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voltmeter offset and accuracy in K2

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Steve Jackson-5

voltmeter offset and accuracy in K2

NB: this posting is about an Elecraft radio

;-)

* * *

I was thinking about doing two things to my vereable
K2 #0771, as I wait for new Elecraft toys:

... one is to re-jigger the divider resistors to
offset the K2 voltmeter so it reads what my actual
input-port voltage is, rather than the "internal"
voltage

and

...two was changing the TO-220 regulator(s) to some
kind of precision type ... could not remember if it
was the 8V LVDO or the plain-old 7805 type that set
the reference voltage for other measurements. Wasn't
obvious from the schematic, at first.

On the matter of the offset, I suppose that the
voltmeter's ultimate accuracy will be reduced a bit
because a fixed offset won't compensate for the
actual, varying drop through the polarity-protect
diode, as that will vary over the IsubF ... however,
I've switched mine to the newer Schottky type -- and
aren't they V-drop-flatter over the current curve than
the original?

And on the second matter of the regulator precision,
maybe I am mistaken about the availability of
more-accurate TO-220 parts. What is the typical load
that the 8V rail has? I'm guessing a 1 amp regulator
will suffice.

Which regulator was the "critical" voltage unit? It
was the +5v, right?

Steve KZ1X/4

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Don Wilhelm-3

RE: voltmeter offset and accuracy in K2

Steve,

Although some have been confused by the readings at first, I don't see any
advantage in modifying the internal voltmeter to read the external supply
voltage - what it reads is the actual voltage the K2 is operating at - but
if changing it is your choice, by all means do it - it should be a simple
task of padding R10 (or lowering its value a bit).

By process of elimination, the more 'critical' voltage regulator is the 8
volt low dropout regulator. The main thing the 5 volts drives is the front
panel and control board digital stuff and as long as the voltage is within
the tolerances for digital components, it will be close enough.

The really critical circuit - the PLL is regulated by a secondary regulator
down to 5 volts driven from the 8 volt line.

The major parts of the K2 driven from the 8 volt regulator are the IF amp
and product detector, the AGC, and the low level transmit stages - the
devices used in those stages are not particularly sensitive to the voltage
level. The only thing really sensitive to the exact voltage level is the
AGC threshold since it is a resistive divider. I would think you could
better 'improve' your K2 by carefully setting the AGC threshold level by
changing the Control Board R1 value and leave the regulator as it is. As I
recall, you may have optimized the AGC Threshold the last time you 'tweaked'
your K2.

73,
Don W3FPR

> -----Original Message-----

>
> NB: this posting is about an Elecraft radio
>
> ;-)
>
> * * *
>
> I was thinking about doing two things to my vereable
> K2 #0771, as I wait for new Elecraft toys:
>
> ... one is to re-jigger the divider resistors to
> offset the K2 voltmeter so it reads what my actual
> input-port voltage is, rather than the "internal"
> voltage
>
> and
>
> ...two was changing the TO-220 regulator(s) to some
> kind of precision type ... could not remember if it
> was the 8V LVDO or the plain-old 7805 type that set
> the reference voltage for other measurements. Wasn't
> obvious from the schematic, at first.
>
> On the matter of the offset, I suppose that the
> voltmeter's ultimate accuracy will be reduced a bit
> because a fixed offset won't compensate for the
> actual, varying drop through the polarity-protect
> diode, as that will vary over the IsubF ... however,
> I've switched mine to the newer Schottky type -- and
> aren't they V-drop-flatter over the current curve than
> the original?
>
> And on the second matter of the regulator precision,
> maybe I am mistaken about the availability of
> more-accurate TO-220 parts. What is the typical load
> that the 8V rail has? I'm guessing a 1 amp regulator
> will suffice.
>
> Which regulator was the "critical" voltage unit? It
> was the +5v, right?
>
> Steve KZ1X/4
>
>

--
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JEAN-FRANCOIS MENARD-3

Re: voltmeter offset and accuracy in K2

Hi Don,

You said : "By lowering the value of R10 ..." Could I just put a
trimmer near that value instead of R10 and tune the trimmer so that
the voltage displayed match more closer the real external voltage ???

And could I do limilar thing with the current display ???

