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Cobra 148- Much Higher Performance Receive Setup Than Shottky Diode Receive Mod

progress update:

the detector circuit is installed, and it does work, but the recovered audio is very low.

The RF is there, as in a 100uV signal still gives an S-9 indication, but the actual audio is very low.

it would seem that i need a bit of gain from this circuit to make up for the inherent losses.

711 and CBphreaker, did you guys just use the other side of the TL082 as an amplifier/buffer?

any tidbits will be very helpful.
LC
 
@loosecannon

I did use the second opamp as a line-in driver. It only required a gain of 4 or 5 so not to much.

I'm sure you already did this, but did you stick your scope probe on the output of the detector?
Many radios will still receive (but generally lower in volume) even if there is no detector.
 
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good point CBP!

i will make sure of this tonight when i get home.

mine will still be connected to the volume control of the radio just like stock, so im wondering if my component values will be much different from the ones you used.
LC
 
well i found the problem with my detector board.

i had unintentionally created a resistor.LOL

when i was cutting the copper off of my strip board to separate the strips in order to accommodate the op amp, i let the knife slip and cut into a trace i was using as the VCC.

so essentially i had put a 1.84 meg ohm resistor in series with the 8 volt supply. OOPS!

soldered across the cut and now the detector works.

im not sure how much i like it yet, and i feel like i need to tweak some component values in order to get what i want.

the receive just seems "dirty", i dont know how to describe it other than that.

i do know that turning the ANL/NB switch on it pretty much completely cuts the audio down to nothing, but i think CBphreaker referred to this in one of his posts.
Ill have to look back at the thread and see what he said about the ANL.

i think the first thing im going to try is halving the value of the input cap, just as an experiment.

i did use schottkys in mine if that matters to anyone. i matched them for resistance.
LC
 
will you tell me what, if anything you connected to pins 5, 6, and 7 of the op amp?

you mentioned something earlier about terminating the IC properly, but i wasnt sure what you were referring to.

also, i am using an NTE op amp because i couldnt find my TL082's.
could be that this NTE op amp is not actually a TL082, and it could be the problem.
LC
 
@loosecannon

I would think the NTE is okay. It really is best to terminate your unused opamp, or use it for something.

Referring to the schematic in the attachment. For Vref you can just use 2 10ks, bypass with poly .1 cap.

CBphreaker
 

Attachments

  • Terminating Op Amp.png
    Terminating Op Amp.png
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Hello CBphreaker and LC:

I found this device for measuring asymetrical modulation with a simple gadget that you can build

The software is a demo for 14 days and very inexpensive
(I am not affiliate with the developer)

I got all the parts just need to find the time to put it together

Keep up the good work,as myself and I believe many Tweaknicians have a good interest following this thread


Thanks you
Andy/Tecnicoloco
 

Attachments

  • Asymod PC Oscilloscope Manual.pdf
    809.4 KB · Views: 49
thanks Andy!
I will look into that.


CBP- so my plan is to ground pin 5 of the TL082 (NTE sub) and then connect pin 6 to pin 7 with a 10K resistor.

does that sound about right?
LC
 
@tecnicoloco

Thanks for the handy information. Ill give it a try here, in the short future. And thanks for your kind thoughts, as well.

@loosecannon

Short pins 6 and 7 - Build a "Vref voltage" which is 1/2 Vcc and apply that voltage to pin 5. Don't for get to by pass pin 5 to ground with a .1 poly cap.

Vref is nothing more than a voltage divider using the same values connected to Vcc- so like 10k and 10k or 47k and 47k. The voltage at junction of the two resistors is fed into pin 5 and by passed to ground via a cap.

@Whisky Flats

Yes, you can use the Precision Diode Detector in a 29. You can use it any AM receiver with a last IF of less than 500khz.

CBphreaker
 
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cbphreaker said:
@tecnicoloco

Thanks for the handy information. Ill give it a try here, in the short future. And thanks for your kind thoughts, as well.

@loosecannon

Short pins 6 and 7 - Build a "Vref voltage" which is 1/2 Vcc and apply that voltage to pin 5. Don't for get to by pass pin 5 to ground with a .1 poly cap.

Vref is nothing more than a voltage divider using the same values connected to Vcc- so like 10k and 10k or 47k and 47k. The voltage at junction of the two resistors is fed into pin 5 and by passed to ground via a cap.

@Whisky Flats

Yes, you can use the Precision Diode Detector in a 29. You can use it any AM receiver with a last IF of less than 500khz.

CBphreaker
Click to expand...


thanks CBPhreaker,

i am still quite a novice when it comes to op amps and really any internals of ICs, so i usually have to use reference texts when putting the external part values and combinations together.

I must've really misunderstood the text i read! LOL

so, i'll put this in the thread for others that might be at my level of understanding or thereabouts.

what CBP is saying is that the unused side of a dual op amp such as the TL082 must be properly terminated to keep noise and other bad possibilities from occurring.

in this configuration, pin 5 needs to be fed with half the supply voltage.
both he and i, and i imagine the others building this circuit are using the 8 volts receive only voltage provided by pin 6 of the MB3756 voltage regulator in the MB8719 chassis.

so, pin 5 of the TL082 needs 4 volts fed to it.
to get this 4 volts, you use a voltage divider, which would consist of two resistors of equal value. let's use 10k ohm resistors.

so you connect one end of a 10k ohm resistor to the 8 volt supply rail (pin 8), and then connect another 10K ohm resistor to the free end of the first resistor. the other end of this second 10K resistor gets connected to ground (pin 4).

so what you have now is two 10K resistors in series, with one end connected to voltage, and the other end connected to ground.

now, where the two resistors meet, this is your 4 volt point, and this is what you would connect to pin 5 of the TL082. (right in between the two resistors)

the way a voltage divider works, if the two resistor values are the same, then the voltage in between the two resistors will be one half of the supply voltage.

now, you also want to add a .1uF poly cap from pin 5 to ground. this is for RF bypass.

then, you just short pins 6 and 7 together.


again, i am still vigilantly learning about op amps and digital circuits and such, so this is not meant to be a lesson, just putting the info out there for the future circuit heads to find.

Thanks again CBP, i believe this is the problem with the way mine sounds.
LC
 
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