Trying to get a handle on these things. Next time you want to bitch about n ot being able to get parts and prints, I work for the factor and can't get shit. I do not mean an ASC, I am talking about the company. Not in manufact uring but my bosses own most of the brand, of course made in China, it seem s mostly by Apex.
Anyway these are wireless microphones and of course there are no prints. Th e receivers use the LA1140 and LA1186 chipset. They are fixed frequency so I am not sure what to do with them because some of them work just fine so I suspect there was interference at the venue. I am going to tell them to se ll them locally so that people can just drop them off and pick up a differe nt frequency set, which should cure some of the problems. This is less prac tical when you ship the thing to bumfuckt Egypt somewhere
Thing is, the chipset appears to be hetrodyne but might not be being used t hat way. Reason I think that is that the frequency marked on the crystals i s exactly the frequency of the mics. You would think it would be offset by the IF frequency, like 10.7 MHz or something. They must be putting the sign al through or across the crystal, depending on which cut it is, series or p arallel resonant. On these, if the mic is labelled 208.95 MHz, so is the cr ystal in the receiver.
But why bother ? There are like four tuned circuits on there per channel so that should give adequate selectivity.
Use a frequency counter to see what frequency the receiver crystal is oscillating. It's probably the receive frequency, plus or minus the IF frequency. It's common to label the receive crystal at the operating frequency and maybe stamp the crystal with the actual crystal frequency. Same with the transmitter, where the can might be stamped with the crystal frequency with the operating frequency being some multiple of the crystal frequency. I dunno how it's done with wireless mics, but in the bad old days of crystal controlled 2-way radios, most (not all) manufacturers crystals were labeled with the operating frequency because the various crystal suppliers were only interested in the crystal frequency and want extra money to stamp the can with the operating frequency. Some, like Motorola did both on the can.
So, what problem are you trying to solve (or create)?
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
In the old two way radio business, it was pretty common for crystals to be marked with the operating frequency, not the crystal frequency.
So a crystal might be marked 146.94, but it's really 6.1225 since the transmitter multiplies the 6Mhz crystal by 24. The receiver crystal would be marked the same frequency, even though it was that frequency -10.7MHz or whatever IF, divided by 24.
CB crystals were often marked with the CB channel (and "T" or "R" for transmit or receive), though of courze they only worked in a specific set, a receiver with a different IF wouldn't be on frequency.
This is much more obvious for the uninformed, so they can go and ask for a pair of crystals for Channel A or whatever the frequency, and they won't be puzzled by the crystal that isn't marked that way.
Since this sort of application might require changing the frequency (because you already have one on that frequency, or someone in the theatre next door is using the first frequency), it makes sense to keep a stock of crystals and let the user change them.
YOu want matching crystals in the transmitter and receiver, the rest doesn't matter to the user.
not being able to get parts and prints, I work for the factor and can't ge t shit. I do not mean an ASC, I am talking about the company. Not in manufa cturing but my bosses own most of the brand, of course made in China, it se ems mostly by Apex.
The receivers use the LA1140 and LA1186 chipset. They are fixed frequency s o I am not sure what to do with them because some of them work just fine so I suspect there was interference at the venue. I am going to tell them to sell them locally so that people can just drop them off and pick up a diffe rent frequency set, which should cure some of the problems. This is less pr actical when you ship the thing to bumfuckt Egypt somewhere
that way. Reason I think that is that the frequency marked on the crystals is exactly the frequency of the mics. You would think it would be offset b y the IF frequency, like 10.7 MHz or something. They must be putting the si gnal through or across the crystal, depending on which cut it is, series or parallel resonant. On these, if the mic is labelled 208.95 MHz, so is the crystal in the receiver.
so that should give adequate selectivity.
** I used to regularly see fixed frequency, VHF radio mics through the 1980 s and 90s until they were replaced by UHF models. Owners had plenty of prob lems with VHF models - but practically none were due to faults in the mics themselves.
As a service tech, all you can do is check the mic is transmitting on the f requency it is labelled and operates with the receiver over a decent range, like 50 metres. You will need a counter that covers the VHF range and a sh ort antenna fitted with a BNC plug. No direct connection is needed, the mic ought to drive the antenna hard enough to get a steady reading from a foot or two away.
The crystals oscillate at a frequency that is either a 1/3 or 1/5 of the VH F frequency and multipliers are used to get to the final result. Rx and Tx crystals are different types and have different actual frequencies - often the Tx crystal is "pulled" to create FM modulation.
Most mic systems use wide band FM, same as broadcast FM but a few use narro w band and their noise performance is much poorer. Practically all use audi o companding to improve s/n ratios under weak signal conditions.
