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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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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. |
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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 |
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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. |
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