Crystal Tuned TRF (I Think) FM Receiver

Trying to get a handle on these things. Next time you want to bitch about not 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 manufacturing but my bosses own most of the brand, of course made in China, it seems mostly by Apex.

Anyway these are wireless microphones and of course there are no prints. The 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 sell them locally so that people can just drop them off and pick up a different frequency set, which should cure some of the problems. This is less practical when you ship the thing to bumfuckt Egypt somewhere

Thing is, the chipset appears to be hetrodyne but might not be being used 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 by the IF frequency, like 10.7 MHz or something. They must be putting the signal 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.

But why bother ? There are like four tuned circuits on there per channel so that should give adequate selectivity.

Anyway, anyone know about these things ?

 

[Phil Allison]

jurb...@gmail.com wrote:

Trying to get a handle on these things. Next time you want to bitch about not 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 manufacturing but my bosses own most of the brand, of course made in China, it seems mostly by Apex.

Anyway these are wireless microphones and of course there are no prints. The 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 sell them locally so that people can just drop them off and pick up a different frequency set, which should cure some of the problems. This is less practical when you ship the thing to bumfuckt Egypt somewhere

Thing is, the chipset appears to be hetrodyne but might not be being used 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 by the IF frequency, like 10.7 MHz or something. They must be putting the signal 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.

But why bother ? There are like four tuned circuits on there per channel so that should give adequate selectivity.

Anyway, anyone know about these things ?

** I used to regularly see fixed frequency, VHF radio mics through the 1980s 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.

As a service tech, all you can do is check the mic is transmitting on the frequency 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 short 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 VHF 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 narrow band and their noise performance is much poorer. Practically all use audio 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 transmitter, 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 front ends - otherwise expect major problems with signal drop outs. A "drop out" is often not simply a loss of audio signal but a burst of white noise as the mic is moved around by even small distances.

Wireless mic receivers must be located close as possible to where the mic is being used and with its antennas well clear of obstructions, especially metalwork. Many owners do no appreciate this and do the opposite - the result is lots of drop outs.

Placing the receiver near a PC or any other source of VHF interference is also 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.


...... Phil

 

[Phil Allison]

Michael Black wrote:

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.

** I have never come across user replaceable crystals in a radio mic.



..... Phil

 

[Michael Black]

On Mon, 13 Jun 2016, jurb6006@gmail.com wrote:

Trying to get a handle on these things. Next time you want to bitch
about not 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 manufacturing but my bosses own most of the brand, of course made
in China, it seems mostly by Apex.

Anyway these are wireless microphones and of course there are no prints.
The 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 sell them locally so that people can just drop
them off and pick up a different frequency set, which should cure some
of the problems. This is less practical when you ship the thing to
bumfuckt Egypt somewhere

Thing is, the chipset appears to be hetrodyne but might not be being
used 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 by the IF frequency, like 10.7 MHz or something. They must be
putting the signal 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.

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.

Michael

But why bother ? There are like four tuned circuits on there per channel
so that should give adequate selectivity.

Anyway, anyone know about these things ?

 

[Jeff Liebermann]

On Mon, 13 Jun 2016 17:42:53 -0700 (PDT), jurb6006@gmail.com wrote:

Thing is, the chipset appears to be hetrodyne but might not be being
used 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 by the IF frequency, like 10.7 MHz or something.
They must be putting the signal 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.

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

 

"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."

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 to speak. Basic LED display and it says the frequency but what it says is marked on the side with a sharpie.

When you look at the back of the board you can see that the terminals are all 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 sticker 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 lower. 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 flathead 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 the money into this you now there is a reason.

 

"
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. "

https://www.youtube.com/watch?v=0evW6v69yF0

I was used to seeing a crystal of 3,579,545 Hertz and that was the damn resonant frequency. In TVs, when it said 4.000 MHz it was.

This shit should be illegal. Where is the FTC ?

 

>"** I used to regularly see fixed frequency, VHF radio mics through >the 1980s 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 never a proponent of digital TV, I was of abolishing VHF. The system here was AM 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 crumbling infrastructure right ?

