So here is the last trick installed in the tricked-out Cobra 142GTL. Converting the radio from 5 pins to 4 can be done four ways. There are two ground pins, 2 and 4 so you can tie them together, and connect to pin 1 on the 4-pin socket. Or you could just connect to pin 2 alone, or to pin 4 alone. All three of those choices will run the risk of feedback "squeal" problems, either on AM transmit or SSB receive, or maybe both.
The fourth choice is to use a relay that connects the pin 4 wire to the other wire for transmit and receive. This keeps the pin 4 ground isolated, like the stock mike does.
This customer wanted to receive with the mike unplugged, or more importantly with his direct-audio setup feeding into the mike jack.
Using a transistor to activate the relay coil isolates the relay's coil current from pin 2, so that it still has just audio current along with less than a milliamp of DC to key the transistor. This prevents the audio-feedback issue. The relay's coil current goes to ground by the pin 4 wire, like the 5-pin mike would do.
Taking the 5-pin socket loose may require a tiny, tiny drop of acetone into the flat of the socket bushing, just under the round nut. This will cause the acetone to wick into the threads, keeping it away from the plastic bezel.
Once you have it loose, the five wires come off.
The audio hot and ground are the shielded wire, of course. Green is receive, black is switching (only) common ground and red is transmit.
Connect the audio first.
Yeah it seems to say "Blue" next to the audio wire. That is for the empty hole next to where the white audio wire is soldered. The "Blue" pad is for the audio wire from an unkey-beep board. The audio wire for those is commonly blue.
There is a long red wire already installed on the adapter. It goes to a source of main power for the relay coil. Best place is the pad at pin 1 of the TA7222P audio power chip.
When you go to poke it into the front-panel hole and replace the socket nut, you will find a slight interference between this board and a plastic pillar on the inside of the bezel. We don't need it, so a pair of dikes takes care of this.
Once it's in place and the round nut tightened, a tiny drop of clear nail polish applied to the flat of the bushing, just against the round nut will "wick" into the threads without fouling the exposed threads where the mike-plug's ring needs to tighten down.
This setup started out long ago as a relay strapped onto the radio's chassis, a transistor scabbed onto it, and appropriate bypass disc caps and RF chokes added. Was ugly, but worked except for a somewhat-loud "POP!" when you unkey.
I'll make a separate post showing the 7 revision levels that came before this design. Got tired of the hand wiring and put the parts on a printed circuit board. Really did serve to speed up installing the mod. But the "POP!" problem continued.
Numerous theories arose from online forums about the cause and/or cure.
And then my idiot light came on. The switch in the mike has a "dead zone". When your thumb lets go of the PTT button, the speaker isn't connected immediately. The transmit shuts off first. This causes a "POP?" from the speaker-amp chip.
But the speaker isn't conneted yet. The button is still halfway between transmit and receive. Around the time the POP dies down, the receive side of the mike switch closes and connects the speaker. No "POP" because it faded before the speaker turned back on.
If you're skeptical about this analysis, take a straight hand mike and slide your thumb off the button sideways. This will release the button to snap back to receive side a lot faster than normal. More than likely you'll hear the "POP", or at least some of it.
Long story short, we use a transistor to key pin 3, and the relay to activate the speaker on pin 5. the relay has a capacitor to delay turning the speaker back on.
Just one new problem. The transistor keys the radio's transmit side too soon, before the relay can shut off the speaker. Now we have a quieter "click" when keying the mike. No "POP" on unkey, but this is not the perfect solution I was seeking.
It's as if this thing is an eternal prototype. Each redesign improves one problem and then causes another.
I'll pick up from here with the evolution from the version 1 in another post.
73
The fourth choice is to use a relay that connects the pin 4 wire to the other wire for transmit and receive. This keeps the pin 4 ground isolated, like the stock mike does.
This customer wanted to receive with the mike unplugged, or more importantly with his direct-audio setup feeding into the mike jack.
Using a transistor to activate the relay coil isolates the relay's coil current from pin 2, so that it still has just audio current along with less than a milliamp of DC to key the transistor. This prevents the audio-feedback issue. The relay's coil current goes to ground by the pin 4 wire, like the 5-pin mike would do.
Taking the 5-pin socket loose may require a tiny, tiny drop of acetone into the flat of the socket bushing, just under the round nut. This will cause the acetone to wick into the threads, keeping it away from the plastic bezel.
Once you have it loose, the five wires come off.
The audio hot and ground are the shielded wire, of course. Green is receive, black is switching (only) common ground and red is transmit.
Connect the audio first.
Yeah it seems to say "Blue" next to the audio wire. That is for the empty hole next to where the white audio wire is soldered. The "Blue" pad is for the audio wire from an unkey-beep board. The audio wire for those is commonly blue.
There is a long red wire already installed on the adapter. It goes to a source of main power for the relay coil. Best place is the pad at pin 1 of the TA7222P audio power chip.
When you go to poke it into the front-panel hole and replace the socket nut, you will find a slight interference between this board and a plastic pillar on the inside of the bezel. We don't need it, so a pair of dikes takes care of this.
Once it's in place and the round nut tightened, a tiny drop of clear nail polish applied to the flat of the bushing, just against the round nut will "wick" into the threads without fouling the exposed threads where the mike-plug's ring needs to tighten down.
This setup started out long ago as a relay strapped onto the radio's chassis, a transistor scabbed onto it, and appropriate bypass disc caps and RF chokes added. Was ugly, but worked except for a somewhat-loud "POP!" when you unkey.
I'll make a separate post showing the 7 revision levels that came before this design. Got tired of the hand wiring and put the parts on a printed circuit board. Really did serve to speed up installing the mod. But the "POP!" problem continued.
Numerous theories arose from online forums about the cause and/or cure.
And then my idiot light came on. The switch in the mike has a "dead zone". When your thumb lets go of the PTT button, the speaker isn't connected immediately. The transmit shuts off first. This causes a "POP?" from the speaker-amp chip.
But the speaker isn't conneted yet. The button is still halfway between transmit and receive. Around the time the POP dies down, the receive side of the mike switch closes and connects the speaker. No "POP" because it faded before the speaker turned back on.
If you're skeptical about this analysis, take a straight hand mike and slide your thumb off the button sideways. This will release the button to snap back to receive side a lot faster than normal. More than likely you'll hear the "POP", or at least some of it.
Long story short, we use a transistor to key pin 3, and the relay to activate the speaker on pin 5. the relay has a capacitor to delay turning the speaker back on.
Just one new problem. The transistor keys the radio's transmit side too soon, before the relay can shut off the speaker. Now we have a quieter "click" when keying the mike. No "POP" on unkey, but this is not the perfect solution I was seeking.
It's as if this thing is an eternal prototype. Each redesign improves one problem and then causes another.
I'll pick up from here with the evolution from the version 1 in another post.
73
