Electrical resistance, that is …
I was finally giving my new Datong FL-3 a workout today with my TS-430S, and I realized that the front-mounted headphone jack on the Datong didn't work.
I've spent a couple of hours, off and on, troubleshooting the problem. As much as it pains me to do this to something I just bought, I field dressed the Datong, its guts and cabling strewn across my operating position. God forbid something roll off the desk onto the floor, I would never find it.
One of the old maxims I learned the hard way as a new electronics troubleshooter “way back in the day” was to never EVER make assumptions. I learned this lesson again today. Let's just say I found the problem after realizing I needed to be a little more basic about my work.
The Datong takes the input from the radio's speaker output. If the Datong is off, the unprocessed audio passes through the Datong either thru the RCA output jack on the back, or through the headphone jack on the front panel. The Datong, when off, is supposed to be transparent.
There was audio out on the rear panel RCA output jack, but nothing coming through the headphone jack. Why? I dug into the Datong to find out.
I did some continuity checks and it took me a while to find that the problem was the headphone jack's audio ground was open. If you hooked a jumper wire from chassis ground to the headphone plug, it worked beautifully.
When the Datong is switched on, it interrupts the audio path, sending the audio through its filtering apparatus before it exits to the rear output jack or the headphone jack. I cleaned the headphone jack with contact spray, but still nothing. I was tempted to simply jumper the audio lines from the rear RCA jack to the headphone jack. That would fix it, but it wouldn't find the problem.
The problem turned out to be a broken resistor lead. It's pretty typical to have a dropping resistor in the audio line going to a headphone jack, and the 680 ohm resistor was neatly clipped at one end. The resistor was an mounted vertically, and the clipped end was the bottom end. It stood up perfectly and was in position, but wasn't making contact with the other half of the lead. Maybe its' my eyesight, but I looked at that resistor half a dozen times and didn't see it was clipped. After I realized it was a ground problem, I should have immediate worked backward from the phone jack. I eventually did, but doing so earlier would have saved me some time.
A trip to Radio Shack will take care of the resistor, or if not, I'll just jumper the thing.
Otherwise the Datong works pretty well. The automatic notch isn't as effective as the one in my Icoms, but I guess that should come as no surprise. It's interesting that it works when using CW — if there's a heterodyne, the notch will not lock in the CW signal, but will instead notch the steady heterodyne. Other than the automatic notch, its exactly the same as my FL-2. The auto notch feature is built on an accessory board that mounts on top of the main printed circuit board inside the unit. I could cut the 4 or 5 wires connecting the notch board and remove it and I would have an FL-2.
Because the notch is external, the volume of the input signal has to be enough for the notch to “find” it. I'll play with it some more, but for now it's in pieces.
That's it for this transmission … de KY4Z … GN … SK … dit dit …
UPDATE, THURSDAY, AUG. 2
After I finished this entry, I decided to go ahead and jumper the snipped resistor. The resistor's function was simply to cut the volume level going to the headphone jack, and without the jumper its not like its too loud in the phones. What's more important is that the FL-3 is reassembled and stamped “DONE.”