Low Hum Coming From SONY ICF-C218 Clock Radio Speaker? Can It Simply Be Fixed?

[montecarlo1987]

Hello. Nice to meet you all here! I have a question to ask you and I am not sure what is causing the issue.

I have an older SONY Dream Machine clock radio Model ICF-C218.

I brought it out of storage after 5 years and now again, using it. However, I notice now coming directly from the single speaker on top of it, a low hum. I mean you have to put your head and ear almost over the speaker to hear the low “hum” (not really a “buzz”). I thought that to be strange, since the radio (both AM and FM frequencies is off), plus the alarm; and yet it is still emitting a low hum through the speaker. There should be no sounds at all being emitted from the clock radio speaker when the radio and alarm are totally off. The only way to eliminate the low hum is literally unplugging the clock radio’s power cord from the wall and shut it down (powered off). NOTE: When the radio is on and the volume is very low, I still can hear the low hum in the background; but when I turn the volume up the low hum is overshadowed or masked by the radio’s own sound like music playing for example. I have been to other rooms with clock radios in them (different brands), and none of the other ones emit a low hum when their radios are off but plugged into an outlet for power.

This SONY clock radio is out of warranty and I am not seeking any repair or replacement from SONY at this time, as obviously the cost of a factory repair technician is going too far exceed the cost of a new or used one ($10-$30); just need to know if this issue is a problem (malfunction) that could result in fire or physical harm in the future. Also, if it is something a novice like me can easily low-cost fix such as a simple part replacement and simple soldering the wired connections I can handle, please let me know or suggest what would need repairing or replacing.

Please reply.

Thank you!

 

[Harald Kapp]

This phenomenon is typical for old electrolytic capacitors that have lost most of their capacity. They no longer filter the AC ripple from mains in the power supply. The hum should be 120 Hz (2* mains frequency due to the rectifier).
Open the unit look for the electrolytics (usually easily identifiable as comparatively large cylindrical components. Find at least the ones in the power supply, get suitable replacements (same or higher capacity, same or higher voltage, size needs to fit the space available) and replace the old capacitors.
Note that eletrolytic capacitors are polarized, The case usually shows clearly where '-' is. Observe correct polarity when inserting the new capacitors (for reference make photos of the placement of the old capacitors before removing them).

Make sure no residual charge is on the mains side capacitors before opening the radio (wait a few minutes after disconnecting from mains power) to avoid an electrical shock.

 

[duke37]

A lot of low powered devices do not switch off the internal power supply when parts are turned off. This is likely to be in the case of a clock where you do not want to lose the time.
The hum may not be coming from the speaker but from the mains transformer. These always vibrate to some extent and some can be worse if they have lose laminations. Make sure that the transformer is bolted down securely or mount on rubber support.

Glue can be used to stop loose laminations vibrating and placing the clock on a surface which does not vibrate helps.

There is unlikely to be a safety problem.

 

[dorke]

Harald,
The hum can be 60Hz(USA ) as well if the rectifier is half-wave.
This isn't the case here since the rectifier is full-wave.

It will be 60Hz if the hum is coming from the transformer duke duke explained.

montecarlo,
can you identify the frequency of the hum in any way?
may be even audio record it and post here.


Here is a schematic of the SONY Model ICF-C218.
The filtering capacitors that may likely cause the problem are marked(C304-6).
The most effect on noise is by C304.
Look for buldges on the caps. (a photo may help).