148 shorts out with antenna ground

[Aztec]

Hey guys,

I got a weird problem with one of my Cobra 148s. Whenever I hook up the antenna ground the channel indicator goes off as if something is shorting. I'm gonna check it out when I get home but in the meanwhile I'd like to hear your ideas as to what may be wrong.

 

[midnight special]

one thing i have seen doing this is the ground wire to radio is disconnected and the positive wire is hot,,, and the radio is getting ground thru antenna,,, but most times it would blow protection diode,,,,,

 

[nomadradio]

The chassis on that kind of radio is not connected to either the red or the black wire of the power cord. It's meant to be a "floating" chassis that uses the chassis ground for RF only, and not for the DC power.

This became a standard way to design a mobile radio back in the bad old days when Mack, White and other brands of Diesel over-the-road tractors that had the positive side of the battery grounded to the vehicle chassis.

The days of "positive ground" trucks is long gone, along with the brass radiators those trucks used. Plastic radiators that have no electrical connection to the chassis have made this practice obsolete. All OTR diesel tractors made for decades have used negative ground, just like your daily driver.

But the radios are still built this way.

Your problem is being caused by some "hot" point in the radio's circuitry having continuity to the chassis.

Most-likely places for this to happen are the TO-220 parts that are bolted to the side rails with a mica or white-ceramic washer under it, and a shoulder washer under the mount screw.

If one of those shoulder washers gets 'canted', and the rim of its shoulder doesn't seat properly in the metal transistor tab the shaft of the metal screw can touch the inside rim of the hole in the metal tab.

Driver, final, modulator or voltage regulator transistors are susceptible to this small installation error.

You can zero in on where to look by leaving the antenna disconnected and grounding your DC Volt meter to the black wire. Now touch the meter's positive probe to the chassis. You'll read a DC voltage. This can suggest which component has the short from transistor tab to ground.

Of course, it could be some other fault that makes the radio's metal chassis 'hot" when it should not be. A stray solder glob, or an attempt to install some accessory by someone who tries to use the chassis as a ground connection, where he should have used a circuit ground instead are all possible.

If your 148 is new enough to have a 3-legged PNP power transistor as the 8-Volt regulator, rather than the MB3756 used in the older radios, this part would be the first place to look. It's generally bolted to the side rail halfway between front and rear on the same side as the audio chip.

73

 

[543_Dallas]

The chassis on that kind of radio is not connected to either the red or the black wire of the power cord. It's meant to be a "floating" chassis that uses the chassis ground for RF only, and not for the DC power.

This became a standard way to design a mobile radio back in the bad old days when Mack, White and other brands of Diesel over-the-road tractors that had the positive side of the battery grounded to the vehicle chassis.

The days of "positive ground" trucks is long gone, along with the brass radiators those trucks used. Plastic radiators that have no electrical connection to the chassis have made this practice obsolete. All OTR diesel tractors made for decades have used negative ground, just like your daily driver.

But the radios are still built this way.

Your problem is being caused by some "hot" point in the radio's circuitry having continuity to the chassis.

Most-likely places for this to happen are the TO-220 parts that are bolted to the side rails with a mica or white-ceramic washer under it, and a shoulder washer under the mount screw.

If one of those shoulder washers gets 'canted', and the rim of its shoulder doesn't seat properly in the metal transistor tab the shaft of the metal screw can touch the inside rim of the hole in the metal tab.

Driver, final, modulator or voltage regulator transistors are susceptible to this small installation error.

You can zero in on where to look by leaving the antenna disconnected and grounding your DC Volt meter to the black wire. Now touch the meter's positive probe to the chassis. You'll read a DC voltage. This can suggest which component has the short from transistor tab to ground.

Of course, it could be some other fault that makes the radio's metal chassis 'hot" when it should not be. A stray solder glob, or an attempt to install some accessory by someone who tries to use the chassis as a ground connection, where he should have used a circuit ground instead are all possible.

If your 148 is new enough to have a 3-legged PNP power transistor as the 8-Volt regulator, rather than the MB3756 used in the older radios, this part would be the first place to look. It's generally bolted to the side rail halfway between front and rear on the same side as the audio chip.

73

Just a note that doesn't apply here. Newer cobra rigs and some exports I've ran across do not have the chassis DC isolated from the board anymore. Not sure when it started but I have a 2013 cobra 29 made this way.

 

[Robb]

I'll bet that it a transistor shorting on the 13.8v rail . . .

 

[nomadradio]

That's a problem with a "Cobra 148".

Like "Ford F-100".

The '65 is different from the '75, is different from the '85, is different from the '95.

The description "Cobra 148" only narrows you down to about a dozen radios made by 9 or 10 different vendors since 1979.

And yeah, newer radios don't bother with the isolated-chassis business. I would guess that the cost of having a new model "Certificated" by a lab for the FCC's approval has made it cheaper to keep old designs in production.

They could have ditched that 'feature' a long time ago.

73