Radio Shack Weather Cube

Does the buzzing follow the volume control? In other words, does turning the volume up and down change both the weather report AND the buzzing, or is the buzzing a constant level?

The problem might a new source of EMI (electromagnetic interference). Try moving the cube around the house and see if the buzzing gets stronger near some new electronic or motorized equipment. Also try moving the cub outside and down the road to see if it goes away with distance.

I once had a weather cube. I found that nearly dead 9V batteries would cause the audio state to "motorboat". It was easier to replace the battery than the fix the problem, so I never troubleshot it down to the failed component. I suspect you might have a similar problem if you're using a rechargeable LiIon 9V battery, some of which only produce 7.2VDC. If you have a new 9V alkaline battery, try it.

The buzzing follows the volume control, it's not constant.

Moving it around the house doesn't affect the buzzing. It stays at a constant level. And no new equipment came online in the house since it's been in use.

My battery was a Duracell alkaline at 8.59V. I replaced it with another Duracell at 9.19V. No change in the buzzing.

Really a strange problem. I'd suspect a bad connection somewhere, but the problem just appeared slowly as it was used on a shelf. There was no movement, or jarring involved that could have knocked something loose. And there is a lot of wax poured over the components - presumably to keep the little coils fixed in shape. I thought the electrolytics finally gave out, as it's a vintage one with faux wood grained sides. Maybe 70's - 80's era.

OK. So it's in the receiver section, before the volume control.

Is the model number 12-174 or 12-181b?

Any chance you have a schematic or a URL with a schematic? I couldn't find anything using Google Image search. Everything seems to point to radio-reference.com, which has some RS information, but nothing on the weather cube.

OK. That eliminates an EMI/RFI source. You're probably on the right track looking for something wrong with the cube.

So much for the almost dead battery theory.

The buzzing sound is something oscillating at an audio rate. Lots of things in the RF and low level audio section could do that. Most likely is a power supply bypass cap. I suggest poking around the PCB with an oscilloscope looking for oscillation, but that's difficult without a schematic.

Try poking around the PCB with your finger. If you find an area where you finger changes the noise, investigate that area for bad parts, residual flux or bad components. Maybe an alcohol and brush cleaning.

A dead electrolytic is a possibility. Instead of removing and testing the caps, find a similar value cap and temporarily connect it across each electrolytics on the PCB, one at a time. Soldering is best but just holding it on the solder pads is sufficient. I like to put a sharp point on the capacitor leads (with a diagonal wire cutter) in order to punch through the protective goo and wax on the PCB.

Below is a relatively long post concerning a lower probability cause of the problem than the cube itself. It was mentioned earlier, but dismissed when it should not have been. I'm posting this because there is a small possibility that there is a safety issue in your home.

You have NOT eliminated all possibilities of EMI/RFI Walk it down the road, as was mentioned. That is, take it somewhere to a place far away from any power lines, houses, buildings etc where electrical devices are. If it still buzzes then you know the problem is within the cube. Sources of EMI/RFI can be in a nearby neighbor's house, a power line fault such as a cracked insulator or even a loose connection in the wiring in your house. It doesn't have to be a new electrical device in your house, even though that is a most likely cause in many cases.

If the cube works properly when walked down the road, but starts buzzing as you near your house, use it as a detector to try to locate the source - perhaps a power line or a neighbor's house. If the source is something in your house, you can shut off breakers until you determine which circuit "hosts" the noise making device, then further isolate by restoring power to the circuit and unplugging/switching off one device at a time. Don't overlook devices that you cannot switch off like doorbell or thermostat transformers hard wired devices and so forth. If the problem is not found in any device you can unplug or switch off, it could be a loose connection in the wiring. That is serious, as a fire could result.

I hope diagnosis/fix turns out to be simple and does not involve safety at all. Good hunting! Ed

Thanks. The schematic is somewhat out of focus but I was able to make it more readable using the "Sharpen" feature in Irfanview.

I know both IC's quite well, having used them in various marine radio designs around 1975. The MC3357 has a large amount of gain and is therefore likes to oscillate with minimal provocation.

In order from most probable: Electrolytics: C28, C15.

Also, check the DC voltage across D4. It should be 5.1VDC

I suggest you bypass the capacitors with a replacement capacitor, one at a time, instead of removing and replacing. It's quicker and easier. If you need to remove the wax, a hair dryer or heat gun will work.

MC3357 datasheet:

I can think of some less likely failures and parts if cap replacements show no improvement. As I previously suggested, probe the PCB with your finger and look for an area that produces a change in the buzzing noise. Whatever failed is nearby.