email]>
> Sent: Monday, November 15, 2004 7:34 PM
> Subject: Re: [Elecraft] Low voltage on AGC threshold
>
>
> > Mike,
> >
> > Since your adjustment range is on the 'edge of the cliff', it would be a
> > good idea to identify what is causing it before it bites you later on.
> > Check all the components for proper values and check solder connections
> very
> > carefully to determine why you are running out of adjustment range.

Look

> at
> > the schematic to determine which components are connected to the AGC
> > threshold pot circuits and check them all with a careful eye, then check
> > again and when you have finished, check it once more - these things are
> > difficult to spot even with careful inspection (I know, I've been there,
> > done that many many times).
> >
> > 3.78 volts may be fine for the AGC Threshold adjustment - I often
> (usually)
> > find that the proper AGC threshold is between 3.6 and 3.8 volts.
> >
> > Just as an afterthought - you do have the RF gain control full clockwise
> do
> > you not?  It will alter the reading on the AGC threshold measurement
> point.
> >
> > 73,
> > Don W3FPR
> >
> > ----- Original Message -----
> >
> > When setting the AGC threshold I was not able to get 3.8 volts on pin 5

of
> > U2 as described in the manual.   The voltage varied from 2.60 to 3.78
when

> > adjusting R1 to the extreme settings.  Is this close enough or should I
> have
> > more room for adjustment?
> >
> > I have a faily good DMM, and it compares very closely to the K2 internal
> > meter, and when checking values on 1% tolerance resisters.  All other
> checks
> > to date have been fine.
> >
> >
> >
> >
>
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>

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[John Lonigro]

Mike:

How accurate is your DMM?  The 8v and 5v regulators should still produce
8 and 5 volts even with a slightly low supply voltage.  Also, if the
K2's internal voltmeter reads the same as your DMM, that tells me your
DMM is reading low because there is a diode in the K2 power line which
will make the K2 internal voltmeter read LESS than an accurate external
DMM.  Only if the external DMM reads low would the two voltmeters agree.

Try to check your voltages with a different DMM.  My guess is your
problem is NOT low voltage all around.  Good luck.

73's

John AA0VE

Mike, W8ENG wrote:

>This is an update to my earlier post.  I touched up a solder joint and
>reassembled the radio.  The voltage on pin 5 of U2 was still 3.78 volts,
>with the RF gain set fully clockwise.  The input voltage from my power
>supply was 12.4 volts, both on my DMM and on the K2's internal voltmeter.
>The voltage out on the 8v regulator was 7.8,a nd on the 5v regulator was
>4.9.
>
>I'm thinking that the problem may be low voltage all around.  I don't have a
>better power supply to check with at a higher voltage.
>
>Any addational help or comments would be appreciated.
>
>73,  Mike, W8ENG
>  
>

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[Geoffrey Mackenzie-Kennedy-2]

Mike,

There is a design problem in this circuit, IMHO, which has to do with
tolerances, and I am having an off list discussion about it. Nothing
serious, and I am sure that it can be fixed easily. I'm letting you know so
that you don't damage anything by too much poking around in K2. The 3.78
volts that you read at U2 pin 5 is 0.053 volts higher, I figure, than you
should get with the 7.8 volts from the 8v regulator, if the RP6 resistors
are 5100 ohms +- zero!! Three terminal voltage regulators also have
tolerance specs on voltage output as measured at the regulator output
terminal, so 8.000 volts is not a sacred cow.

I had exactly the same sort of problems, slightly different voltages. But K2
was VERY useable with AGC on, but not quite up to full sensitivity being a
little bit "AGC limited".

73,     Geoff    GM4ESD

PS : The low voltage that you get is 2.6 volts  -  I figure that you should
get 2.667 volts - nit pickey fussy eh?



----- Original Message -----
From: "Mike, W8ENG" <[hidden email]>
To: <[hidden email]>
Sent: Wednesday, November 17, 2004 12:44 AM
Subject: Re: [Elecraft] Low voltage on AGC threshold


> This is an update to my earlier post.  I touched up a solder joint and
> reassembled the radio.  The voltage on pin 5 of U2 was still 3.78 volts,
> with the RF gain set fully clockwise.  The input voltage from my power
> supply was 12.4 volts, both on my DMM and on the K2's internal voltmeter.
> The voltage out on the 8v regulator was 7.8,a nd on the 5v regulator was
> 4.9.
>
> I'm thinking that the problem may be low voltage all around.  I don't have
a
> better power supply to check with at a higher voltage.
>
> Any addational help or comments would be appreciated.
>
> 73,  Mike, W8ENG
>

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[Geoffrey Mackenzie-Kennedy-2]

Mike ,
Typing error in my post, last line.     Low voltage should be 2.600  not
2.667 - same as you get.  Sorry.
73,
Geoff   GM4ESD


----- Original Message -----
From: "Geoffrey Mackenzie-Kennedy" <[hidden email]>
To: "Mike, W8ENG" <[hidden email]>
Cc: "Elecraft Discussion List" <[hidden email]>
Sent: Wednesday, November 17, 2004 10:53 AM
Subject: Re: [Elecraft] Low voltage on AGC threshold


