K3 Caution when using an external monaural speaker

[T Gahagan]

Most of the K3 owners probably already know this but for the benefit of
those that don't please be aware that you may damage the audio amplifier
chip on the DSP board if you enable Config Speaker 2 and then plug a
monaural speaker plug into the rear panel speaker jack.  This overloads the
audio amp.

If running a monaural speaker the Speaker setting must be set to 1.  I would
recommend always using a set of stereo speakers no matter what the
configuration setting for speakers.  I am somewhat embarrassed to say that
the audio amp chip is really not much fun to replace and I will be more
careful in the future.  :-(

Todd, WA7U

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[wayne burdick]

Thanks for your observations on this, Todd.

Just to be safe, we recommend using a stereo plug even when only a
single external speaker is being used. This is discussed in detail in
the owner's manual (page 20 in the case of revision D). Just use the
TIP contact of the plug for the speaker, leaving RING unconnected.

Of course we want to reduce the possibility of damaging the AF amp to
zero -- even with SPKRS=2 and a mono plug. To this end, we're working
on a very simple modification to the KIO3 audio daughter board. This
will be the subject of a modification document soon, and is already
being phased into production.

73,
Wayne
N6KR


TGahagan wrote:

> Most of the K3 owners probably already know this but for the benefit
> of those that don't please be aware that you may damage the audio
> amplifier chip on the DSP board if you enable Config Speaker 2 and
> then plug a monaural speaker plug into the rear panel speaker jack.


---

http://www.elecraft.com

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[David Cutter]

Wayne

I will be doing all the mods for Christmas. Hope you can give us the info
really soon so I don't have to take the K3 apart again.  Can you just point
to the circuit and tell us what you intend ahead of the official mod
instruction?

73
David
G3UNA
>
 To this end, we're working
> on a very simple modification to the KIO3 audio daughter board. This will
> be the subject of a modification document soon, and is already being
> phased into production.
>
> 73,
> Wayne
> N6KR
>
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[wayne burdick]

Hi David,

Lyle, KK7P (who designed the KIO3 board set) will create an application
note and post it as soon as possible.

The short version is: put 1/8-W (or larger), 470-ohm resistors from
each speaker output to ground. (The actual value isn't critical;
anything from 330 to 1000 ohms would probably work just as well.) This
can be done conveniently on the KIO3's audio daughter board, at its
single-row, right-angle male connector.

Technical details:

The resistors hold the (-) leads of the AF amp's large electrolytic
coupling capacitors at ground potential (RF board, C2 and C3, 470 uF).
Thus if you accidentally ground *either* of the speaker outputs, you'll
be changing only the AC load to the amplifier -- not the DC voltage.
This means you won't get a big current spike backward, into the AF
amp's output, through the electrolytic; that's what we think can damage
the chip, because the caps can dump a lot of energy very quickly.

An astute observer would note that a similar current spike should be
generated on power-up, if a short at either speaker jack was present at
that time. But the AF amp chip protects itself during power-up,
limiting current at its output pins. If it didn't, even the presence of
a low-Z speaker might cause chip damage.

With the resistors in place, Lyle repeatedly shoved a mono plug into
the speaker jack, under both small and large-signal AF output
conditions. He couldn't break the amp chip.

Credit for this "bleeder resistor" idea goes to another one of our
staff engineers, Bob, N6CM. Lyle and I both wish we'd thought of it.

73,
Wayne
N6KR

On Dec 20, 2008, at 12:59 AM, David Cutter wrote:

> Wayne
>
> I will be doing all the mods for Christmas. Hope you can give us the
> info really soon so I don't have to take the K3 apart again.  Can you
> just point to the circuit and tell us what you intend ahead of the
> official mod instruction?
>
> 73
> David
> G3UNA
>>
> To this end, we're working
>> on a very simple modification to the KIO3 audio daughter board. This
>> will be the subject of a modification document soon, and is already
>> being phased into production.
>>
>> 73,
>> Wayne
>> N6KR
>
>

---

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[David Pratt]

In a recent message, wayne burdick <[hidden email]> wrote ...
>The short version is: put 1/8-W (or larger), 470-ohm resistors from
>each speaker output to ground. (The actual value isn't critical;
>anything from 330 to 1000 ohms would probably work just as well.) This
>can be done conveniently on the KIO3's audio daughter board, at its
>single-row, right-angle male connector.

