Re: VHF radio mic problems

"John Legon" <john@[EMAIL PROTECTED]
> wrote in message
news:$nqlWBA4fK7HFw6W@[EMAIL PROTECTED]
> Tim Perry <timperry@[EMAIL PROTECTED]
> wrote in article <47ec6b0b$0$1
> 6646$4c368faf@[EMAIL PROTECTED]
>:
>
> >Please list the make and model, the number of units and the frequencies
if
> >possible.
>
> Audio-Technica ATW-R11 receivers x 2, 174.4 and 174.8 MHz
>
> >Assuming both transmitter and receiver are operating to spec you may
improve
> >operation by adjusting the squelch threshold.
>
> I've looked at the squelch trimmers and they're fully anti-clockwise,
> presumably giving maximum sensitivity.
>
> >Are you using alkaline or rechargeable batteries?  Often rechargeable
have a
> >lower terminal voltage even when fully charged.
>
> We always start off with fresh alkaline batteries, but it makes no
> difference.
>
> >Depending on model, often VHF body packs use the mic cable as an
antenna.
It
> >should be extended as much as possible and not coiled up.
>
> The body packs have separate aerial wires.
> >
> >About how old is the instilation? Is it recient or years old?
> >
> Must be nearly ten years old. (circa 1999)
>
> -- 
> John Legon

Well,  I just got back from setting up sound for Rogers and Hammerstein's
Cinderella.

My comments are based in the assumption that this used to work and over
the
years it has slowly degraded to the point you have now.

On a ten year old wireless system pretty much anything could be broken. My
feeling it the most likely suspect is crystals - over time crystals slowly
drift in frequency.  The transmitter and/or the recover could be off
frequency to the point that the receivers AFC circuit is on the edge of it
capture range.

Other suspects are: hairline cracks in the transmitter PC board
particularly
near the antenna circuit.

After years of proximity to actors sweat, the enitre PC board can become
slightly conductive do to contaminants finding thier way in.

Occasionally receiver sensitivity suffers when the first RF stage goes low
gain, wheather from age or power surges.

Transmitters can become detuned to the point that most of the power is is
reflected back into final amplifier (high VSWR).

If you have "line of sight" from transmitter to receiver, do not blame the
presence of an audience. Look for some other factor. As an example a few
years back I did a Disney show in a public mall which employed 9 brand new
wireless HTs. everything was fine for sound check but just as each show
started dozens of police and security guards all started using two-ways
intensively.

I do not agree that extending the antennas will necessarily be the "magic
bullet" you are looking for. Cable losses at these frequencies are pretty
hefty.
example: RG -58  (depending on brand) about 5.5 dB per 100 feet.
http://www.ocarc.ca/coax.htm
a 6 dB low noise preamp would be needed to make up the loss for a coax
this
long. That or using a Yagi antenna which is problematic due to size at
these
frequencies.
However changing from a 1/4 wave whip antenna to a 5/8 wave loaded antenna
will yield about 3 dB gain.

Based on the listed MSRP these units probably sold new for $200 to $300
each.  My recommendation is to contact the 2-way guy who services the bus
radios, security radios ect. have them do a bench check and then determine
whether repair or replacement is applicable. as these are discontinued and
specialized parts will be unavailable. In general, it is difficult if not
impossible to obtain service information in items like this.

My recommendation on antenna orientations for diversity receivers is one
straight up and one at 90 degrees (if it can be perpendicular to the line
of
sight to the receiver.)