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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