And now, about channel 12 TV (even though we do not have that here at the moment) now the TV is digital and the sidebands are like, who knows. Seriously.

They should have abolished VHF TV decades ago and gave more high fidelity radio stations. Or something. In fact, that VHF range is about where RC planes 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 ?

 

[Jeff Liebermann]

On Tue, 14 Jun 2016 02:26:57 -0700 (PDT), jurb6006@gmail.com wrote:

I was used to seeing a crystal of 3,579,545 Hertz and that
was the damn resonant frequency. In TVs, when it said
4.000 MHz it was.

3.579545MHz is the color burst frequency. You'll see such crystals in
TV sets, not wireless mics.

>This shit should be illegal. Where is the FTC ?

I think you mean the FCC. Congress cut their budget some years ago,
so the FCC simply reduced the enforcement burro.
<http://www.arrl.org/news/fcc-announces-enforcement-bureau-field-office-reorganization-plans>
Field offices deal primarily with enforcement issues.

FCC enforcement is now generally controlled by Homeland Security,
<https://www.fcc.gov/public-safety-and-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.
<http://transition.fcc.gov/eb/>

--
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

 

[Phil Allison]

jurb...@gmail.com wrote:

OH ! I have another reason to believe these are TRFs,
they are two channel, dual mics.

** So they are cheap and nasty piles of Chinese junk that sell for like $40 ?

No way are they TRF cos that is impossible.

The side with the lower frequency, some are down like 174 MHz,

** A 34.825MHz crystal can produce both 208.95 & 174.125 if multiplied by 6 and 5.

To multiply by 6 takes two steps, first by 3 and then by 2.


..... Phil

 

>"** So they are cheap and nasty piles of Chinese junk that sell for >like $40 ? "

No, they go for quite a bit more than that, but I bet you are probably close to the actual cost of them. And damnear everything is Chinese junk. Any company publicly traded in the US is pretty much governed by Dodge v Ford.

>"No way are they TRF cos that is impossible. "

Why ? I tellya I seen the cores lower in the adjustable coil/transformers lower in the channels with the lower frequency. Are you saying that is all front end and they still downconvert ?

Maybe they do, leaving little for the IF to do in the way of selectivity and letting the previous stages do it. When you have like an audio type FM tuner, 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 front panel are burnt in.

Yup, cheap Chinese shit made by Apex. But what ain't ? Everything is junk these days.

>"** A 34.825MHz crystal can produce both 208.95 & 174.125 if >multiplied by 6 and 5. "

This chipset has no such function as far as I can tell. These are very simple 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 frequency, 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 not a bad approach. Saves money and there is absolutely no radiation to worry about. So the FCC or whatever you have as its equivalent will not give you 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. And 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 hobbyists, rich kids who want to feel useful. Of course folks like that can be useful to me.

 

[Phil Allison]

jurb...@gmail.com wrote:

"** So they are cheap and nasty piles of Chinese junk that sell for
like $40 ? "

No, they go for quite a bit more than that, but I bet you are probably close to the actual cost of them. And damnear everything is Chinese junk. Any company publicly traded in the US is pretty much governed by Dodge v Ford.

"No way are they TRF cos that is impossible. "

Why ? I tellya I seen the cores lower in the adjustable coil/
transformers lower in the channels with the lower frequency.
Are you saying that is all front end and they still downconvert ?

** A "TRF" receiver has no local oscillator, mixer or IF amplifier - which your examples clearly do.

Yup, cheap Chinese shit made by Apex. But what ain't ?
Everything is junk these days.

** 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.

"** A 34.825MHz crystal can produce both 208.95 & 174.125 if multiplied by 6 > > and 5. "

This chipset has no such function as far as I can tell.

** 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.

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
frequency, I am going to pay special attention to the last one, which
would be a coil for the quadrature detector.

** The quadrature coil should be next to the IF amplifier, the LA1140 chip.

http://obrazki.elektroda.pl/2812242800_1353270059.jpg

If those are buried deeper that indicates TRF. For fixed frequency TRF
is not a bad approach. Saves money and there is absolutely no radiation
to worry about. So the FCC or whatever you have as its equivalent will
not give you any trouble.