I don't want to be more catholic than the Pope, but is it possible to
do those mods ???

Best 73 from a junior ;-)

Le 05-07-23 à 18:56, W3FPR - Don Wilhelm a écrit :

> Steve,
>
> Although some have been confused by the readings at first, I don't
> see any
> advantage in modifying the internal voltmeter to read the external
> supply
> voltage - what it reads is the actual voltage the K2 is operating
> at - but
> if changing it is your choice, by all means do it - it should be a
> simple
> task of padding R10 (or lowering its value a bit).

======

Jean-François Ménard / VA2VYZ

======

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JEAN-FRANCOIS MENARD-3

Re: voltmeter offset and accuracy in K2

Don Wilhelm-3

RE: voltmeter offset and accuracy in K2

In reply to this post by JEAN-FRANCOIS MENARD-3

Jean-Francois,

You certainly could do that - it should not alter any other function that I
am aware of. The current display can be trimmed by changing Control Board
R7.

R10 and R7 are 1% resistors, and that provides repeaability from one K2 to
another - one could hand-pick from 5% resistors just as well.
I personally would be concerned about leaving a trimmeer in place because of
the possibility of the setting being changed, but certainly using a trimmer
to determine the exact value required would be an easy method.

73,
Don W3FPR

> -----Original Message-----
>
> Hi Don,
>
> You said : "By lowering the value of R10 ..." Could I just put a
> trimmer near that value instead of R10 and tune the trimmer so that
> the voltage displayed match more closer the real external voltage ???
>
> And could I do limilar thing with the current display ???
>
> I don't want to be more catholic than the Pope, but is it possible to
> do those mods ???
>
> Best 73 from a junior ;-)
>
> Le 05-07-23 à 18:56, W3FPR - Don Wilhelm a écrit :
>
> > Steve,
> >
> > Although some have been confused by the readings at first, I don't
> > see any
> > advantage in modifying the internal voltmeter to read the external
> > supply
> > voltage - what it reads is the actual voltage the K2 is operating
> > at - but
> > if changing it is your choice, by all means do it - it should be a
> > simple
> > task of padding R10 (or lowering its value a bit).
>
>
> ======
>
> Jean-François Ménard / VA2VYZ
>
> ======
>

JEAN-FRANCOIS MENARD-3

Re: voltmeter offset and accuracy in K2

Thanks, I will try and let you know my result !!!

73

Le 05-07-24 à 07:32, W3FPR - Don Wilhelm a écrit :

> Jean-Francois,
>
> You certainly could do that - it should not alter any other
> function that I
> am aware of. The current display can be trimmed by changing
> Control Board
> R7.
>
> R10 and R7 are 1% resistors, and that provides repeaability from
> one K2 to
> another - one could hand-pick from 5% resistors just as well.
> I personally would be concerned about leaving a trimmeer in place
> because of
> the possibility of the setting being changed, but certainly using a
> trimmer
> to determine the exact value required would be an easy method.
>
> 73,
> Don W3FPR
>
>
>> -----Original Message-----
>>
>> Hi Don,
>>
>> You said : "By lowering the value of R10 ..." Could I just put a
>> trimmer near that value instead of R10 and tune the trimmer so that
>> the voltage displayed match more closer the real external voltage ???
>>
>> And could I do limilar thing with the current display ???
>>
>> I don't want to be more catholic than the Pope, but is it possible to
>> do those mods ???
>>
>> Best 73 from a junior ;-)
>>
>> Le 05-07-23 à 18:56, W3FPR - Don Wilhelm a écrit :
>>
>>
>>> Steve,
>>>
>>> Although some have been confused by the readings at first, I don't
>>> see any
>>> advantage in modifying the internal voltmeter to read the external
>>> supply
>>> voltage - what it reads is the actual voltage the K2 is operating
>>> at - but
>>> if changing it is your choice, by all means do it - it should be a
>>> simple
>>> task of padding R10 (or lowering its value a bit).
>>>
>>
>>
>> ======
>>
>> Jean-François Ménard / VA2VYZ
>>
>> ======
>>
>>
> --
> No virus found in this outgoing message.
> Checked by AVG Anti-Virus.
> Version: 7.0.338 / Virus Database: 267.9.4/57 - Release Date:
> 7/22/2005
>
>

Don Wilhelm-3

RE: voltmeter offset and accuracy in K2

In reply to this post by Don Wilhelm-3

Folks,

After thinking about this voltmeter 'adjustment' a bit more, I discovered I
had erred. The value of R10 should be INCREASED to make the reading higher.
Sorry - I guess I had a 'senior moment' and got my head turned around wrong.

