Re: VHF radio mic problems

On Sat, 29 Mar 2008 04:09:42 -0000, Tim Perry  
<timperry@[EMAIL PROTECTED]
> wrote:

>
> "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.

Or use Rg213 ot CT125. Plus it tends to attenuate the noise as well as the
 
signal & 20' of cable in this situation shold help a lot.

> 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.)
>
>