I was working on another of our passel of Alinco DR-1200T's. This one is our spare radio which I recapped, aligned (as best I could) and set aside in case our primary APRS radio failed. Well, a different radio failed, and we attempted to use this one in its place. Unfortunately, it didn't work.
The presented problem was "Won't TX". I brought it into the shack and ran it through a bevy of tests. It was transmitting and was pretty much right on frequency. I ran a tone through it, and it made audio. I chalked it up to "testing failure" when we tried to use it, and asked our APRS guru to give it another go.
He did, and it still didn't work. This time, he was able to give me a little more information. While he attempted to transmit, he had a Baofeng on the APRS frequency, and was listening for the tone. He noted that it WAS, indeed, transmitting a tone. However, in his mind it seemed a bit short. He sent me a short video clip. When I heard it, It definitely sounded like the tones were WAY off.
So, back to the bench it came. I hooked it up to my service monitor and confirmed I could still get a tone through it. I busted out my function generator, and had it transmit tones from 300hz on up to 5khz. It sounded OK to me. I was confused.
After noodling on it for a few days, I had an epiphany. What if the deviation is WAY off? Perhaps my service monitor is being very forgiving with a very wide, or narrow, deviation and I still hear the sound, but when it's transmitting, it's not working well with APRS receivers?
I commenced some additional testing. I've got an old IFR 1200S service monitor, but I've never trusted the deviation meter. It was time to try something new. I researched using the "MIN/MAX" method on a spectrum analyzer.
Well, the results were pretty striking. The radio's deviation was 10khz! I didn't manage to get a screen shot, unfortunately. It would appear that my service monitor's deviation meter is WAY off.
So, I decided to adjust the deviation without using my service monitor. Here's what I had to do. First, the alignment procedure calls for a 1khz tone with a P2P voltage of 25 millivolts for deviation alignment, and 4 millivolts for the mic alignment. My service monitor doesn't go that low, and it doesn't really provide for reading the voltage. So, I opted to use function generator.
This crusty old Instek does the job. The amplitude of the signal was still way too high. So, I had to attenuate it even more than the function generator's internal attenuators could do. I bodged together a line of attenuators to get the tone's voltage down where it was needed:
That was sufficient to get the P2P voltage down to what I needed. I tested it with my oscilloscope:
Now, confident that I was feeding the signal in according to the alignment procedure, I proceeded to do the MIN / MAX method to measure the deviation on my Spectrum Analyzer. I pumped the radio's RF output into a tapped dummy load, and fed the tap into the analyzer.
After some trial and error, I was able to measure the deviation. This is a screen shot after I finally got the trim pot adjusted such that the deviation was the required 4.9 khz, according to the service manual.
Once it was properly aligned, I went ahead and plugged it into my IFR 1200S. The deviation meter was reading 3khz, with all the same settings. That would explain why my FM Deviation alignments always seemed off. The IFR must have a problem in the deviation meter. That also explains why an APRS tracker I built, years ago, never managed to work. I was using the IFR to adjust the deviation. Whoopsie. I guess I'll have to use this method from now on.
So, I ended up fully aligning the deviation on this radio using MIN/MAX. The proof will be in the pudding when we hook this up and try to beacon with it. I suspect it'll be MUCH better, since the deviation is now less than 1/2 what it was. While I was at it, I readjusted the deviation on the other of these radios I had on hand. The deviation wasn't AS far off - 6.9 khz where it should have been 4.9, but i was definitely worth doing.
I'll hand both radios back off to our APRS guru, and see if they're working better now. I'm hoping that he won't have to readjust his TNCs to compensate for the changes in the radios. However, these are "in spec" now, so if the TNC needs adjusting, it's probably best to do it there.
