Uniden Madison - channel selector noise

[jrd426]

Hello guys, i have a uniden madison that when changing channels makes a clunking noise heard from speaker (internal or external). If i am not dreaming it, i think my cobra 2k does the same. Its not a huge deal but wondering why. Is it poor audio ground somewhere? Not sure if this is common or not.
Thx. 73's

 

[walterjn]

Hello guys, i have a uniden madison that when changing channels makes a clunking noise heard from speaker (internal or external). If i am not dreaming it, i think my cobra 2k does the same. Its not a huge deal but wondering why. Is it poor audio ground somewhere? Not sure if this is common or not.
Thx. 73's

I think most of the older Uniden/ Cobra cb radios will do that. @loosecannon or Xit13, plus a few others should be able to answer that for you.
Loosecannon did a few mods on my Grant XL and removed that noise when I change channels.
They should be along soon to answer your question for you.

 

[jrd426]

Thanks walter. Come to think of it my uniden grant xl does it as well. 73s

 

[jrd426]

Without the stock mic plugged in all quiet spinning the dial. Plug the mic in to receive audio and the annoying thud is back changing channels. Wondering if its some ground loop problem?? Thx for any info . Enjoy the weekend

 

[nomadradio]

It's the channel selector itself. The FCC figured out that by making the channel knob "hesitate" between detent clicks you could get binary numbers out of the selector that gave you extra (not legal) channels. After around 1979 only a channel selector that locks out the PLL when it's "between" clicks is legal when certain PLL types are used in the radio. A PLL that locks out any binary input that's not a legal channel won't need to use this kind of selector. The MB8719 and MB8734 PLL chips don't have a lockout built into them, and the FCC required this type of channel selector to use with those two chips.

The channel selector switch has a contact point inside it that closes when the selector is between clicks. Naturally it closes just briefly every time you turn the knob. It is connected to the "out of lock" line that disables the radio any time the PLL loses control of the channel frequency. This prevents an out-of-lock PLL chip from transmitting outside the legal 40 channels. Every PLL-controlled CB has this function in it. The between-channels lockout is connected to it. Pretty sure you can cut a foil trace to defeat this, but I don't have a procedure in the can for doing it.

73

 

[loosecannon]

Hi all,

in some 8719 dual conversion radios you can remove C232 in order to get rid of the muting between channels.
in others (like the madison/grant xl) you would remove TR47.

I probably have the numbers for the single conversion chassis written down somewhere...
LC

 

[nomadradio]

There is a transistor we learned to remove from 1990s-era Uniden Washington radios. If you turned up the AM audio it would cause a "boop!" sound in the transmit audio when you keyed the mike. The transistor in question was fed from the out-of-lock line and served to mute the receiver audio. Wasn't on the original Sams diagram, but somewhere I have a pic of the offending part.

Somewhere.

73

 

[Dr. Frankenstein]

I remember reading it on one of the posts here a while ago.

On the Cobra 2000GTL and Madison 8719 schematic it was TR47 (PNP 2SA733P), causing the problem, just lift one end of R225 (100K). It happens when the channel selector is in between making (channel) contact causing the Out-Of-Lock (ground signal) from pin-6 of the PLL through diode D31.

 

[9C1Driver]

My Madison has always made this sound, it's never really been a problem.

 

[jrd426]

I guess you get used to it after a bit. At least i am anyways with my Madison and grant. Been looking for a while for a correct speaker for the madison but not much luck so far. Thx guys for the info and replies.