73,
Don W3FPR

> -----Original Message-----
>
> Jean-Francois,
>
> You certainly could do that - it should not alter any other
> function that I
> am aware of. The current display can be trimmed by changing Control Board
> R7.
>
> R10 and R7 are 1% resistors, and that provides repeaability from one K2 to
> another - one could hand-pick from 5% resistors just as well.
> I personally would be concerned about leaving a trimmeer in place
> because of
> the possibility of the setting being changed, but certainly using
> a trimmer
> to determine the exact value required would be an easy method.
>
> 73,
> Don W3FPR
>
> > -----Original Message-----
> >
> > Hi Don,
> >
> > You said : "By lowering the value of R10 ..." Could I just put a
> > trimmer near that value instead of R10 and tune the trimmer so that
> > the voltage displayed match more closer the real external voltage ???
> >
> > And could I do limilar thing with the current display ???
> >
> > I don't want to be more catholic than the Pope, but is it possible to
> > do those mods ???
> >
> > Best 73 from a junior ;-)
> >
> > Le 05-07-23 à 18:56, W3FPR - Don Wilhelm a écrit :
> >
> > > Steve,
> > >
> > > Although some have been confused by the readings at first, I don't
> > > see any
> > > advantage in modifying the internal voltmeter to read the external
> > > supply
> > > voltage - what it reads is the actual voltage the K2 is operating
> > > at - but
> > > if changing it is your choice, by all means do it - it should be a
> > > simple
> > > task of padding R10 (or lowering its value a bit).
> >
> >
> > ======
> >
> > Jean-François Ménard / VA2VYZ
> >
>

JEAN-FRANCOIS MENARD-3

Re: voltmeter offset and accuracy in K2

I'm planning to use a little higher value trimmer, so I would be
able to be more or less in a second. I let you know wich one I use...
I would like the really smaller one (in size) that it can take not
much space as the actual resistor.

Le 05-07-24 à 09:03, W3FPR - Don Wilhelm a écrit :

> Folks,
>
> After thinking about this voltmeter 'adjustment' a bit more, I
> discovered I
> had erred. The value of R10 should be INCREASED to make the
> reading higher.
> Sorry - I guess I had a 'senior moment' and got my head turned
> around wrong.
>
> 73,
> Don W3FPR
>
>
>> -----Original Message-----
>>
>> Jean-Francois,
>>
>> You certainly could do that - it should not alter any other
>> function that I
>> am aware of. The current display can be trimmed by changing
>> Control Board
>> R7.
>>
>> R10 and R7 are 1% resistors, and that provides repeaability from
>> one K2 to
>> another - one could hand-pick from 5% resistors just as well.
>> I personally would be concerned about leaving a trimmeer in place
>> because of
>> the possibility of the setting being changed, but certainly using
>> a trimmer
>> to determine the exact value required would be an easy method.
>>
>> 73,
>> Don W3FPR
>>
>>
>>> -----Original Message-----
>>>
>>> Hi Don,
>>>
>>> You said : "By lowering the value of R10 ..." Could I just put a
>>> trimmer near that value instead of R10 and tune the trimmer so that
>>> the voltage displayed match more closer the real external
>>> voltage ???
>>>
>>> And could I do limilar thing with the current display ???
>>>
>>> I don't want to be more catholic than the Pope, but is it
>>> possible to
>>> do those mods ???
>>>
>>> Best 73 from a junior ;-)
>>>
>>> Le 05-07-23 à 18:56, W3FPR - Don Wilhelm a écrit :
>>>
>>>
>>>> Steve,
>>>>
>>>> Although some have been confused by the readings at first, I don't
>>>> see any
>>>> advantage in modifying the internal voltmeter to read the external
>>>> supply
>>>> voltage - what it reads is the actual voltage the K2 is operating
>>>> at - but
>>>> if changing it is your choice, by all means do it - it should be a
>>>> simple
>>>> task of padding R10 (or lowering its value a bit).
>>>>
>>>
>>>
>>> ======
>>>
>>> Jean-François Ménard / VA2VYZ
>>>
>>>
>>
>>
> --
> No virus found in this outgoing message.
> Checked by AVG Anti-Virus.
> Version: 7.0.338 / Virus Database: 267.9.4/57 - Release Date:
> 7/22/2005
>
> _______________________________________________
> Elecraft mailing list
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> Elecraft web page: http://www.elecraft.com
>

