Tx Rx fault finding HELP

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I need help with fault finding a car alarm key fob transmitter and its
receiver.
If this is not the right group please point me in the right direction.

I have adapted an old no name car alarm Tx Rx to tell my computer to open my
front gate and or the garage door.

For some time the operating range has been decreasing and my fix was to
adjust the tuning capacitor in the Tx.  Not the way to go I know.

At short range I can see the code being received on a CRO and the voltages
around both ccts seem ok using a DMM.

Question 1. How do I determine which cct is at fault ?

Question 2. Is there a URL for cross-referencing transistors ?

I guess I'm asking if anyone has a crystal ball but any help would be
appreciated.

Gordon


Re: Tx Rx fault finding HELP


Replace the battery in the transmitter.

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my



Re: Tx Rx fault finding HELP


first thing I did

gordon


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open
voltages


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I went and bought a copy of the transistor substitution guides.  Sometimes
paper is better.....

Mike



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at $43 for a one time use maybe

gordon


...
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Re: Tx Rx fault finding HELP


: Question 1. How do I determine which cct is at fault ?

REPLACE THE TX
REPLACE THE RX

then you will know if the problem is tx or rx related

no spare? then move the rx away from noise sources - ie the computer

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no replacements.
and I have the Rx off the computer and on a power supply and the CRO shows a
nice clean code at the output of the Rx but only at short range (2 to 3 ft)
I'm sorry I don't understand what noise might have to do with it, I thought
my problem was decreasing sensitivity, i.e. transistors reaching end of
life, hence my request for a cross-reference URL.
I'd be happy to change all the transistors in both the TX and Rx and get
this thing back on the road but I'd really like to have a fault finding
procedure for now and the future and a cross-reference of parts.

Gordon

PS: my wife says "another day without HER beeper is divorce material"




Re: Tx Rx fault finding HELP



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a
ft)
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thought
Gordon, unless I've missed something here changing the components is
unlikely to help. These components are more likely to fail outright rather
than decrease in performance IMHO.
I'm unfamiliar with the devices but your twiddling of the tx cap suggests
some drifing in frequency in the rx or tx. Is the system radio, ultrasonic
or whatever ?
If one of the latter two, voltage or contamination of the sensors would be
my first port of call. If its radio, then alignment would be the go.

Cheers
Jim




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shows

Thanks Jim, it's radio and the transistor in the Tx has an ft of 1GHz so I
imagine the operating freq is around 300-500MHz or so.  It has a coder chip
feeding the osc/output transistor and the appropriate coil, caps and
resistors.  The Rx is similar but has more transistors and an opamp for gain
to the decoder chip.  Really simple I guess but with only an old CRO and a
DMM and no knowledge I'm stuck with tweaking so goodness knows where the
alignment is now if both the Tx and Rx have wandered off freq.  If you or
anyone has an idea how I can get this back on track I'd be most thankful.

Gordon


Re: Tx Rx fault finding HELP



"Gordon W"
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**  The main issue is you have no idea what the Tx frequency is -  can you
obtain any help from the makers ??

Very simple command transmitters are not generally crystal locked but simply
adjusted with a ferrite slug in the tank circuit coil. If it were my
problem,  I would try using my radio scanner ( AR 1000 XLT), set it to 303.9
MHz and "wide FM" and place it a few metres away  -  then tune the Tx coil
for maximum audio level on the scanner. Next switch the scanner to AM and
check the signal that way too.  If all goes OK , the Rx is then tuned to
give best range.

Frequencies in the range from 273 MHz  to 328 MHz  might also be tried if
304 MHz disappoints.





.............    Phil



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simply
303.9

Thanks Phil, I'm trying to do that but with the Rx as I haven't got a
scanner.
For a long time now I've been able to tune the Tx to get back the range but
it didn't work this time.  Therefore I thought some component must have been
moving outside of its specs and should be replaced.  I haven't touched the
Rx yet so maybe that's where I should head next ie use the Tx as the
standard.

Anyway, thank you and all the other people for your help.

Gordon


Re: Tx Rx fault finding HELP



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3
of
get
finding
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suggests
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gain
Gordon, no idea sorry. I use a comms test set .




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