Homebrew Tone Board Using Arduino Works, Then Doesn't

[TMA-1]

I'm back, with another problem child.  Please forgive the length of this post.

Recently I picked up a Kenwood TR-7950, a very nice (1980's?) 2-meter VHF "for a song" at a thrift store.  Testing it out it appeared fine, but lacked a tone board--a rather essential thing with today's repeaters.

A little research on the Net revealed the original "optional" TU-79 tone board is either outright unobtainium, or laughably expensive today.  An aftermarket is available, but was $100 USD before shipping, duty to Canada, and taxes.  Not happening.

What to do, what to do?  So I had an idea.  (This happens occasionally, and while generally considered a bad thing leading to chaos, has been known on rare occasion to have positive end results.)  Arduinos are capable of generating tones down to a few Hertz.  What if I MADE one instead?

The original tone board connects to the radio via a connector.  Using the FLUKE A8000 multimeter, (that I repaired with the assistance of some helpful members on this forum), I mapped out a truth table of the inputs to the tone board.

The original tone board option supported 3 tones.  You selected the 3 you wanted to use by snipping diodes on each of 3 "banks" of the tone board itself, before installing it.  The radio then allows you to assign "Tone 1", 2, or 3 to the channel you are using or programming.  The 1/2/3 selection is passed to the tone board using logic pins on the connector.



So my Arduino programming takes those inputs and similarly digests them into an actual tone frequency, and then fires up that tone on an output pin headed back to the radio.  Square wave.

A little more research on the Net revealed a nifty 3-stage RC filter network to shape the square wave into a fair approximation of a sine wave.



I am happy to report that the makeshift prototype worked like a charm!  I made a contact on the local repeater, and my signal was reported as good.

Feeling generally pleased with myself, I programmed another Arduino Nano (without pin headers), wired it up with solder this time, and installed it and the filter into the radio where the original TU-79 would have gone.



It doesn't work.

It doesn't work, and I can't figure out why.  The waveform going into the radio looks just as it had before, nice and clean.  Connecting the Arduino back to my PC for the Serial Monitor feature so I can monitor diagnostic messages, the logic of the Arduino program appears to operating perfectly, and the correct tone is being sent.

A second radio confirms my transmission is going out.  Yet all the neighborhood repeaters are elevating their noses in the opposite direction.  How rude.

If RF was somehow interfering with the Arduino, I'd expect to see a compromised signal on the scope.  I've tried adjusting the amplitude with a shunt resistor.  Nothin'.  If it's not amplitude, frequency, or phase (which shouldn't matter here), then what is it?

Why did the prototype wiring work well, and the "permanent" installation not?  I can't help thinking I'm going to be dreadfully embarrassed when I find out the answer.

Any ideas?

[DeepLink]

CTCSS boards for old radios can be replicated, I made one for the Yaesu FT-736R

My guess is that your signal could be way to distorted (looking at your scope pictures)
Or your frequency could be off

The original Kenwood TU-79 uses a MX315 chip, which has a distortion of ~2%

If you rely on the onboard RC clock of the Arduino, this is only accurate within +/-2% (+/-14% over temperature range)

Try to use an external crystal / oscillator, and a better filter for the output

/Hans

[p.larner]

would this help?

[TMA-1]

Thank-you for the responses, but I think I've figured it out, somewhat.

Through experimentation with a little off-the-shelf op amp circuit, I've decided that the problem wasn't too much amplitude in the sub-audible tone, but rather too little.

The earlier contact I made through the repeater must have just been a borderline fluke.  I had fair winds blowing horseshoes and rabbits' feet at me that day.

I played with it more tonight, adding the little amplifier, and again made a contact through a local repeater.  Another evening I will organize and solidify the wiring, and again pack it all back into the radio.  Hopefully there won't be any more issues.

Assuming that that is successful, I will have established a tone board for this radio at a total cost of about five bucks.  (An Arduino Nano clone from China on eBay is about $3.50 CAD, and the amplifier another dollar.)  And of course a bunch of time.  But that's all hobby "fun" time, right?

[TMA-1]

Epilogue:  A little more tweaking has resulted in a successful mod.  I found a better place to steal a few milliamps for the amplifier, added a filter capacitor to strain out some DC that the amp seemed to inject, and packed the whole deal back into the radio.  It works.  The tone signal is now clean and bright, and the repeaters love it.  Happy camper.  Thanks for the indulgence.