Adding an external digital frequency display that reads both receive and transmit frequency requires that you tap into the radio's frequency synthesizer. There are possible pitfalls. A shielded cable or coax must be used for this, and every foot of that cable has capacitance. That sum total of that capacitance can be enough to load down the output of the radio's local oscillator signal and reduce receive and/or transmit performance. And sometimes tapping into the radio's PLL causes no compromise at all. Kinda hard to predict, and not all external counter displays are created equal.
The other risk is that the drive level to the counter display's input will be too weak to achieve a stable display. Or it will be weak enough that transmitting causes RF from the final to leak into the display's input and disrupt the stable display during transmit.
Either way, this radio is getting a VERY simple buffer amplifier added. It doesn't boost the signal level at all, but isolates the counter's input and the coax from the signal we're tapping into. Think of it as a current amplifier, not a voltage amplifier.
This customer will be using an old PDC256 display on his Cobra 142GTL. The input on those is quite sensitive, but since I can't test the two items together, this buffer serves to play it safe and prevent gripes after the customer gets it back from us.
Bear in mind that all Galaxy/RCI/Connex radios that have either an internal counter display, or a socket on the back for a FC390 contains a separate amplifier circuit to deliver the input signal for the counter.
The Cobra 142GTL doesn't.
Yet.
Bill of materials:
About 10 inches (forgot to measure it) shielded cable. Type is not critical.
1) 330 ohm 1/4W resistor. Not critical any value from 220 to 470, maybe more will work okay.
2) .01uf disc cap. This example uses monolithic ceramic caps for this. Same as a disc cap, just smaller.
1) 470pf disc cap. Not a critical value. As low as 220pf may work. We used a .001 (1000pf, 1nf) cap for years, but I seem to have run out of that one.
1) J310 junction FET transistor. Numerous substitutes abound, including 2N3819, MPF102, 2SK19 and others. The pin arrangement will be different from one type to the next. With any luck your choice of a substitute FET will be listed on a data sheet showing the particular pinout for that part. This J310 is arranged with the drain on the left, source in the middle and gate on the right.
1) RCA jack (phono plug socket) or a match for whatever plug you want to put on the input wire of the counter display.
The pinout for this J310 FET is drain on the left, source in the center, and gate on the right.
I tend to start with holes that need to get drilled. This chassis has holes in the rear apron of the steel chassis that are covered by the serial number plate. Drilling the aluminum plate is just easier than steel.
The left socket is for another accessory we'll cover later. The black one on the right will be for the display output.
I use teflon cable for this kind of job, since it ignores soldering heat. I prefer to wrap a thin bare wire around the shield braid and clip it off short. A solid wire is always easier to connect than the shield braid.
The FET gets prepped with one of the .01uf caps and the 330 ohm resistor. The blue cap's position could be clearer, but one leg is wrapped around the FET's drain lead on the left. This capacitor lead is long enough it will reach to the source of 8 Volts that runs the circuit. The other leg of this cap will get grounded.
Eventually.
So here we are looking at the underside of L14, the output from the radio's PLL/VCO. There is a spare hole in the foil pad, where the test point would go if the factory still installed them. This is where the FET's gate lead goes. Showing that lead connected alone is the only way to show clearly where it goes. Hooking up the other two legs of the FET will fisually obscure this connection.
Next is to put the free end of the 330 ohm resistor into another spare hole, this time in a ground foil.
You'll see what I meant about hiding the gate connection.
The long end of the blue capacitor already soldered to the FET's drain lead gets soldered to the 8 Volt DC pad that powers the PLL. Ignore the orange wire, it activates the front-panel clarifier control for receive and transmit both. The free end of this cap goes to a handy ground pad as seen.
The output socket gets the center wire of the shielded lead connected directly to it. The shield braid gets connected to the socket's ground lug by way of the other .01uf cap. This keeps the circuit-board's DC ground circuit isolated, same as the other half-dozen disc caps that the factory placed around the edge of the main pc board. Looks quirky, but it's necessary.
So here's the inside end of the output coax. There is another handy unused hole in the foil for a ground wire. One side of the 470pf cap gets lap-soldered to the center conductor.
Last step is to trim and lap-solder the other side of the 470pf disc to the FET's source lead, where the 330 ohm resistor is already soldered.
Now it's ready to rock and roll. Well, not the displayed frequency. It should be rock solid. If the radio is, anyway.