JEAN-FRANCOIS MENARD-3

Re: voltmeter offset and accuracy in K2

Don, Do you think after the voltage will be "adjust" according to an
external DC voltage and an accurate external volt meter, if we change
later the DC source supply, like a battery with a little bit
different voltage, instead of a standard power supply by example, do
you think there will be or might be another voltage offset or it will
be calibrate for any kind of DC supply in the range of, let say, 12V
to 14V ???

Maybe an error offset will be anyway present, and by changing the DC
source it could increase the offset error and indicate an improper
value again....

Sorry for the clumsy question.... ;-)

Le 05-07-24 à 09:13, JEAN-FRANCOIS MENARD a écrit :

> I'm planning to use a little higher value trimmer, so I would be
> able to be more or less in a second. I let you know wich one I
> use... I would like the really smaller one (in size) that it can
> take not much space as the actual resistor.
>
> Le 05-07-24 à 09:03, W3FPR - Don Wilhelm a écrit :
>
>
>> Folks,
>>
>> After thinking about this voltmeter 'adjustment' a bit more, I
>> discovered I
>> had erred. The value of R10 should be INCREASED to make the
>> reading higher.
>> Sorry - I guess I had a 'senior moment' and got my head turned
>> around wrong.
>>
>> 73,
>> Don W3FPR
>>
>>
>>
>>> -----Original Message-----
>>>
>>> Jean-Francois,
>>>
>>> You certainly could do that - it should not alter any other
>>> function that I
>>> am aware of. The current display can be trimmed by changing
>>> Control Board
>>> R7.
>>>
>>> R10 and R7 are 1% resistors, and that provides repeaability from
>>> one K2 to
>>> another - one could hand-pick from 5% resistors just as well.
>>> I personally would be concerned about leaving a trimmeer in place
>>> because of
>>> the possibility of the setting being changed, but certainly using
>>> a trimmer
>>> to determine the exact value required would be an easy method.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>>
>>>
>>>> -----Original Message-----
>>>>
>>>> Hi Don,
>>>>
>>>> You said : "By lowering the value of R10 ..." Could I just put a
>>>> trimmer near that value instead of R10 and tune the trimmer so that
>>>> the voltage displayed match more closer the real external
>>>> voltage ???
>>>>
>>>> And could I do limilar thing with the current display ???
>>>>
>>>> I don't want to be more catholic than the Pope, but is it
>>>> possible to
>>>> do those mods ???
>>>>
>>>> Best 73 from a junior ;-)
>>>>
>>>> Le 05-07-23 à 18:56, W3FPR - Don Wilhelm a écrit :
>>>>
>>>>
>>>>
>>>>> Steve,
>>>>>
>>>>> Although some have been confused by the readings at first, I don't
>>>>> see any
>>>>> advantage in modifying the internal voltmeter to read the external
>>>>> supply
>>>>> voltage - what it reads is the actual voltage the K2 is operating
>>>>> at - but
>>>>> if changing it is your choice, by all means do it - it should be a
>>>>> simple
>>>>> task of padding R10 (or lowering its value a bit).
>>>>>
>>>>>
>>>>
>>>>
>>>> ======
>>>>
>>>> Jean-François Ménard / VA2VYZ
>>>>
>>>>
>>>>
>>>
>>>
>>>
>> --
>> No virus found in this outgoing message.
>> Checked by AVG Anti-Virus.
>> Version: 7.0.338 / Virus Database: 267.9.4/57 - Release Date:
>> 7/22/2005
>>
>> _______________________________________________
>> Elecraft mailing list
>> Post to: [hidden email]
>> You must be a subscriber to post to the list.
>> Subscriber Info (Addr. Change, sub, unsub etc.):
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>>
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>> Elecraft web page: http://www.elecraft.com
>>
>>
>
> ======
>
> Jean-François Ménard / VA2VYZ
>
> ======
>
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
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>

