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E. Dusina W4NVK
571 Orange Ave. W
Melbourn e FL 32901
Now tha t phone patches are legal,
amateurs are given the responsibility
to adj ust th e levels they feed into the lines
of th ose phone syste ms that do no t fu rnish
or have not got int er face black boxes to
automatically adjust patc h injection levels.
Failur e to adjust the levels may overdrive
th e pho ne lines and cause undesirable
cross talk on other phones sharing t he same
cable. Since this is usually yo ur neighb ors,
cro ssta lk, like TVI , is close to home.
The pro cedure for adjusting your patch
injection level is relatively simp le, and may
be done to a pr acti cal degree of accur acy
with any ac voltmet er which can read as
low as 1.5V full scale.
First , you should und erstand that there
are two line wires and a floa ting ground in
most phon e systems. You are only interested
in th e line wires, and these are
generally coded green and red . In an y case
you can find th ese tw o with a voltme ter,
since there is a residual d e voltage on the
line.
To calibrate your ac voltme te r to yo ur
phone's referen ce level, connect a 0 .1 pF
capacitor in series with one lead, to block
the de but pass the ac. If your multime ter
has an OUT PUT sett ing, use that sinc e such
a capacito r is insert ed internally . Use your
lowest ac scale. Then take th e phone off
the hook and dia l the first 3 digits of your
own phone number. This will dr op the dial
tone and give yo u a clear audio line. Talk
into the phone in a loud voice ~ a little
louder than yo u talk on th e ph one normally
. You will see the meter needle kick
Phone Patch Lecel
Adjustments and
Manproofing
up to about 0.5V . Notice whe re it kicks up
to as yo u ta lk, and then hang up the
phone. You have now esta blished your
referen ce po int.
To adjust the pa tch level, tune yo ur
receiver to a good QR M-free signal on the
band, choosing one tha t causes your
S-meter to at least show some valu e larger
than S2 or so . Again dial th e first 3 digits
of your number, and flip the patch into th e
line. Adjust the receiver audio gain until
the ac volt me te r on the phone line kic ks up
about th e same ' as it did when you calibrated
it with your own voice.
When completed, mar k the audio gain
no w at th e sett ing you fou nd pro per fo r
your patch. This will be the corr ect level
for all pat ch use, regardless of signal S
valu e, because any decent SSB rig ho lds t he
audio ou t put level sufficiently accurately,
through it's normal age r-ction , to suffice
for phone pat ching use. If yo u want more
precision , however, you can leave some
form of capacito r-isolated meter on the
pat ch , as a sort of VU met er.
Do no t omit th e capacitor, and preferably
use one in each lead to prevent
shor ting th e phone line if you have an
accident with th e leads.
Manproofing the Patch
If you want to make your patch foolproof,
you can do thi s to the ex te nt that
nothing you do on your patch will cause
your phone to be out of service, nor
prevent its use in a no rma l manner. To do
th is, do not connec t your attachment wires
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73 MAGA Z I NE