** 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.


..... Phil

 

>"** 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."

I might find out tomorrow or Friday. For what they pay I work when I damnwell please. I make three times as much at home actually. Actually, it is time for me to get back into that 465B and find a suitable replacement for Q4565. Thing is most of this stuff is stock which means it is really hard to put any pressure on them.

Anyway, so what you're saying is that the crystals are labelled for the assemblers with a number that is offset from the IF frequency, and since this is a standard chip with the quadrature detector that would mean it most likely operates at 10.7 MHz. Do I got that right ? If so, the frequency of the crystal should be off by 10.7 MHz then.

This is kinda just a matter of interest, the only use of it I might get is if I need to change frequencies. If I oculd just change the crystal that would be great but I doubt it. That 1186 or whatever is for the front end. That means that if I decide to change the frequency of one of the channels I have to align it. Ugh. I think it is time to find out what these things sell for. I'm sure it is not forty bucks but if it isn't at least close to $200 I have to think about how much time to put into them.

"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 think, at least with the VHF ones, more other interference fucks them up than the other way around. Security systems with wireless cameras and the like. We had one that knocked out all of the cordless phones in the house, had to get rid of it. That band around 200 MHz is not the place to be anymore.

Of course now they sell UHF wireless mics. That is my next stack to get into actually. But I am thinking of telling them just to sell the VHF ones locally so people can bring them back ad they can pick one up running on a different frequency. Of course that is gong to require a note or whatever on the box denoting that frequency.

Other than that they can throw them in the fucking dumpster. Can't be shipping these things to bumfukt Egypt for a trial to see if they work.

 

[Michael Black]

On Wed, 15 Jun 2016, jurb6006@gmail.com wrote:

"** So they are cheap and nasty piles of Chinese junk that sell for
like $40 ? "

No, they go for quite a bit more than that, but I bet you are probably
close to the actual cost of them. And damnear everything is Chinese
junk. Any company publicly traded in the US is pretty much governed by
Dodge v Ford.

"No way are they TRF cos that is impossible. "

Why ? I tellya I seen the cores lower in the adjustable
coil/transformers lower in the channels with the lower frequency. Are
you saying that is all front end and they still downconvert ?

How wide is the deviation?

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.

Michael

 

"I thought you said these could be changed to
another frequency,..."

No, exactly the opposite. Maybe I mistypoed or you misread, but these are absolutely fixed frequency as far as I can tell. The frequency readout in the 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 nothing 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 DO 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 already 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 just want to get out of the house.

Personally I would rather work for a place that wants to make money. In fact HAS to make money because that makes me more valuable.

 

>"How wide is the deviation? "

If you mean the base carrier frequencies they range from about 174 to 210 MHz on these models. I do not now the modulation but I know each frequency ends in a 0.05. Every one of them, like 210.95, 174,15, they all end in a 0.05. 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 problems that are dropped in my lap. Three year old unit we can't get any parts for, and there are still sellers on Amazon and eBay selling them. WTF.

 

[Phil Allison]

jurb...@gmail.com wrote:

"How wide is the deviation? "

If you mean the base carrier frequencies they range from about 174
to 210 MHz on these models. I do not now the modulation but I know
each frequency ends in a 0.05. Every one of them, like 210.95,
174,15, they all end in a 0.05. That leads me to believe they run maybe
50KHz deviation ? Europe uses it for FM radio. The US runs 75 KHz.

** 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.

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.

** 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.



..... Phil

 

[isw]

In article <4aa2c184-028c-4103-ab9b-1b2d88b411c9@googlegroups.com>,
jurb6006@gmail.com wrote:

"I thought you said these could be changed to
another frequency,..."

No, exactly the opposite. Maybe I mistypoed or you misread, but these are
absolutely fixed frequency as far as I can tell. The frequency readout in the
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 nothing
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 DO
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 already
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 just
want to get out of the house.

Personally I would rather work for a place that wants to make money. In fact
HAS to make money because that makes me more valuable.

Why don't you just write up the purchase order and take it to be
approved, instead of carping about it?

Isaac