[CQ-Contest] MFJ Voice Keyer

David A. Pruett k8cc at ix.netcom.com
Sun Apr 11 00:57:56 EDT 1999


There may be three versions!  As I recall, when we were at WP3X in 1997 my
(then brand new) MFJ-432 had a different schematic and PCB layout from the
older examples owned by some of the other operators.  I don't think any of
these had relays.

Its enlightening to note that the W9XT Contest Card, which uses essentially
the same chip, has exhibited NO hum problems here, regardless of the radio
type.  The reason?  Gary isolated the output with a 600:600 transformer. 
Gee, good engineering works!

BTW, I just bought a new Contest Card right before WPX SSB and they now
come with the larger ISD chip, so that each memory is 16 seconds long.  No
need to juggle messages for length now!



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>From Dick Green" <dick.green at valley.net  Sun Apr 11 06:27:47 1999
From: Dick Green" <dick.green at valley.net (Dick Green)
Date: Sun, 11 Apr 1999 01:27:47 -0400
Subject: [CQ-Contest] Re: MFJ Voice Keyer
References: <199904110356.WAA06567 at dfw-ix1.ix.netcom.com>
Message-ID: <009601be83dc$7f9d1f80$2ca473c6 at tpk.net>

> One thing I found strange was that with my particular MFJ-432, the +5V
> for the internal mic is also applied to the external mic as well.  I don't
> know if this is a problem, but I doubt that Heil headsets are designed to
> have DC bias on them at all times.
> My solution was to cut out the resistor providing the bias.  This renders
> the internal mic useless, but that seemed to be a reasonable tradeoff.
> I've never had any problem getting a Heil DXer element (that's the 4,
> right?) to drive the MFJ-432 correctly.

Yikes! I think we have the same vintage and I didn't notice that +5V bias. I
would agree that it shouldn't be there for Heil mics. I have a vague
recollection that Icom mics have a positive voltage on the mic element.
Unfortunately, I tried cutting off the +5V bias and that had no effect on
recording with the Heil. It's not the mic, either, because the stock Kenwood
handheld mic for the 950 has unacceptable gain as well. I don't think it's
the chip or the mic reference voltage components because the internal mic
works reasonably well. The gain isn't killer, but it's way above what I'm
getting with external mics. That leads me to believe it could be something
amiss in the external mic input chain, like a bad connection at the header
or a bad solder joint somewhere. I'll have to check that out.

73, Dick, WC1M

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