[CQ-Contest] Voice Keyer

[Dave Pruett]

Tom,

The comments about the MFJ-432 came from me in response to KK9A's note.  My 
unit was purchased in March 1997.  The manual says "Version 2B" on the 
cover.  The manual included a schematic dated 11/09/95, PCB P/N 862-0432.

>To correct some misinformation about the MFJ432, it has been
>ground isolated since I was given the job of reworking it a few years
>ago. The small chokes were NEVER for ground isolation, they were
>for RF isolation.

My schematic shows four 1000uH chokes.  There is one each in the PTT, 
MIC_OUT and MIC_IN lines which appear to be for RF isolation.  There is a 
fourth, L6 which connects the MIC_GND/PTT_GND (which are tied together at 
connector J5) to chassis ground.  This choke (in series with ground) would 
isolate the PCB and make for *worse* RF characteristics, I would expect.

Tying MIC_GND and PTT_GND together violates the recommendations from the 
manufacturers of all the radios I own.  Sticking a choke from this 
connection to chassis, where both the mic and PTT currents have to flow 
through it makes no sense.  When the MFJ-432 was connected to my TS-930, 
there was severe hum on the transmit audio both for live and recorded 
audio.  Shorting out L6 made this problem go away completely.

>The original MFJ VK switching scheme was FET's (duh)

The schematic for my unit shows a DPDT relay RY1 which switches the MIC OUT 
line from MIC IN and the output of the recorder for playback, and also 
switches the PTT line to ground.

>It was also hand wired, it is now surface mount.

Mine has a PC board, with through hole components.  The only wires are from 
the PC board to output cable.

>As far as I am aware, the only recent problems come from a batch
>that may still be out at dealers where an assembly line inspector
>let units out that were not stuffed with all the parts, or were not
>soldered at the correct temperature in the wave solder machine.
>Since MFJ does not serial number units there is no way to trace
>these other than popping the cover and looking next to the jumper
>terminals for missing or partially soldered ferrite beads.

My unit may have been manufactured before your involvement.  Mine has no 
quality problems at all - it has worked perfectly one L6 was jumpered over.

The other change I made was to cut out R3 which provides 5VDC to run the 
internal electret mic.  However, this also put 5 VDC on the MIC IN whenever 
a message was being recorded - I did not think the Heil element would 
appreciate this.

73,

Dave/K8CC


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