> Mike,
>
> There is a design problem in this circuit, IMHO, which has to do with
> tolerances, and I am having an off list discussion about it. Nothing
> serious, and I am sure that it can be fixed easily. I'm letting you know
so
> that you don't damage anything by too much poking around in K2. The 3.78
> volts that you read at U2 pin 5 is 0.053 volts higher, I figure, than you
> should get with the 7.8 volts from the 8v regulator, if the RP6 resistors
> are 5100 ohms +- zero!! Three terminal voltage regulators also have
> tolerance specs on voltage output as measured at the regulator output
> terminal, so 8.000 volts is not a sacred cow.
>
> I had exactly the same sort of problems, slightly different voltages. But
K2
> was VERY useable with AGC on, but not quite up to full sensitivity being a
> little bit "AGC limited".
>
> 73,     Geoff    GM4ESD
>
> PS : The low voltage that you get is 2.6 volts  -  I figure that you
should

> get 2.667 volts - nit pickey fussy eh?
>
>
>
> ----- Original Message -----
> From: "Mike, W8ENG" <[hidden email]>
> To: <[hidden email]>
> Sent: Wednesday, November 17, 2004 12:44 AM
> Subject: Re: [Elecraft] Low voltage on AGC threshold
>
>
> > This is an update to my earlier post.  I touched up a solder joint and
> > reassembled the radio.  The voltage on pin 5 of U2 was still 3.78 volts,
> > with the RF gain set fully clockwise.  The input voltage from my pow

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[Mike, W8ENG]

John,   My DMM is a $30 Radio Shack unit.  I do not have anything handy to
calibrate or check the accuracy.  It does report DC voiltage to 2 decimal
points, but I doubt that it is that accurate.  Any way, for now I'm
proceeding with construction.

Mike, W8ENG


----- Original Message -----
From: "John R. Lonigro" <[hidden email]>
To: "Mike, W8ENG" <[hidden email]>
Cc: <[hidden email]>
Sent: Tuesday, November 16, 2004 10:29 PM
Subject: Re: [Elecraft] Low voltage on AGC threshold

> Mike:
>
> How accurate is your DMM?  The 8v and 5v regulators should still produce
> 8 and 5 volts even with a slightly low supply voltage.  Also, if the
> K2's internal voltmeter reads the same as your DMM, that tells me your
> DMM is reading low because there is a diode in the K2 power line which
> will make the K2 internal voltmeter read LESS than an accurate external
> DMM.  Only if the external DMM reads low would the two voltmeters agree.
>
> Try to check your voltages with a different DMM.  My guess is your
> problem is NOT low voltage all around.  Good luck.
>
> 73's
>
> John AA0VE
>
> Mike, W8ENG wrote:
>
> >This is an update to my earlier post.  I touched up a solder joint and
> >reassembled the radio.  The voltage on pin 5 of U2 was still 3.78 volts,
> >with the RF gain set fully clockwise.  The input voltage from my power
> >supply was 12.4 volts, both on my DMM and on the K2's internal voltmeter.
> >The voltage out on the 8v regulator was 7.8,a nd on the 5v regulator was
> >4.9.
> >
> >I'm thinking that the problem may be low voltage all around.  I don't

have a
> >better power supply to check with at a higher voltage.
> >
> >Any addational help or comments would be appreciated.
> >
> >73,  Mike, W8ENG
> >
> >
>

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[Douglas Westover]

Don't confuse precision with accuracy. Why not measure some
fresh 1.5 volt batteries (which should actually measure around
 1.6 volts), that should give you a quick check of your meter's
accuracy.

73,
Doug
W6JD

----- Original Message -----
From: "Mike, W8ENG" <[hidden email]>
To: <[hidden email]>
Cc: <[hidden email]>
Sent: Wednesday, November 17, 2004 4:15 PM
Subject: Re: [Elecraft] Low voltage on AGC threshold

> John,   My DMM is a $30 Radio Shack unit.  I do not have anything handy to
> calibrate or check the accuracy.  It does report DC voiltage to 2 decimal
> points, but I doubt that it is that accurate.  Any way, for now I'm
> proceeding with construction.
>
> Mike, W8ENG
>
>
> ----- Original Message -----
> From: "John R. Lonigro" <[hidden email]>
> To: "Mike, W8ENG" <[hidden email]>
> Cc: <[hidden email]>
> Sent: Tuesday, November 16, 2004 10:29 PM
> Subject: Re: [Elecraft] Low voltage on AGC threshold
>
>
> > Mike:
> >
> > How accurate is your DMM?  The 8v and 5v regulators should still produce
> > 8 and 5 volts even with a slightly low supply voltage.  Also, if the
> > K2's internal voltmeter reads the same as your DMM, that tells me your
> > DMM is reading low because there is a diode in the K2 power line which
> > will make the K2 internal voltmeter read LESS than an accurate external
> > DMM.  Only if the external DMM reads low would the two voltmeters agree.
> >
> > Try to check your voltages with a different DMM.  My guess is your
> > problem is NOT low voltage all around.  Good luck.
> >
> > 73's
> >
> > John AA0VE
> >
> > Mike, W8ENG wrote:
> >
> > >This is an update to my earlier post.  I touched up a solder joint and
> > >reassembled the radio.  The voltage on pin 5 of U2 was still 3.78

volts,
> > >with the RF gain set fully clockwise.  The input voltage from my power
> > >supply was 12.4 volts, both on my DMM and on the K2's internal
voltmeter.
> > >The voltage out on the 8v regulator was 7.8,a nd on the 5v regulator
was

> > >4.9.
> > >
> > >I'm thinking that the problem may be low voltage all around.  I don't
> have a
> > >better power supply to check with at a higher voltage.
> > >
> > >Any addational help or comments would be appreciated.
> > >
> > >73,  Mike, W8ENG
> > >
> > >
> >
>
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