JEAN-FRANCOIS MENARD-3

Fwd: voltmeter offset and accuracy in K2

> I'm planning to use a little higher value trimmer, so I would be
> able to be more or less in a second. I let you know wich one I
> use... I would like the really smaller one (in size) that it can
> take not much space as the actual resistor.
>
> Le 05-07-24 à 09:03, W3FPR - Don Wilhelm a écrit :
>
>
>
>> Folks,
>>
>> After thinking about this voltmeter 'adjustment' a bit more, I
>> discovered I
>> had erred. The value of R10 should be INCREASED to make the
>> reading higher.
>> Sorry - I guess I had a 'senior moment' and got my head turned
>> around wrong.
>>
>> 73,
>> Don W3FPR
>>
>>
>>
>>
>>> -----Original Message-----
>>>
>>> Jean-Francois,
>>>
>>> You certainly could do that - it should not alter any other
>>> function that I
>>> am aware of. The current display can be trimmed by changing
>>> Control Board
>>> R7.
>>>
>>> R10 and R7 are 1% resistors, and that provides repeaability from
>>> one K2 to
>>> another - one could hand-pick from 5% resistors just as well.
>>> I personally would be concerned about leaving a trimmeer in place
>>> because of
>>> the possibility of the setting being changed, but certainly using
>>> a trimmer
>>> to determine the exact value required would be an easy method.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>>
>>>
>>>
>>>> -----Original Message-----
>>>>
>>>> Hi Don,
>>>>
>>>> You said : "By lowering the value of R10 ..." Could I just put a
>>>> trimmer near that value instead of R10 and tune the trimmer so that
>>>> the voltage displayed match more closer the real external
>>>> voltage ???
>>>>
>>>> And could I do limilar thing with the current display ???
>>>>
>>>> I don't want to be more catholic than the Pope, but is it
>>>> possible to
>>>> do those mods ???
>>>>
>>>> Best 73 from a junior ;-)
>>>>
>>>> Le 05-07-23 à 18:56, W3FPR - Don Wilhelm a écrit :
>>>>
>>>>
>>>>
>>>>
>>>>> Steve,
>>>>>
>>>>> Although some have been confused by the readings at first, I don't
>>>>> see any
>>>>> advantage in modifying the internal voltmeter to read the external
>>>>> supply
>>>>> voltage - what it reads is the actual voltage the K2 is operating
>>>>> at - but
>>>>> if changing it is your choice, by all means do it - it should be a
>>>>> simple
>>>>> task of padding R10 (or lowering its value a bit).
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>> ======
>>>>
>>>> Jean-François Ménard / VA2VYZ
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>>
>> --
>> No virus found in this outgoing message.
>> Checked by AVG Anti-Virus.
>> Version: 7.0.338 / Virus Database: 267.9.4/57 - Release Date:
>> 7/22/2005
>>
>> _______________________________________________
>> Elecraft mailing list
>> Post to: [hidden email]
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>>
>>
>>
>
> ======
>
> Jean-François Ménard / VA2VYZ
>
> ======
>
>
> _______________________________________________
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>

======

Jean-François Ménard / VA2VYZ

======

======

Jean-François Ménard / VA2VYZ

======

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n5ib

Re: voltmeter offset and accuracy in K2

In reply to this post by JEAN-FRANCOIS MENARD-3

Please forgive me for putting on my "physics teacher hat" and doubly
forgive the algebra which is to come :^))

I'd argue against trying to recalibrate the K2 internal voltmeter to
compensate for the diode drop in series with the supply. How come?
Because changing the resistor values in the voltage divider is a
multiplier correction, while the problem is an additive offset.

The algebra:

if the supply voltage is Vs and the metered voltage is Vm then

Vm = Vs - Vf where Vf is the forward drop of a diode

if you fiddle with the voltage divider you can get

Vm = k (Vs - Vf) where k is the factor for the "new"
divider.

