Two PCBs off of one power source

[CALAHAN]

Audioguru,

I inserted the new amplifier into the communicator tonight. All works great. I tweaked the resistors a bit to mix the volume levels as I wanted and no issues. Thank you for you time and patience on this - and thank you for helping me realize this childhood dream of having an actual "working" Star Trek communicator. Now I have one and will be making more. I'll be the talk of the campground.


Bruce

P.S. now to tweak the antenna a bit more. I picked up a helical 2.5" duck stubby tuned to my frequency range, and now to figure out how to hack this thing to fit in my shells and create a proper ground plane. But I have had a bit of experience in this already.

 

[CALAHAN]

Audioguru,

Now that am using this amp circuit, I have lost my volume pot on the FRS. I get all, or nothing.  The wipe of the resistance has been cut to immediate if you know what I mean. As I had stated early on, this FRS uses a resistor pot with thumb wheel. Any fixes?

I am also wondering how this problem will lay out when I use the circuit on an FRS with digital volume control instead of analog.

Bruce

 

[audioguru2]

The input to the MC34119 amplifier is from where the speaker/headphone jack of the FRS used to be. Nothing has changed how the volume control works. It should adjust the volume smoothly from zero to maximum.

 

[CALAHAN]

I changed out the main 3.3k resistor at pin 4 to 5 to get a bit more volume on the whole unit per your suggestions prior. Could this be the issue?

Bruce   

 

[audioguru2]

I changed out the main 3.3k resistor at pin 4 to 5 to get a bit more volume on the whole unit per your suggestions prior. Could this be the issue?

Please attach the schematic of what "changed out" means.
Before you said the volume was too high and you would reduce the value of the 3.3k resistor between pins 4 and 5 to reduce the volume. I don't know what value you tried and I don't know what value you have now.

 

[CALAHAN]

I changed the resistor from pin 4 to 5 with a 10k instead of the 3.3k because I needed the extra volume.  I tried the with both value resistors tonight, and both are the same - all or nothing. 


Bruce

 

[audioguru2]

I tried the with both value resistors tonight, and both are the same - all or nothing.

Then the volume control resistance is cracked. When its slider is above the crack it is much too loud. Below the crack is nothing.

The amplifier for the MC34119 should be like this:

 

[CALAHAN]

The volume/switch pot is round and there is no damage on it.  I have the device wired as you have it already. All to pin 7.


Bruce

 

[audioguru2]

The 10k resistor between pins 4 and 5 of the MV34119 has colour bands brown, black then orange. Then the circuit has a signal gain of 7.
Maybe you used a 100k resistor instead which makes a signal gain of 61. A 100k resistor has colour bands brown, black then yellow. Then the volume control would act like an on-off switch.

 

[CALAHAN]

Audioguru,

No it's a 10k alright.  I took it apart last night to try both the 3.3k and 10k. 

Bruce

 

[CALAHAN]

Audioguru,

This is the issue I saw coming. Remember one of my initial posts on this subject? I saw a red flag but I thought it was just my musings of a novice. Perhaps I may be right on this one.

The FRS volume is, for lack of a better term, "double cut". The original volume pot controls the audio signal  at line level (I assume), and when the volume is all the way up, the pot is letting through probably what .3 volts? (for full line level), and when reducing the volume by turning the thumbwheel,  that voltage is reduced to even less (and there isn't much to reduce). Since I have tapped off the headphone jack, instead of the speaker - am I correct in assuming that I have effectively bypassed the on board audio amplifier?

With an unamplified line level out of the FRS, "cut again" by the 3.3k resistor in the new circuit we created, I will only get volume when the FRS pot is at full volume allowing the full signal to pass, because any less then full volume is such a small amount of signal, it is inaudible.

Is this summation correct?  and if so, do I really need the 3.3k resistor on the FRS end of the new amplifier circuit, since the original onboard volume pot is already supplying line level?  I measured the volume pot some time back and I think it is 10k rated already.

Bruce 

 

[audioguru2]

Hi Bruce,
The volume control in the FRS radio feeds its audio amp (the NJM2073 has a voltage gain of 100) which has an output for the speaker, or for the earphone and for your MC34119 mixer/amp.

 

[CALAHAN]

O.K. so I was wrong and the headphone output which I am using is amplified. How do I fix my problem do you think?

Maybe tonight I will snap an image of the internals of that headphone jack. I noticed when I was breadboarding it, the MC34119 circuit worked with the FRS on either pin of barrel of the jack I plugged in it. The jack does not look like the schematic. When pushing the jack in, both contacts move off of posts inside. Perhaps I have the output to the MC34119 linked off the wrong end of the jack. 

Thanks,

Bruce

 

[CALAHAN]

O.K. here is the jack. Right now I have the wire to the amplifier fed off the head of the jack. Perhaps I should have it attached to the shaft? and that is why I don't have volume control?

Bruce

 

[audioguru2]

Your pic appears to be a completely different circuit than the FRS schematic you posted. Where is its NJR2073 audio power amplifier IC?

 

[CALAHAN]

Audioguru,

I have no idea where the amp is. The schematic I posted is off the FCC web page and is the original submitted drawing for this model radio. They do state that the production models may differ from the submitted drawing to some extent.

Bruce

 

[CALAHAN]

Audioguru,

This issue was my error. I opened the comm back up, and I had the 10uf capacitor backwards which caused chaos with the batteries. After resolving this issue, the volume pot works, but there does seem to be some damage in the middle of the sweep which was my mistake also.


Out of curiosity, will this amplifier schematic you provided work "as is" with 6 volts run into it instead of 4.5, as I will need to do it with my next build?

And, is there any quick and simple way to knock the 6 volts down to 3 to power the sound effects? two sets of diodes perhaps instead of one, or a voltage split?

Regards,

Bruce

 

[audioguru2]

The MC34119 will get very hot with a 6V supply and an 8 ohm speaker. It will waste a lot of battery power. Use an LM386 amplifier IC instead.
Connect four diodes in series to drop 6V down to 3.2V.

 

[CALAHAN]

Can you give me a bit more on the diodes. You gave me the way to hook up two diodes earlier in this thread to get 3v from 4.5.  So now I do this procedure twice using four diodes?

Bruce

 

[audioguru2]

A silicon diode has about 0.7V across it. So four in series from 6V produces 3.2V.
When the battery voltage drops to 4V then the output from the diodes will be only 1.2V.