Radio Shack Weather Cube

[John Crane]

Bought one in excellent shape.
Worked great for about a month, then developed a buzzing background
noise in the audio - but you can still hear the weather reports. Just
annoying.

It's been sitting on a shelf the whole time. And 9V battery powered.

I tried replacing all the electrolytic caps, but no go. In fact, the
buzz is slightly louder.

Any ideas as to what could cause this?

[Jeff Liebermann]

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.

--
Jeff Liebermann je...@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

[John Crane]

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.

[Jeff Liebermann]

On Fri, 21 Oct 2022 23:01:20 -0500, John Crane
<john_c...@yahoo.com> wrote:

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

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.

>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.

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

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

So much for the almost dead battery theory.

>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.

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.

[ehsjr]

On 10/22/2022 11:57 PM, John Crane wrote:

> It's a 12-181b.
>
> As luck would have it, it came with a little paper schematic and user's
> guide tucked in the battery compartment....attaching a jpg of it.
>
>
>

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

[Jeff Liebermann]

On Sat, 22 Oct 2022 22:57:45 -0500, John Crane
<john_c...@yahoo.com> wrote:

>It's a 12-181b.
>
>As luck would have it, it came with a little paper schematic and user's
>guide tucked in the battery compartment....attaching a jpg of it.

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.
0.01uF ceramics: C22, C30, C10.
My best guess(tm) is 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:
<https://www.digchip.com/datasheets/download_datasheet.php?id=643405&part-number=MC3357>

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.