I see that the AF mod application note at
http://www.elecraft.com/K3/mods/AF_Output_Mod_Rev_A.pdf
is somewhat different to what you describe, Wayne.  The 470 ohm
resistors appear to be in different places of the circuit.  Does this
come to the same thing, or are the two resistors from each speaker
output to ground also necessary?

73
--
David G4DMP
Leeds, England, UK
------


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[Bruce Beford]

Notice that the app note (rev A) for this has an error. Figure 4 legend
reads " Installing the 470K Ohm resistors" Should read "... 470 Ohm
resistors".

-Bruce N1RX

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[David Pratt]

In a recent message, Ron D'Eau Claire <[hidden email]> wrote ...
>The version shown in the app note mimics what Aptos is doing as they modify
>the boards in inventory. The resistors are in the same place electrically,
>but they chose to put them on the main KIO3 board instead.

Thank you, Ron, for your explanation. I have carried out the mod as in
the App note.  I was particularly puzzled by the resistor across pins 29
(TXD) & 35 of P76 which appears to put the resistor between pin 14 of U1
and ground. I had difficulty in relating this to the original note.

73
--
David G4DMP
Leeds, England, UK
------


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[David Pratt]

In a recent message, David Pratt <[hidden email]> wrote ...
>Thank you, Ron, for your explanation. I have carried out the mod as in
>the App note.  I was particularly puzzled by the resistor across pins 29
>(TXD) & 35 of P76 which appears to put the resistor between pin 14 of U1
>and ground. I had difficulty in relating this to the original note.

My mistake, chaps, I think I have misinterpreted the pin numbers. They
are evidently pins 4 & 16 of P76 which places the resistor across SPKR
R. Sorry!

73
--
David G4DMP
Leeds, England, UK
------


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[NE0T]

David Pratt-4 wrote

In a recent message, wayne burdick <[e-mail]> wrote ...
>The short version is: put 1/8-W (or larger), 470-ohm resistors from
>each speaker output to ground. (The actual value isn't critical;
>anything from 330 to 1000 ohms would probably work just as well.) This
>can be done conveniently on the KIO3's audio daughter board, at its
>single-row, right-angle male connector.

I see that the AF mod application note at
http://www.elecraft.com/K3/mods/AF_Output_Mod_Rev_A.pdf
is somewhat different to what you describe, Wayne.  The 470 ohm
resistors appear to be in different places of the circuit.  Does this
come to the same thing, or are the two resistors from each speaker
output to ground also necessary?

73
--
David G4DMP
Leeds, England, UK
------


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[K2QI]

Hi Ron,

I just ordered my K3 on Thursday.  Do you know if this modification has already been phased in to new kits?

73,
James KC2UEE

Ron D'Eau Claire wrote

Hello David:

Wayne asked me to reply since I prepared the application note. Only two
resistors are needed, so if you've done the mod using the daughter board
locations Wayne originally mentioned, you're all set.

The version shown in the app note mimics what Aptos is doing as they modify
the boards in inventory. The resistors are in the same place electrically,
but they chose to put them on the main KIO3 board instead.

We followed that in the application note so it'd be easy to tell if the
modification has been made either in the field or at the factory.

Ron AC7AC

[Bob Cunnings NW8L]

According to the Enhancements and App Notes page at:

http://www.elecraft.com/K3/k3_app_notes.htm

"This modification is needed for all K3s shipped approximately before
01/12/09 containing either a Revision "XB" or Revision "A" KIO3
board."

Bob NW8L

On Sat, Feb 14, 2009 at 5:41 PM, KC2UEE <[hidden email]> wrote:

>
> Hi Ron,
>
> I just ordered my K3 on Thursday.  Do you know if this modification has
> already been phased in to new kits?
>
> 73,
> James KC2UEE
>
>
> Ron D'Eau Claire wrote:
>>
>> Hello David:
>>
>> Wayne asked me to reply since I prepared the application note. Only two
>> resistors are needed, so if you've done the mod using the daughter board
>> locations Wayne originally mentioned, you're all set.
>>
>> The version shown in the app note mimics what Aptos is doing as they
>> modify
>> the boards in inventory. The resistors are in the same place electrically,
>> but they chose to put them on the main KIO3 board instead.
>>
>> We followed that in the application note so it'd be easy to tell if the
>> modification has been made either in the field or at the factory.
>>
>> Ron AC7AC
>>
>>
> --
> View this message in context: http://n2.nabble.com/K3-Caution-when-using-an-external-monaural-speaker-tp1680829p2328299.html
> Sent from the Elecraft mailing list archive at Nabble.com.
>
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