It is possible to change the operating frequency by swapping both crystals, but the new frequency must be less than 2% away from the old to avoid the need to do a major re-tune.
You mention 208.95MHz, which is covered by TV station 12 in the USA. If the mic is being used inside a venue and which is not close to a Ch12 transmit ter, it will work OK. If used outdoors, it will likely have poor range.
I assume the receivers are "true diversity" types with twin antennas and fr ont ends - otherwise expect major problems with signal drop outs. A "drop o ut" is often not simply a loss of audio signal but a burst of white noise a s the mic is moved around by even small distances.
Wireless mic receivers must be located close as possible to where the mic i s being used and with its antennas well clear of obstructions, especially m etalwork. Many owners do no appreciate this and do the opposite - the resul t is lots of drop outs.
Placing the receiver near a PC or any other source of VHF interference is a lso a big no-no. DJs may well do this if they use a PC as part of their set up. The other annoying issue is with batteries, but I figure you can work them out for yourself.
Oh boy, I can't believe I didn't figure that out. I am slipping, but I got an excuse. Not going into that right now.
Doing that, printing the operating frequency on the crystals would not only facilitate service, if the friggin told us, but assembly.
Another thing I found out about the construction of these marvels of (down( s)) engineering is that the frequency displays in the front are fixed. So t o speak. Basic LED display and it says the frequency but what it says is ma rked on the side with a sharpie.
When you look at the back of the board you can see that the terminals are a ll connected together. Obviously they had the vendor purposely burn out the segments they did not want on. This is not changeable. And I got unit that the frequency does not match either the display on the front or the sticke r on the back. (which do match each other though, and the mics were changed to match)
Anyway thanks for your input, you are one of the sharper people on here.
OH ! I have another reason to believe these are TRFs, they are two channel, dual mics. The side with the lower frequency, some are down like 174 MHz, that channel I have noticed usually has the cores in the tuned circuits low er. As a matter of fact, the construction of those coils or whatever seems pretty stout these days. The look like they have brass cores and take flath ead screwdrivers. They didn't put that money there fro nothing. Nobody does that these days. And I am pretty sure it was built by Apex, if Apex puts t he money into this you now there is a reason.
980s and 90s until they were replaced by UHF models. Owners had >plenty of problems with VHF models - but practically none were due >to faults in the mics themselves."
I have labelled a bunch of them "SUSPECT INTERFERENCE IN VENUE".
I don't know if you read when I spouted off about it, but though I was neve r a proponent of digital TV, I was of abolishing VHF. The system here was A M and a mess, and every time it rained on a power line you get hum bars in the picture from the arcing. At least in some places. I did mention the cru mbling infrastructure right ?
And now, about channel 12 TV (even though we do not have that here at the m oment) now the TV is digital and the sidebands are like, who knows. Seriou sly.
They should have abolished VHF TV decades ago and gave more high fidelity r adio stations. Or something. In fact, that VHF range is about where RC plan es are. Now can you imagine some broad singing "stand by your man" at a bar making an RC plane or copter take the wrong turn and kill someone ?
3.579545MHz is the color burst frequency. You'll see such crystals in TV sets, not wireless mics.
I think you mean the FCC. Congress cut their budget some years ago, so the FCC simply reduced the enforcement burro. Field offices deal primarily with enforcement issues.
FCC enforcement is now generally controlled by Homeland Security, which means you will need to complain about an act of terrorism before you'll get the FCC's attention. If there's substantial potential revenue from fines "notices of apparent liability", the FCC might think about acting.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
No, they go for quite a bit more than that, but I bet you are probably clos e to the actual cost of them. And damnear everything is Chinese junk. Any c ompany publicly traded in the US is pretty much governed by Dodge v Ford.
Why ? I tellya I seen the cores lower in the adjustable coil/transformers l ower in the channels with the lower frequency. Are you saying that is all f ront end and they still downconvert ?
Maybe they do, leaving little for the IF to do in the way of selectivity an d letting the previous stages do it. When you have like an audio type FM tu ner, they use superhetrodyne because the customer want to tune from channel to channel at will and the IF provides most of the selectivity. But these are fixed. I believe I have mentioned that the frequency readout on the fro nt panel are burnt in.
Yup, cheap Chinese shit made by Apex. But what ain't ? Everything is junk t hese days.
y 6 and 5. "
This chipset has no such function as far as I can tell. These are very simp le units, and cheaply build. There is no chip on the board up front.
What I am going to do next is look at more of these and while I know I will find the cores buried deeper in the coils associated with the lower freque ncy, I am going to pay special attention to the last one, which would be a coil for the quadrature detector.
