Uniden pc68xl /cobra 25ltd DIRECT AUDIO INJECTION POINT?

[025]

I I am trying to set up audio direct injection on both of these radios and all I'm finding on the Internet is information on the cobra 29 The 29 board and these boards look similar but I don't understand the process enough to understand to figure out where to inject on either one of these radios. if someone could help, point out how it works and where it's actually entering at and any other information that would be great.
73

 

[nomadradio]

Biggest issue is to decide if you want a one-way modification. Meaning that a mike won't work any more, only the 'direct' input for transmit audio.

You'll need a switch if you want to select one or the other.

What audio source will you use for the 'direct' transmit audio? Might want an isolation transformer, especially if the radio will be driving an amplifier.

The term "direct injection" doesn't narrow down just where the 'direct' input will go. Not the same for every radio. The diagram above feeds into the radio's mike amplifier transistor. You'll find that your audio source also comes through the receiver speaker unless you shut it down for receive mode. A relay that operates only while the mike is keyed will prevent this if it's wired to feed the external audio only while transmitting.

73

 

[025]

Biggest issue is to decide if you want a one-way modification. Meaning that a mike won't work any more, only the 'direct' input for transmit audio.

You'll need a switch if you want to select one or the other.

What audio source will you use for the 'direct' transmit audio? Might want an isolation transformer, especially if the radio will be driving an amplifier.

The term "direct injection" doesn't narrow down just where the 'direct' input will go. Not the same for every radio. The diagram above feeds into the radio's mike amplifier transistor. You'll find that your audio source also comes through the receiver speaker unless you shut it down for receive mode. A relay that operates only while the mike is keyed will prevent this if it's wired to feed the external audio only while transmitting.

73

I dont plan to use the radios rx. I will use a sdr reciever. I want to bypass the radios mic pre amp i believe. I guess it has to go in directly before the audio amplifier? Or can you go in after it?

 

[025]

I tried injection at the pin 4 of the audio chip . Not sure it much different than the front mic in

 

[nomadradio]

Pin 4 is the farthest "downstream" from the radio's mike circuits you can go. Should be a good choice.

73

 

[025]

Im just sending the mic audio into that pin and using turner plus 3 . It doesn’t sound impressive. I havent tried to do anything with the pc yet. I tried to go to pin 5 the output but I guess the tuner doesnt have enough drive?
I would like to de couple the stuff before the amp.
No feedback on rx so maybe i did it wrong?
The pc68xl must have the chip backwards from the 25?

 

[Handy Andy]

For a PC68 - this may help...

Not sure of "Direct inject" is really going to work well for the concept of fidelity - but if you can use any of this older stuff I posted on another forum years ago, - you're welcome...

They're attached, but one thing I wanted to warn you about...

That "summing area" where the 100K and the 15K come together at the Audio chip...

​

That may create an poor balance for tapping.

You may want to "split up" the R53 100K resistor into two 47K resistors - then tie your live of your direct feed into where the two 47K's meet in the middle...

​

This would help for loading of both halves of the tie point that sums the Receive and Mic audio together.

At least you won't have to worry so much about loading down the audio involved in all the sections trying up and working against each other...

 

[bob85]

When you direct inject you need some form of eq & limiter to get a decent sound,
without eq you sound terrible in my experience.

 

[025]

For a PC68 - this may help...

Not sure of "Direct inject" is really going to work well for the concept of fidelity - but if you can use any of this older stuff I posted on another forum years ago, - you're welcome...

They're attached, but one thing I wanted to warn you about...

That "summing area" where the 100K and the 15K come together at the Audio chip...

View attachment 39864​

That may create an poor balance for tapping.

You may want to "split up" the R53 100K resistor into two 47K resistors - then tie your live of your direct feed into where the two 47K's meet in the middle...

View attachment 39865​

This would help for loading of both halves of the tie point that sums the Receive and Mic audio together.

At least you won't have to worry so much about loading down the audio involved in all the sections trying up and working against each other...

Awesome information. I havent found any problem feeding it directly at pin 4. But i havent tried a different audio source yet.

 

[025]

When you direct inject you need some form of eq & limiter to get a decent sound,
without eq you sound terrible in my experience.

I cant find my old mixer, but im going to see what I can do with computer software and a usb sound card

 

[bob85]

i gave my mixer to a local when i lost interest in tripping over cables & went back to hand mics,
now i want to play again all i have is an old pc & half decent sound card.

 

[025]

i gave my mixer to a local when i lost interest in tripping over cables & went back to hand mics,
now i want to play again all i have is an old pc & half decent sound card.

I hear ya. I had a big mixer i found at goodwill but gave it away. But somewhere i have behringer its like a odd ball 4 input stereo and 4 mono inputs but no decent mics to use. I have a crap load of ham gear but im just burned out on that. I miss the days of CB when i was a kid back in the 80s. And i just prefer AM. And am on 3.870 is just no fun. These guys key down for like 20 mins at a time. Same with 7.290 . I have my extra but after a few years im not so entertained. Lol back to playing with cb radios. I would rather work on stuff than talk anyway.

 

[025]

For a PC68 - this may help...

Not sure of "Direct inject" is really going to work well for the concept of fidelity - but if you can use any of this older stuff I posted on another forum years ago, - you're welcome...

They're attached, but one thing I wanted to warn you about...

That "summing area" where the 100K and the 15K come together at the Audio chip...

View attachment 39864​

That may create an poor balance for tapping.

You may want to "split up" the R53 100K resistor into two 47K resistors - then tie your live of your direct feed into where the two 47K's meet in the middle...

View attachment 39865​

This would help for loading of both halves of the tie point that sums the Receive and Mic audio together.

At least you won't have to worry so much about loading down the audio involved in all the sections trying up and working against each other...

The cobra 25ltd im unable to get to work without a squeel on rx. And i cant go directly. I fed it how you said. Replaced r53 with 2 47k resistors. Tx audio sounds very nice but that squeel. The pc68xl im feeding directly at pin 4 no issues but the rx is Intermittent on that radio. You know the deal when you push on the board and you get full rx lol.
So anyway how do i stop the squealing on rx

 

[sp5it]

For direct inject I would lift anode of D10 and put audio input there. Also limiter need to be disabled, all audio settings should be done externally.
Audio level must be high for proper drive level.
Mike

 

[Handy Andy]

Well, this may sound stupid, but when you're working on the RX side, remember to, that the MIC amp is on all the time. It's powered directly thru the Power supply, so you don't really have a "squeal" on RX, but a feedback loop.

Ok, this may be due to several problems but let's look at this one component or two at a time..

Your "squeal" may be from too much INPUT capacitance - not necessarily at your tap, but the OVERALL input cap level of farads is "gathering up" any noises and "echoes" (yes echoes causing that squeal because of the always on Mic amp section including the limtier) So yes, that includes making adjustments' to the limiter input from the OUTPUT tap it uses just past the transformer. Smaller level of feedback cap helps at C70 reduced to even 560pF can reduce the loop effects.

So if you ADDED capacitance not at C89 but elsewhere - near Pin 4 - back it off - at least near and around that input pin - use a lower value cap and swap until you don't have the squeal - it's not necessarily the mod, the level of drive and capacitance used - is what generates the squeal loop..

Can not go past a total series capacitance of 103 (0.01uF) else the whole thing will squeal - RX and TX inclusive. This is the bane of RF...