This radio got tricked out. Two more mods remain to show. An internal relay to key a ham-type amplifier without a foot switch, and an adapter that changes the mike socket from five pins to 4, without the feedback issues.
Film at 11.
Or when I get a 'round tuit'.
73
The other risk is that the drive level to the counter display's input will be too weak to achieve a stable display. Or it will be weak enough that transmitting causes RF from the final to leak into the display's input and disrupt the stable display during transmit.
Either way, this radio is getting a VERY simple buffer amplifier added. It doesn't boost the signal level at all, but isolates the counter's input and the coax from the signal we're tapping into. Think of it as a current amplifier, not a voltage amplifier.
This customer will be using an old PDC256 display on his Cobra 142GTL. The input on those is quite sensitive, but since I can't test the two items together, this buffer serves to play it safe and prevent gripes after the customer gets it back from us.
Bear in mind that all Galaxy/RCI/Connex radios that have either an internal counter display, or a socket on the back for a FC390 contains a separate amplifier circuit to deliver the input signal for the counter.
The Cobra 142GTL doesn't.
Yet.
Bill of materials:
About 10 inches (forgot to measure it) shielded cable. Type is not critical.
1) 330 ohm 1/4W resistor. Not critical any value from 220 to 470, maybe more will work okay.
2) .01uf disc cap. This example uses monolithic ceramic caps for this. Same as a disc cap, just smaller.
1) 470pf disc cap. Not a critical value. As low as 220pf may work. We used a .001 (1000pf, 1nf) cap for years, but I seem to have run out of that one.
1) J310 junction FET transistor. Numerous substitutes abound, including 2N3819, MPF102, 2SK19 and others. The pin arrangement will be different from one type to the next. With any luck your choice of a substitute FET will be listed on a data sheet showing the particular pinout for that part. This J310 is arranged with the drain on the left, source in the middle and gate on the right.
1) RCA jack (phono plug socket) or a match for whatever plug you want to put on the input wire of the counter display.
The pinout for this J310 FET is drain on the left, source in the center, and gate on the right.
I tend to start with holes that need to get drilled. This chassis has holes in the rear apron of the steel chassis that are covered by the serial number plate. Drilling the aluminum plate is just easier than steel.
The left socket is for another accessory we'll cover later. The black one on the right will be for the display output.
I use teflon cable for this kind of job, since it ignores soldering heat. I prefer to wrap a thin bare wire around the shield braid and clip it off short. A solid wire is always easier to connect than the shield braid.
The FET gets prepped with one of the .01uf caps and the 330 ohm resistor. The blue cap's position could be clearer, but one leg is wrapped around the FET's drain lead on the left. This capacitor lead is long enough it will reach to the source of 8 Volts that runs the circuit. The other leg of this cap will get grounded.
Eventually.
So here we are looking at the underside of L14, the output from the radio's PLL/VCO. There is a spare hole in the foil pad, where the test point would go if the factory still installed them. This is where the FET's gate lead goes. Showing that lead connected alone is the only way to show clearly where it goes. Hooking up the other two legs of the FET will fisually obscure this connection.
Next is to put the free end of the 330 ohm resistor into another spare hole, this time in a ground foil.
You'll see what I meant about hiding the gate connection.
The long end of the blue capacitor already soldered to the FET's drain lead gets soldered to the 8 Volt DC pad that powers the PLL. Ignore the orange wire, it activates the front-panel clarifier control for receive and transmit both. The free end of this cap goes to a handy ground pad as seen.
The output socket gets the center wire of the shielded lead connected directly to it. The shield braid gets connected to the socket's ground lug by way of the other .01uf cap. This keeps the circuit-board's DC ground circuit isolated, same as the other half-dozen disc caps that the factory placed around the edge of the main pc board. Looks quirky, but it's necessary.
So here's the inside end of the output coax. There is another handy unused hole in the foil for a ground wire. One side of the 470pf cap gets lap-soldered to the center conductor.
Last step is to trim and lap-solder the other side of the 470pf disc to the FET's source lead, where the 330 ohm resistor is already soldered.
Now it's ready to rock and roll. Well, not the displayed frequency. It should be rock solid. If the radio is, anyway.
This radio got tricked out. Two more mods remain to show. An internal relay to key a ham-type amplifier without a foot switch, and an adapter that changes the mike socket from five pins to 4, without the feedback issues.
Film at 11.
Or when I get a 'round tuit'.
73
