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COBRA 2000GTL questions
- Thread starter Crambone
- Start date
The technical basis of this "ghostly glow" problem with the channel digits is a bit obscure but it's built into the design of this particular base station. Doesn't share this quirk with any other radio I have ever seen.
It starts with the alarm-clock function. By 1978 Uniden had decided that they don't like relays. All their radios designed after that used solid-state devices to do all the switching functions in their products, including CB radios. But how to make the computerized clock/frequency counter turn the radio on when the alarm activates?
The answer is a component called a SCR, or silicon controlled rectifier. A brief pulse of current from the digital-clock circuit turns it on, and it will stay switched on until the power button shorts across the SCR when you push it in. This resets the SCR for the next time you use the alarm to power up the radio. Just one problem. It's far easier to switch the ground side of a circuit with the SCR than the hot side. As a result, the Cobra 2000 has a power switch that doesn't interrupt the hot side of the power supply like EVERY other radio you or I have ever seen. This radio has the power button wired into the ground side of the radio circuit board's power hookup. The hot side of the power supply is always connected.
You can't just switch the AC power from the wall on and off. The clock needs power all the time, whether the radio is running or not.
And this is where it gets tricky. A solid-state CB with the power switch in the hot side is pretty simple. You turn it off, and any place where you poke a voltmeter inside the radio will read zero Volts.
Likewise, if you switch the ground wire, that same meter should read 13.8 Volts everywhere inside the radio where you poke your meter probe when the switch is turned off.
What upsets this applecart is the frequency counter hookup, The ground inside the frequency counter is "sorta" shared with the ground in the radio. The two ground circuits have to be separate, but you need to feed two tiny RF signals into the counter from the radio circuit board. The preamp transistors in the counter that amplify the radio's two internal frequencies are needed to drive the counter circuit. They are powered from 8 Volts DC inside the counter. The radio circuits feeding these two preamp circuits in the counter are 'joined' where the two shielded wires feed into the counter. As a result the ground circuit on the radio circuit board doesn't quite come all the way up to 13.8 Volts when the power button is "off". The radio circuit board's ground drifts up to around 8 Volts, by way of the counter's preamp circuit.
Textbook name for this is a "current sneak path". A backdoor that prevents the radio circuit board from shutting totally off. The hot side of the power supply is still feeding into the radio circuit board, and the ground side of the circuit board now floats between 8 and 9 Volts DC. The difference between these is around 5 Volts. This is low enough that none of the radio circuits will run, and the radio appears to be shut down. But the channel digits only need about 3 Volts to 'just barely' light up.
What the current path through the meter lights will do is to "pull up" the radio board's ground side above that 8 or 9-Volt "sneak" voltage. Once this difference voltage drops below about 2.5 Volts the channel LEDs will remain dark. They need at least that much voltage to do anything.
Once those meter lights burn out, that "pull up" current goes away, and the voltage difference rises JUST high enough to make the channel digits glow. Just barely, but enough to see. Installing LEDs to illuminate the meters has the same effect. They just won't pull up the ground side of the circuit board the way the resistance path through the original incandescent meter lamps would do.
Is this a design flaw, or a "feature"? Depends on your point of view. But in 45 years in this business, the Cobra 2000 is the only base radio I have ever seen built this way. It's a quirk that's totally unique to this model.
And that's why.
If you're still awake, congratulations!
73
It starts with the alarm-clock function. By 1978 Uniden had decided that they don't like relays. All their radios designed after that used solid-state devices to do all the switching functions in their products, including CB radios. But how to make the computerized clock/frequency counter turn the radio on when the alarm activates?
The answer is a component called a SCR, or silicon controlled rectifier. A brief pulse of current from the digital-clock circuit turns it on, and it will stay switched on until the power button shorts across the SCR when you push it in. This resets the SCR for the next time you use the alarm to power up the radio. Just one problem. It's far easier to switch the ground side of a circuit with the SCR than the hot side. As a result, the Cobra 2000 has a power switch that doesn't interrupt the hot side of the power supply like EVERY other radio you or I have ever seen. This radio has the power button wired into the ground side of the radio circuit board's power hookup. The hot side of the power supply is always connected.
You can't just switch the AC power from the wall on and off. The clock needs power all the time, whether the radio is running or not.
And this is where it gets tricky. A solid-state CB with the power switch in the hot side is pretty simple. You turn it off, and any place where you poke a voltmeter inside the radio will read zero Volts.
Likewise, if you switch the ground wire, that same meter should read 13.8 Volts everywhere inside the radio where you poke your meter probe when the switch is turned off.
What upsets this applecart is the frequency counter hookup, The ground inside the frequency counter is "sorta" shared with the ground in the radio. The two ground circuits have to be separate, but you need to feed two tiny RF signals into the counter from the radio circuit board. The preamp transistors in the counter that amplify the radio's two internal frequencies are needed to drive the counter circuit. They are powered from 8 Volts DC inside the counter. The radio circuits feeding these two preamp circuits in the counter are 'joined' where the two shielded wires feed into the counter. As a result the ground circuit on the radio circuit board doesn't quite come all the way up to 13.8 Volts when the power button is "off". The radio circuit board's ground drifts up to around 8 Volts, by way of the counter's preamp circuit.
Textbook name for this is a "current sneak path". A backdoor that prevents the radio circuit board from shutting totally off. The hot side of the power supply is still feeding into the radio circuit board, and the ground side of the circuit board now floats between 8 and 9 Volts DC. The difference between these is around 5 Volts. This is low enough that none of the radio circuits will run, and the radio appears to be shut down. But the channel digits only need about 3 Volts to 'just barely' light up.
What the current path through the meter lights will do is to "pull up" the radio board's ground side above that 8 or 9-Volt "sneak" voltage. Once this difference voltage drops below about 2.5 Volts the channel LEDs will remain dark. They need at least that much voltage to do anything.
Once those meter lights burn out, that "pull up" current goes away, and the voltage difference rises JUST high enough to make the channel digits glow. Just barely, but enough to see. Installing LEDs to illuminate the meters has the same effect. They just won't pull up the ground side of the circuit board the way the resistance path through the original incandescent meter lamps would do.
Is this a design flaw, or a "feature"? Depends on your point of view. But in 45 years in this business, the Cobra 2000 is the only base radio I have ever seen built this way. It's a quirk that's totally unique to this model.
And that's why.
If you're still awake, congratulations!
73
So, maybe I should have included that the counter's input-preamp transistors are powered from 8 Volts DC inside the counter module?
Or does that just make the mud muddier?
73
Or does that just make the mud muddier?
73
