Re: VHF radio mic problems

Tim Perry <timperry@[EMAIL PROTECTED]
> wrote in article <47edc141$0$2
4099$4c368faf@[EMAIL PROTECTED]
>:
>"John Legon" <john@[EMAIL PROTECTED]
> wrote in message
>news:$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
>> Audio-Technica ATW-R11 receivers x 2, 174.4 and 174.8 MHz
>> >
>> >About how old is the instilation? Is it recient or years old?
>> >
>> Must be nearly ten years old. (circa 1999)

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

Thanks for taking the time to post your suggestions.  You've given me
something to think about.  To pick up on a few points:

>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.

I've been using the equipment for the past four years and don't think it
has ever been 100% reliable. At present, the mics can run for an hour or
so without any problem, and then for no obvious reason the signal drops
out for a second or two and a few words are lost. Sometimes this happens
near the beginning and then again at the end. It's annoying...

>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.

I'm guessing there will be trimmers in the receivers for fine-tuning the
crystal frequency, and probably an AGC circuit with a voltage that can
be monitored for signal strength when lining up the receivers with the
transmitters?

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

It seems significant to me that both transmitter/receivers suffer from
the same intermittent fault, which doesn't seem to be the result of any
physical movement of the antenna wires or body packs.

>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.

We do have an approximate line of sight over the heads of an audience on
raked seating between the stage and control room.  It's also true that
having a full house doesn't necessarily cause any problem.

It's the "other factor" that I'm trying to pin down. Could the diversity
switching be confused by external interference, e.g. radio taxis working
on the same frequency?  This might cause the totally unpredictable loss
of signal.

>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.

Thanks for this data.  I'd guess that a length of cable just sufficient
to mount the aerials on the other side of the control room wall might
solve the problem, assuming the aerials were suitable... Extending the
cables over the stage itself could be counter-productive without the use
of a preamp.

>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.

Things being as they are, there's a good chance that a service engineer
would recommend the replacement of the entire system...

>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.)

Well, that's something I haven't tried yet.  Problem is, the receivers
are mounted sideways-on to the line of sight, so a perpendicular aerial
would stick out across the sound mixing desk!  I've put up a photo to
show you what I mean:

http://www.legon.demon.co.uk/rack.jpg

-- 
John Legon