Help! Sennheiser mic problems

[TimAV]

I've got a bank of 4 almost brand new sennheiser radio mics, (2 x lavs, 2 x hand helds), and I'm having major nightmares with my second lav mic occasionally cutting out (very briefly), and giving mute signal on receiver, although, the cut out and mute warning don't always happen at the same time?!?

I've tried all the obvious I can think of, I.e loose/bad connection on lav cord; pilot tone is on; squelch is low; freqs are miles apart; battery is full.

Just to confuse things even further, my second lav receiver did the same thing, although only once.

Couldn't get the fault to repeat after the gig either, which is annoying. Any ideas? Am I missing something?

 

Ta,

[Bobbsy]

Well, besides the obvious issue of an actual electronic or wiring fault, the first place I'd be looking is at the possibility of RF interference. Just because frequencies are some distance apart, it doesn't mean that you can't be getting Intermod problems from the harmonics. Similarly, it could be interference from either a broadcast TV channel or another radio mic user.

 

As an aside, you should either be using specific licence-exempt frequencies, or ones assigned to you by JFMG when you get a licence. The upside of this is that working this way should help ensure you're on frequencies that can co-exist.

 

For more information of frequencies and licencing, visit www.jfmg.co.uk .

 

Bob

[Ynot]

you should either be using specific licence-exempt frequencies, or ones assigned to you by JFMG when you get a licence. The upside of this is that working this way should help ensure you're on frequencies that can co-exist.

I'd echo this - tell us a) if you're using licence free or licenced frequencies and b) what they are set on the mics.

 

Another thing to check, daft as it sounds, is that the transmitters & receivers are set to EXACTLY the same freq's.

 

TD

[TimAV]

Yup, they're definitly set to EXACTLY the same freq's, and as far as I know they're licensed, although I'm currently in the process of confirming this with the powers that be, (I must admit, I thought they were exempt!). I'm currently trawling through relevant websites to educate myself about this. (thanks for the link by the way.. :()

 

Any other ideas of obvious things to check?

I'm reluctant to believe that it's a hardware fault, as I've already had to send a receiver back with a fault, only 3 days after buying them!

Surely not two dodgy units in a row!

[Bobbsy]

Well, it would be useful if you could tell us which frequencies you ARE using and perhaps where you are. I have an intermod calculation programme and I could at least check and see if the freqs you're on are compatible.

 

Bob

[Ynot]

Yup, they're definitly set to EXACTLY the same freq's, and as far as I know they're licensed, although I'm currently in the process of confirming this with the powers that be, (I must admit, I thought they were exempt!). I'm currently trawling through relevant websites to educate myself about this. (thanks for the link by the way.. :D)

Sorry if this sounds blunt, but as the operator, it's YOUR responsibility to KNOW whether you're using licenced kit. The venue should possess the paperwork if they're in-house, or if they're hired, whoever booked them on hire should have a document stating the usable frequencies - that is a term of the contract with JFMG, who provide the licencing.

 

TD

[dbuckley]

Yeah, second the intermod theory, first thing to check. Even if you are on assigned legal frequencies, whoever assigned them may have assigned incompatible frequencies, legal but useless... If thats all clear, then try reprogramming the mics so see if the problem is with the microphone or the receiver.

[dosxuk]

Have you been able to repeat the fault yet? Our building cause all sorts of wonders to radio mics :D Non-diversity is a definate no-go and analogue diversity can be troublesome. The only reliable radio mic's we've used have been digital ones.

 

Why? (It's the building in my avatar) All the rooms in the venue are surrounded completely by stainless steel and other general metal work. The RF reflections are wonderful! Some parts of the building, if you're lucky, you can get a WIFI signal through two floors and on the other side of the building, in others it plays up 4m from the access point.

 

If you can't recreate the fault, was it something to do with that particular gig?

[tr7ster]

Hmmm.....this all sounds familiar!

 

Like you I have Sennheiser 300 G2 mics, similarly two hald-held and two lavalier. And I have experienced momentary drop-out on one of the hand-helds before - just occasional, but enough for me to get the AV contractor (it's quite a new install) to extend the aerials.

