Wanted: Radio Shack Weatheradio tech manual
 

I have two Radio Shack 12-250 weather radios here. Both are functional
except that they don't trip on the weekly SAME test. I conclude that
the SAME decoder is not working in either, and needs to be realigned.

Radio Shack reports that the tech manual for these is no longer
available.

If anyone has a tech manual for a 12-250 or the substantially similar
12-251, or knows the SAME circuitry alignment procedure, I would like
to arrange to obtain a copy.

Dr. Klahn wrote:
I have two Radio Shack 12-250 weather radios here. Both are functional
except that they don't trip on the weekly SAME test. I conclude that
the SAME decoder is not working in either, and needs to be realigned.

Have you tried reprogramming the SAME codes in them, or maybe
programming them for an adjacent county served by your NWS transmitter?

I was under the impression that the SAME data is transmitted with a
standard-ish modulation scheme, like Bell 103 or something. It seems
like they'd use a standard chip for this that would run off a crystal
and not have anything to "align", other than the crystal frequency.
Does the normal audio sound OK?

The Rat Shock parts list http://support.radioshack.com/parts//51031.htm
mentions a 10.240 MHz crystal (probably part of the tuner) and what
is probably a 4.0 MHz ceramic resonator, which probably drives whatever
microprocessor it has. If this resonator is off, I could see it working
well enough to respond to keypresses and drive the display, but not well
enough to understand the data stream.

Matt Roberds

wrote:
Have you tried reprogramming the SAME codes in them, or maybe
programming them for an adjacent county served by your NWS
transmitter?

Yep. It doesn't trip on any of those tests or alerts.

I was under the impression that the SAME data is transmitted with a
standard-ish modulation scheme, like Bell 103 or something. It seems
like they'd use a standard chip for this that would run off a crystal
remainder clipped ...

I would tend to agree with that, except that this is an old design.
The
parts list does list a couple of trimpots, which presumably adjust one
or both frequencies of an AFSK data stream. Without the schematic,
it is difficult to know what's going on in there.

I have obtained the tech manual from Radio Shack. There
are several alignments required. In case anyone runs
into the same problem, here is the alignment procedure.

An FM signal generator, SINAD meter, voltmeter, THD /
distortion meter, and oscilloscope are required.

For steps 1 to 3, a 39 nH inductor is required at the L1
position on the receiver board.

1. With channel 4 active: measure the DC voltage on TP4
and align L8 to 2.5 V.

2. With channel 4 active: Set FM RF signal generator
to 100 uV and 162.475 MHz signal: Adjust L2 for max
output level and minimum THD.

3: With channel 4 active: Set FM RF signal generator
to 5 uV and 162.475 MHz signal: Adjust L1, L3, L4
and L6, L8 in sequence for maximum SINAD output.

4: With channel 4 active: Set FM RF signal generator
to 0.6 uV and 162.475 MHz signal: Adjust L1, L3
L4 and L6, L8 for maximum SINAD output (better than
15 dB SINAD).

5: With channel 4 active: Set FM RF signal generator
to 0.4 uV and 162.475 MHz signal: Slightly adjust
L2 for the best SINAD output on.

6: Repeat steps 3 to 5 to obtain the best SINAD output
(better than 15 dB SINAD).

7: With channel 4 active: Set FM RF signal generator
to 100 uV and 162.475 MHz signal with 1.4 KHz
audio modulation: Adjust VR3 to the point where
the voltage on TP3 to ground changes from 0 V to 5V.

8: With channel 4 active: Set FM RF signal generator
to 100 uV and 162.475 MHz signal with 2.3 KHz
audio modulation: Slightly adjust VR2, if necessary,
to the point where the voltage on TP3 to ground
changes from 0 V to 5 V.