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
---
http://www.elecraft.com
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