WeatherRadio Repair

[montef]

I have a Radio Shack "Realistic" N.O.A.A. weather radio. Model 12-240.
I would like to repair it, instead of tossing it in the bin and buying a new one.

Yes, I can buy an exact replacement for $15.00 on eBay.  But, repair is better than replacement. :-)

It has a 9V battery in it, which leaked and corroded the pins on the crystal.
As a result, the pins came off of the crystal, exactly flush with the bottom of the crystal, leaving nothing to be able to solder to.

The local (Atlanta, Georgia, U.S.A.) Weather Radio transmitter is on 162.550Mhz.

The crystal in the receiver has these markings on it.
16.202
UNI
93-G

I presume that 16.202 is the frequency of the crystal in Mhz. So 16.202Mhz 
The rest of the markings, I don't recognize. "UNI" is perhaps a manufacturer?
My Google Fu is not strong enough to have turned up any other information as of yet.

Searching on sites like Digikey, Mouser, Jameco and Newark; I did not find an exact "16.202Mhz" crystal.  But I found several that were close.
My theory was that if I got close enough, I would be able to at least hear *something*.   The transmitter site for the NWS station KEC80, is on top of Stone Mountain; which is less than 4 miles from me.
It has an ERP of 500 watts, so I'm not in a fringe area, trying to pull a signal way out of the noise.

But, none of the new crystals I ordered (about a dozen or so....they were cheap, and I was grasping at straws, hoping I'd get lucky) pick up anything.
One of the new crystals I ordered was a "16.150 Mhz" one.  That's a difference of .052 Hz  (.202 - .150 = .052). 
I'd have thought I could have at least detected a signal, even if it was a tad off frequency...... nada...


I soldered a couple of wires into where the crystal should be.  If I touch the nubs of where the leads on the original crystal should be, I can pick up the signal.
So that tells me that there is nothing wrong with the rest of the receiver.

Anyone have any suggestions, other than "just toss it in the garbage and buy a new one"?

Thanks,

  -Monte

[fzabkar]

This weather station uses a 162.020 MHz crystal:

https://fccid.io/EMOT248
https://fccid.io/EMOT248/Operational-Description/Technical-Description-577604.pdf

Maybe your unit has a PLL which multiplies the frequency by 10

[TheMG]

Assuming a typical superheterodyne receiver, the crystal frequency is going to be multiplied several times to obtain the LO (local oscillator) frequency. Consequently, the difference/error in crystal frequency is also multiplied!

[fzabkar]

https://fccid.io/EMOT248/Schematics/Circuit-Diagram-577611.pdf
https://fccid.io/EMOT248/Block-Diagram/Block-Diagram-577610.pdf

I just checked the circuit diagram. It shows a 16.2020 MHz crystal. But T1 and T7 are tuned to 162.5 MHz and 162.02 MHz. I'm not an RF guy, so someone else would be best to follow this up, assuming it's pertinent.

[xrunner]

There are five capacitors (three variable) and three inductors (variable) which are switched into the crystal oscillator circuit. These change the xtal freq a little to mix with each WX channel freq. to produce a 455 kHz IF. I think the xtal freq can be a bit different than the 16.202 xtal and you might be able to re-tune the variable components to get those channels, and replace the other caps C21 & C22 to get the other two channels.

Or just buy a cheap new radio. 

[CaptDon]

You're not off by .052hz!!!! You're off by 52Khz at the fundamental which in itself is at least 5 narrow band channels off frequency. Consider that crystal is multiplied 9 or 10 times and you are 50 narrowband channels off frequency. As Rocky the squirrel says "That trick never works!!!"
 

[montef]

Oh!
Great information.  I didn't find that in my searching.

Thanks very much for those links!

[montef]

Apparently my math is wrong. :-(
Thanks for the explanation!

[montef]

ok. So I am reading the frequency on the crystal correctly as printed then.

It's a 16.202 Mhz crystal, and not a 162.02 Mhz crystal.
Correct?

Anyone care to speculate about that frequency.... Did Tandy/Radio Shack/Whomever, have these crystals custom made for this product?  I ask, because I can't seem to find a direct drop-in replacement for it anywhere.
In this day and age, does anyone make custom crystals in a "one-off" quantity like I would need to repair this radio?
Is there a place other than Digikey, Mouser, Newark, Amazon; where I could get a crystal tuned to the 16.202 Mhz that this circuit seems to need?

I saw the tunable T1 and T7, but haven't made any attempt at adjustments there; not knowing how much range there was to play with.

