BOD20 Posted February 7, 2014 Share Posted February 7, 2014 I wonder if anyone can shed light on this curious problem. The setup is as follows - 4 x Sennheiser EM300 (1st generation) radio mic receivers operating within Channel 70 and connected to mixing desk mic channels using XLRs, with 4 x Sennheiser SK300 pocket transmitters, and two Sennheiser "paddle" antennae. There is a problem with one, but only one, of the receivers. When the corresponding transmitter is switched ON everything works perfectly - no issues at all. However, when the transmitter is OFF, the receiver generates "white noise" at maximum volume and into the mixer. In this situation the RF display (as you would expect) shows "no signal" but the AF display is full on - maximum signal - and the signal level at the mixer is strong enough to cause the channel peak indicator to stay on regardless of the input gain setting. I noticed this at a show rehearsal but the limited investigation I carried out could find no issues. But this wasn't a problem - I simply ensured that the transmitter was on and everything was ok. But during one show the battery in the transmitter suddenly died (I will insist on Duracells from now on !) the transmitter therefore switched off, and the sudden blast of noise through the PA certainly woke up everyone in the audience and probably scared small children within a radius of 2 miles !! Tracked down the dead battery, replaced it, and all worked perfectly for the rest of the show. The problem receiver is the 4th in the "chain" and set to work at 864.500 MHz.. The other receivers are tuned to 863.00 MHz, 863.500 MHz and 864.00 MHz. There is no indication of any interfering transmissions and, notably, there is no RF signal showing on the display when the noise is present. Thanks for looking Link to comment Share on other sites More sharing options...
bruce Posted February 7, 2014 Share Posted February 7, 2014 Check the "Squelch" settings on that receiver. Link to comment Share on other sites More sharing options...
sam.spoons Posted February 7, 2014 Share Posted February 7, 2014 You don't mention the Squelch setting on the offending receiver, if it's set too low it would cause exactly this problem you describe (and too high could block the wanted signal from the transmitter). Link to comment Share on other sites More sharing options...
BOD20 Posted February 7, 2014 Author Share Posted February 7, 2014 Squelch - that's the problem. I just managed to retrieve the gear from the hall and brought it home to have a look at. The "Squelch" setting on that receiver was wrong (I've also downloaded a user manual from the web for future reference !) Problem solved. Many thanks to bruce and sam.spoons for the quick replies. I've also just "Locked" the receivers so that the settings can't be changed by accident (or inquisitive fingers !) Link to comment Share on other sites More sharing options...
smalljoshua Posted February 7, 2014 Share Posted February 7, 2014 The problem receiver is the 4th in the "chain" and set to work at 864.500 MHz.. The other receivers are tuned to 863.00 MHz, 863.500 MHz and 864.00 MHz.You're not actually operating fully in Ch70 there, your first and 4th mics will be straying outside of Ch70 and into areas that are prohibited for use by PMSE (Radio Mics). Sennhesier's recommended Frequencies are: 863.100863.500864.300864.900 Josh Link to comment Share on other sites More sharing options...
Shez Posted February 7, 2014 Share Posted February 7, 2014 864.5 is still within the band but 863.0 is indeed naughty. Aside from the licencing implications, the oither reason for using those recommended frequencies is that they won't interfere with each other. Your equally spaced choice of frequencies won't work well together - there's a bit more to choosing frequencies than just spacing them out. Look in to "intermodulation" if you want to learn more. Link to comment Share on other sites More sharing options...
BOD20 Posted February 7, 2014 Author Share Posted February 7, 2014 864.5 is still within the band but 863.0 is indeed naughty. Aside from the licencing implications, the oither reason for using those recommended frequencies is that they won't interfere with each other. Your equally spaced choice of frequencies won't work well together - there's a bit more to choosing frequencies than just spacing them out. Look in to "intermodulation" if you want to learn more. It's a school who bought the gear second hand from a local theatre company after hiring it from them a few times. All the gear was supplied "pre-configured" by the theatre company and these frequencies were already programmed in when I arrived to do the sound. Didn't have much time to familiarise myself with anything and certainly no time to start changing the frequencies when everything seemed to be working. But thanks for the "heads up". I have the gear at home now and with no pressure on I'll look into changing and optimising things. Link to comment Share on other sites More sharing options...
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