If those are buried deeper that indicates TRF. For fixed frequency TRF is n ot a bad approach. Saves money and there is absolutely no radiation to worr y about. So the FCC or whatever you have as its equivalent will not give yo u any trouble.
Anyway, from the gist of this whole thread I get that the crystal frequency is not necessarily what is printed on it. What if resistors and capacitors were like that ?
I might just have to take a freq counter in there but that is above my pay scale. That may change.
The scope they gave me is like 20 MHz, so I could see 10.7 MHz IF on it. An d the chipset datasheets do indicate they are for FM, mobile FM in fact.
It probably doesn't matter, I think the people who own that company are hob byists, rich kids who want to feel useful. Of course folks like that can be useful to me.
** A "TRF" receiver has no local oscillator, mixer or IF amplifier - which your examples clearly do.
** Radio mic systems by the likes of Shure, Sennheiser and AKG are very well made products intended for professional use - all operating on UHF nowadays.
What you have is a Karioke / DJ system.
** Frequency multiplying is done with a tuned filter, often one transistor and a coil that selects a single harmonic from the crystal oscillator's output. The oscillator circuit is designed to be rich in harmonics.
** The quadrature coil should be next to the IF amplifier, the LA1140 chip.
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** You said there WAS a crystal in each receiver, labelled with the same frequency as one of the mics, so the receiver is a superhet.
FYI:
Radio mics use such low power levels they cannot operate successfully unless a clear frequency exists at the location where they are being used - having that means they interfere with nothing.
A shit awful load of poot is spoken about radio mics *causing* interference.
I finally took apart some RCA Carphones last fall, 2M tube receivers with a bunch of coils in the front end. Yet it's still wider than the deviation on that thing, and of course they had to downconvert for enough selectivity.
There's a good reason TRF receivers are long in the past. You don't get enough selectivity (and these things are operating much much higher than the Am broadcast band), you have to retune all the tuned circuits if the frequency is changed, and gain has to be at the signal frequency. This latter means that you might have trouble getting enough gain, and making it stable may be an issue. I thought you said these could be changed to another frequency, the last thing you want is an end user having to retune the front end.
Some things still use superregenerative receivers, that provide lots of gain but the quenching makes it stable. But those are as wide as the barn door, and FM is detected by slope detection, ie enough selectivity so there's a frequency to amplitude conversion, and then the signal is received as AM. Lots of noise if you lose the transmitter, you also don't get the limiters (and advantages of those limiters) that you see in a proper FM receiver.
And as pointed out, there are crystals in there, so it must be a superhet.
No, exactly the opposite. Maybe I mistypoed or you misread, but these are a bsolutely fixed frequency as far as I can tell. The frequency readout in th e front has all the segments connected.
I also got a bunch of UHF ones that sync to the receiver by IR. I told them to get me an IR card. Nothing happened so that stack, well there in nothin g happening to that either.
If I work for you and come up and say "We need this", it does not mean "Oh gee golly wiz it would really cool if we have this. It means EVERY DAY WE D O NOT HAVE THIS COSTS YOU MONEY. I do not play at work.
If they ever say "How come these units are not fixed" I am right there with "Because I asked for this and that and did not get it". And this has alrea dy happened at other places so I am starting to think I work for a bunch of hobbyists, I mean hobbyists in business. Probably got a ton of money and j ust want to get out of the house.
Personally I would rather work for a place that wants to make money. In fac t HAS to make money because that makes me more valuable.
If you mean the base carrier frequencies they range from about 174 to 210 M Hz on these models. I do not now the modulation but I know each frequency e nds in a 0.05. Every one of them, like 210.95, 174,15, they all end in a 0.
That leads me to believe they run maybe 50 KHz deviation ? Europe uses it for FM radio. The US runs 75 KHz. Of course there are sidebands but what are the odds ? And the ones I have seen, the two channels are displaced by about 10 MHz or more.
I am probably going to become much bigger in that company. We got other pro blems that are dropped in my lap. Three year old unit we can't get any part s for, and there are still sellers on Amazon and eBay selling them. WTF.
** Europe also uses 75kHz deviation, its the pre-emphasis that differs.
50uS in Europe and most other places, 75uS in the US, Canada and Japan.
** The channel spacing coincides with US, VHF TV stations.
In a given location, prior to digital broadcasting, every second channel was blank. So you had chs 7,9,11 & 13 on the air OR chs 8,10 & 12. This allowed the class licensed use of several radio mics in each of the black bands.
The deviation on such models was usually 30kHz with 75uS pre-emphasis and some form of audio companding.
The best models from Shure, Nady or Vega had exceptional audio quality.
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