Now, for any particular Vs, and knowing that Vf is pretty much constant,
you can choose a k that will make the meter read perfectly, that is Vm =
Vs if k = Vs/(Vs-Vf)

But if Vs changes, you'll need a new k. Granted, since Vs will probably
vary over only a small range, say 11 V to 15 V, the difference won't be
large. But ... the diference now won't be known, whereas before there was
a known, predictable offset. The k value will only need to change by 2%
or so, but 2% of 14V is a couple of tenths.

You went from *knowing* the reading was about a half volt too low, to
having a reading that could be a couple of tenths high or low depending
on where in the range you are.

What you really needed to do was add Vf to both sides:

Vm (new) = Vm + Vf = (Vs - Vf) +Vf

And that you can't do by changing the divider ratio. Could it be done in
the firmware? Sure, just add in the requisite counts to put back the
diode offset. But I'll betcha Wayne and Eric figured it was better to
know the actual voltage the rig is seeing (after the protection diode AND
any voltage drop in the power cord).

OK, physics teacher hat off, QRPer hat back on.... If it ain't broke....

73
Jim N5IB

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Don Wilhelm-3

RE: voltmeter offset and accuracy in K2

Jim,

You are exactly right - it will read a bit high for all voltages. I don't
believe that would be a problem for most folks since they seldom use the
voltmeter for anything other than monitoring the K2 voltage. If you use the
K2 metering circuit in its "EXT" mode by changing the jumper and using a
probe, you will find those readings a bit high after making the change.
Proper compensation COULD be done in firmware - but the firmware would have
to know exactly the diode drop and that varies from diode to diode, and it
is not a good solution for those using the EXT voltmeter function either
because there is no input to the firmware to indicate when the voltmeter is
in the INT or the EXT position.

I hope I did not give the wrong impression - I am not advocating nor
recommending this as a change, I was simply responding to those folks who
wanted to know how to do it.

73,
Don W3FPR

> -----Original Message-----
>
> Please forgive me for putting on my "physics teacher hat" and doubly
> forgive the algebra which is to come :^))
>
> I'd argue against trying to recalibrate the K2 internal voltmeter to
> compensate for the diode drop in series with the supply. How come?
> Because changing the resistor values in the voltage divider is a
> multiplier correction, while the problem is an additive offset.
>
> The algebra:
>
> if the supply voltage is Vs and the metered voltage is Vm then
>
> Vm = Vs - Vf where Vf is the forward drop of a diode
>
> if you fiddle with the voltage divider you can get
>
> Vm = k (Vs - Vf) where k is the factor for the "new"
> divider.
>
> Now, for any particular Vs, and knowing that Vf is pretty much constant,
> you can choose a k that will make the meter read perfectly, that is Vm =
> Vs if k = Vs/(Vs-Vf)
>
> But if Vs changes, you'll need a new k. Granted, since Vs will probably
> vary over only a small range, say 11 V to 15 V, the difference won't be
> large. But ... the diference now won't be known, whereas before there was
> a known, predictable offset. The k value will only need to change by 2%
> or so, but 2% of 14V is a couple of tenths.
>
> You went from *knowing* the reading was about a half volt too low, to
> having a reading that could be a couple of tenths high or low depending
> on where in the range you are.
>
> What you really needed to do was add Vf to both sides:
>
> Vm (new) = Vm + Vf = (Vs - Vf) +Vf
>
> And that you can't do by changing the divider ratio. Could it be done in
> the firmware? Sure, just add in the requisite counts to put back the
> diode offset. But I'll betcha Wayne and Eric figured it was better to
> know the actual voltage the rig is seeing (after the protection diode AND
> any voltage drop in the power cord).
>
> OK, physics teacher hat off, QRPer hat back on.... If it ain't broke....
>
> 73
> Jim N5IB
>
>

Sverre Holm-2

RE: voltmeter offset and accuracy in K2

If you want the K2 voltmeter to read the correct external voltage, why not
connect directly from the input power jack to the external input of the
voltmeter, and slide the switch/strap over to the EXT position? In this way
you bypass the diode loss and there is no need for recalibration. Or are
there some constraints in the original posting that I am missing with this
rather simple solution?

73

Sverre
LA3ZA
http://www.qslnet.de/la3za/

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