At a guess, how far off of the 16.202 Mhz crystal would you think I could go, and have enough tuning adjustment on those chokes to be able to get back on target?

[fzabkar]

The circuit diagram which I found is for a different weather radio. Do not make any adjustments based on that circuit.

The Radioshack radio doesn't have any useful FCC filings:

https://fccid.io/AAO12-240

Thirty years ago I had custom crystals cut by Hy-Q at $25 each. I don't know where to source these today.

Here is another weather radio with the same crystal:

https://fccid.io/GJM9912WA
https://fccid.io/GJM9912WA/Schematics/Schematics-96167.pdf

"Quartz crystal, 49U type, 16.202 MHz, +/- 30ppm"

[SunnyDay]

ok. So I am reading the frequency on the crystal correctly as printed then.

It's a 16.202 Mhz crystal, and not a 162.02 Mhz crystal.
Correct?

Anyone care to speculate about that frequency.... Did Tandy/Radio Shack/Whomever, have these crystals custom made for this product?  I ask, because I can't seem to find a direct drop-in replacement for it anywhere.
In this day and age, does anyone make custom crystals in a "one-off" quantity like I would need to repair this radio?
Is there a place other than Digikey, Mouser, Newark, Amazon; where I could get a crystal tuned to the 16.202 Mhz that this circuit seems to need?

I saw the tunable T1 and T7, but haven't made any attempt at adjustments there; not knowing how much range there was to play with.

At a guess, how far off of the 16.202 Mhz crystal would you think I could go, and have enough tuning adjustment on those chokes to be able to get back on target?

Yes, they are custom made for weather radios, and wouldn't be sold as an off the shelf part.

The fundamental frequency is multiplied 10x by a transistor multiplier stage, to 162.020 MHz, creating the LO for the mixer, when set to receive the 162.475 channel, which in the mixer produces a 455 KHz IF (162.475-162.020 MHz = 455 KHz) for the demodulator. To receive 162.400 or 162.550 MHz channels, the crystal is shifted in frequency by 7.5 KHz down or up, by switching a capacitor or inductor into the circuit, across the crystal.

[fzabkar]

If I understand these circuits correctly, the crystal is made to oscillate at its 10th harmonic frequency. Is that how it works?

[CaptDon]

I doubt it oscillates on the tenth. Usually at best they run on a third overtone. Wonder if yours somehow runs on the fifth overtone and then run through a doubler stage to form the local oscillator injection. Don't try retuning the thing. That is a drawn out process to do it right. It may hear a 500 watt station a mile away but it will never hear anything less if you really botch up the alignment. There are tons of weatherband crystals around for the older scanners, Bearcats, Unidens, and many other brands. Sadly your radio was a cheap design stretching one crystal to cover more than one channel. It works but would be considered a low budget hack by most radio guys.

[fzabkar]

I still think that the 10th overtone is the only one that makes sense.

[floobydust]

It is puzzling. Overtones use odd harmonics 3,5,7,9
If this is RX 162.475MHz with 455kHz IF, LO=162.02MHz I would try 9th overtone with 18.00222MHz crystal?
NOAA Weather Radio is 162.40 to 162.55 MHz.

[BrokenYugo]

Flush with the crystal? You might be able to tack solder some fine wire on and then bury it in a blob of epoxy for stability. The bit of extra lead length may throw off the frequency a bit but I think that's what VC4 is for (get it on a station and peak it up, don't touch the other adjustments). You've got nothing to lose...

[SunnyDay]

It is puzzling. Overtones use odd harmonics 3,5,7,9
If this is RX 162.475MHz with 455kHz IF, LO=162.02MHz I would try 9th overtone with 18.00222MHz crystal?
NOAA Weather Radio is 162.40 to 162.55 MHz.

An overtone crystal is made to oscillate on a harmonic of the fundamental frequency it is cut for, but that's not what they are doing here, I don't think. I think this crystal is oscillating on its fundamental cut frequency of 16.202 MHz, and is followed by a single transistor 10x multiplier, which is just an amplifier designed to have a high amount of harmonic distortion, coupled with a high-Q tank circuit on its output tuned to the tenth harmonic on 162.020 MHz, so that it selectively amplifies that frequency, and passes it to the mixer.

That's what they are doing in this schematic of a similar Realistic Weather Radio, shown on Page 24, where Q3 is the fundamental crystal oscillator, and Q4 is the multiplier: https://www.arimi.it/wp-content/73/01_January_1983.pdf