 

After reading everyone's comments on frequency licensing, I admit I was under the common misconception that I was operating in the license-free bands....I just checked on the jfmg website, cross-referenced it to my receivers....ooops, way off! I have now re-set them so they are legal, and will have to see how that goes. If I have problems, then the bosses here shall be getting a £96 bill!

 

Mentions of intermodulation and frequency harmonics...can anyone explain/post a good link what exactly the symptoms are of this and how it actually occurs? Bit of a grey area in my head (kinda like my brain then).

 

Cheers,

 

 

Ben

[TimAV]

Well, it would be useful if you could tell us which frequencies you ARE using and perhaps where you are. I have an intermod calculation programme and I could at least check and see if the freqs you're on are compatible.

 

Bob

 

the frequencies I'm using are:

850.975 - 831.300 - 835.400 - 849.400

 

(all now registered and paid for, since yesterday incidently! Being a new op in the venue, I had mistakenly assumed licensing had been previously taken care of. Oops)

 

By the way, is the intermod program you have freeware and available?

 

Thanks for everybodys' help so far btw :D

 

The gig in question is still in progress until later today, and I'm still getting the same problem.

I've noticed that I drastically lose RF signal sharply just before the cut out, which explains the cut out/mute problem, but still leaves me puzzled as to why I'm losing the RF so suddenly.

I've been watching the speakers closely in each instance to see if there's a particular blackspot I haven't noticed before, but it seems to occur without rhyme or reason.

I've got boosters/auxilliary antennae installed too, so wouldn't expect there to be RF blackspot problems to the degree I'm getting them anyway...

There ARE a few laptops in the vicinity, but surely this wouldn't cause problems to this extent?

 

To echo Bens' request, can anyone explain in a bit more detail about intermod problems; theory, symptoms and cures etc...

 

Cheers folks......

[Bobbsy]

I've checked those frequencies and, alas, they should be okay in intermod terms. I say "alas" because changing frequencies would be an easy fix!

 

BTW, the programme I use to check intermod is something I wrote myself in Excel. I hesitate to offer to share it because I've never got round to making it tidy and easy to use...fields for frequency entry aren't properly labelled, and so on. Maybe this should become a project!

 

However, there is a simpler on-line checker on the Audio Technica site. Have a look at: http://ff.audiotechnica.com/cgi-bin/freqcompat.cgi .

 

One thing I noticed on your last post was the mention of "signal boosters". These shouldn't normally be necessary in a properly designed and wired system, and I've seen these cause problems as often as they cure them. I'm guessing that they're introducing phase errors and cancellations, but I've never really researched it. Some generic device would be to try without these and simply use two properly specced antenna, wired with good quality, low loss cable of the proper impedence.

 

Finally, your request for background info: I used to know a good site but the bookmark is on a computer at home and a quick google hasn't helped me find it. I'll see if it's still there later on and post a link if I can find it.

 

Bob

[david.elsbury]

Finally, your request for background info: I used to know a good site but the bookmark is on a computer at home and a quick google hasn't helped me find it. I'll see if it's still there later on and post a link if I can find it.

Don't know if this is the site you're referring to, but Kai Harada's Sound Handbook taught me some stuff a while back, and is quite interesting reading.

 

David

[TimAV]

Kai Harada's Sound Handbook taught me some stuff a while back,

That handbook is a damn good read! Thanks Dave. much appreciated...

 

Edit:- Quote fixed

[Bobbsy]

Yup, Kai's site is a good one and was one of the two I was thinking of.

 

The other one, which get's into more technical detail, is: http://audiosystemsgroup.com/wireless.pdf

 

A few of the details are specific to the American situation, but it's a very useful paper by an extremely knowledgeable professional.

 

Bob

[paulears]

have you tried swapping the channel with a problem, to one of the other ok frequencies, and putting the good receiver combination onto the problem frequency? If the good kit remains good, the other one is duff - if the previously reliable kit plays up, it is a problem on that channel - not the equipment.