The Radio Information Board

Hi, all. I have an old Bearcat "250" scanner, model# BC-250, that has a problem. As far as reception, everything seems OK, good sensitivity, on frequency, etc. All the buttons also work. My problem is that there is a loud "hum" in the received audio. This hum varies in intensity with the volume, and is only present with a received signal. I don't hear the hum with white noise produced by open squelch. It may be there, but I can't hear it over the white noise if it is. This is not a power supply filtering problem, as I can run it on battery power (it has a 12vdc input on the back), and the hum is still there, unchanged. The hum doesn't seem to be present on weaker signals, I only seem to hear it on a "full quieting" signal. This is a fairly old scanner, but new enough to be senthisized. If I am reading what looks like a date code on the back panel correctly, it was manufactured in '79. Is anyone familiar with a "problem part" in these units, or maybe have a schematic of it they could send me? It may be a shorted filter or dc blocking capacitor in the audio section, but it would be a real pain to try to trace the circuit without a schematic. Thanks for any help!

Does it happen on all frequencies? Does it sound like a CTCSS tone?

Brett

As per Scotts reply, it is going to CTCSS that is being used and causing the hum. Also known as tone, private line, channel gaurd, to name a few is a subaudible tone in the frequency range of 67Hz to 260MHz. It is sent during the full time of the transmit carrier.

The only thing you can do is bare with it, or install a tone board to use it as an audio filter, or purchase a newer scanner that has the filter.

Regards,

Bill

Well, I would agree with ya'll, but it didn't used to do this. Just started hearing the hum a few months ago. Also, I hear it on CSQ frequencies.

I would suggest using an oscilloscope on the RX audio output to check for noise on the audio on one of the CSQ channels. You should be able to see it on white noise as well. If you see any noise or distortion riding the signal you could have an open capacitor on the audio amplifier, or a bad amp. Also check the B+ to the audio amp.

If you see a good waveform:

Do you have a VHF or UHF portable transceiver to use a signal source?

Let me know if you need to go further and the results of your testing.

Bill

Wow! Somebody else has the same problem! 20 years ago I had a Bearcat, I believe it was a 210 that I had mounted in my car. It had that sloped front on it which was nice for a tabletop, but didn't work too good as a car mounted model. I took the LED display out and turned it upside down, pulled out all the buttons and turned them around as well.

When I got all done with the modifications, I mounted it on the roof of my old Trans-Am, right between the T-tops. It looked great! But for some unknown reason I started getting this darn hum in it when I picked up strong signals. I knew it wasn't PL because it was coming from the department that I worked at and I knew there was no PL.

I messed with that darn thing for a month putting filters on the power wire, doing everything I could think of to make it work right. I finally gave up and sent it back to Bearcat to repair. I figured they'd look at my mods and tell me it was unfixable, due to consumer input to re-engineer.

I got it back in the mail a couple weeks later, with (as memory serves me) about a $50 billing. They charged me for a new cap and labor to replace the cap. But what was really nice was that they actually replaced the ribbon cable to the LED display as well and left my mods in place!

So, my guess would be just that, a bad cap!

Kel

I'm far from knowledgable in electronics... BUT... is it next to your computer...? if so move it away and see if that helps... I found that sometimes the RFI produced by a computer/other elctronics is picked up on other elctronics... similar to how my computer speakers chirp just b4 my cell phone rings...lol Bizzaro...

While the computer is a very probable cause, it's not that. It does it anywhere in the room. Thanks for the suggestion, though!

Is it more like a low howling sound rather than a hum that chnges intensity with the audio setting ? I had almost the same problem years ago with a BC-220, mine was the VCO cover making poor contact. Remove the top cover and see if the screws holding the RF shield are coming loose.

No, it's a hum, app. 60Hz, that stays the same all the time. I will, however, check the VCO cover.

OK 60 hz hum, probably a power supply filter cap. If I remember correctly those receivers used Large radial leaded electrolytics.


Mike