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Michael Black[_2_]

On Mon, 13 Jun 2016, 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 ****. 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
bum****t 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 ?