[NLRS] ft-817 mic repair

[Dr. Gerald N. Johnson]

On Thu, 2006-08-24 at 18:11 -0400, Glen Overby wrote:
> 
> 
> Last weekend during the 10GHz contest I had reports of something wrong with
> the microphone on my Yaesu FT-817.  I switched to my 'spare' mic (the one from
> my 'spare' IF rig) for a while, then switched back when I discovered (I think)
> a loose PTT connection.
> 
> I don't know much about microphone failure modes.  I wonder if it could be an
> intermittent connection on the cord, not a failure of the mic itself (the mic
> and cable are separate, and the connection to the mic and radio is with an
> 8-pin "RJ-45" type connector).  I opened up the case and inspected the wiring
> inside, to see if it was an easy problem like a broken wire.  It's not.
> 
> I'm looking for ways to diagnose the problem further, before just replacing
> "the part".  In particular, I'd like to make sure that it's not the cable.
> 
> Any ideas?
> 
> Glen
 Mobile hand mics always have tended to fail at a connector. That's
where the wire flexes the most, and if there's a strain relief around
the cable at the connector the strands are most likely to break at the
out end of that strain relief.

In the past week I've been into several Yaesu microphones, mostly MH-31
as supplied for the FT-847 and the FT-857. The cable for the 847 ends in
an 8 pin round connector with soldered connections. WB0BQV has been
having intermittent audio with it for a couple year. Last Sunday
evening, he found he could cause it by tugging on the microphone but
that didn't show which connector it was or whether it was radio or cable
or microphone. I attacked the connector end (after having to grind a
special screwdriver to remove the 1.6 mm x 3 mm flat head self tapping
screw) and under magnification, there was a rosin joint on one of the
wires (yellow color code). And when I hit it with a hot iron there was a
puff of smoke as the rosin burned out of the connection. So I hit all
but the center pin.

Inside that microphone, which uses through hole parts (unlike my new
MH-36 for the FT-90 that's all surface mount), I saw about half the
connections had bulbous solder lumps, not convex fillets. For all my
soldering life, I've always found a bulbous solder lump (shaped like
certain features of one Dolly Parton) hid a disconnected pair of wires
or circuit board and wire that were separated by rosin. Sure enough,
several of those released puffs of rosin smoke when heated, and every
two or three I had to wipe my soldering iron as it accumulated some of
the excess solder. In these microphones, the hand soldering was NOT
good.

Crimped cable ends handle flexing better than soldered ends, but the
modular connector hasn't much back up for the single surface connection.
Soldered flex wires tend to break where the solder wicks up them unless
much care is taken to prevent that wicking (like a heat sink fitted to
the wire at the end of the insulation).

Its possible that a one tenth drop of DeoxIT on each wire of the modular
connector can fix corrosion there, but it won't fix a truly broken wire
or crimp (idc isn't really much of a crimp) where moving the wire
creates an air gap.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer