06-22-2013, 10:33 PM
Yreka!! (it's just a bit north and east of Eureka).
It was a complete accident. I went to connect my scope to the cathode of the 1st Detector so that I could see exactly when the LO dropped out on Band 5. Strange thing, as soon as I connected the scope probe, first to ground, then to the cathode, Band 5 came literally ROARING to life. I found that I could tune clear down to 10.5 MHz and all the way up to 23 MHz using my sig gen as a source (of course there were also all those pesky shortwave stations in there.)
So, I thought.. OK, it's radiation off the probe getting back to the LO. I tested this theory by putting my finger on the cathode. No dice.
Next, I thought.. well, it's not loaded properly. I tested the cathode resistor (5K), and it showed 4.56K. I replaced this. No dice.
OK.. so what else could it possibly be? AHA! Stray capacitance caused by the probe being connected! This theory I tested by inserting a 1200pf mica cap across the cathode resistor. Now, you would think that such a large capacitance would swamp the incoming signal from the LO.. BUT IT DIDN'T! Instead, Band 5 started working beautifully! From the bottom of the dial to the top, everything was loud and clear.
All right.. what did that do to the other bands? I ran though each and found that each was still working very well as it already had. Of course, the calibration is off, but I can't do a lot about that (and it was already off) until I get the rubber mounts in for the tuner and can properly set everything up. I suspect that it will be even better once calibrated and properly aligned.
Now, I'm going to see if I can still make this work with much smaller shunt caps, which should in theory improve the S/N ratio due to more LO injection.
I'm also going to try the same trick on my other 16b's and see if that solves the same issue for those. Would be great if it did.
Now, my question is, why should I even have to do this? I've replaced every component around the LO circuit in all of these radios, but this is the first one I've been able to make work below 14 Mhz on Band 5. Any ideas?
It was a complete accident. I went to connect my scope to the cathode of the 1st Detector so that I could see exactly when the LO dropped out on Band 5. Strange thing, as soon as I connected the scope probe, first to ground, then to the cathode, Band 5 came literally ROARING to life. I found that I could tune clear down to 10.5 MHz and all the way up to 23 MHz using my sig gen as a source (of course there were also all those pesky shortwave stations in there.)
So, I thought.. OK, it's radiation off the probe getting back to the LO. I tested this theory by putting my finger on the cathode. No dice.
Next, I thought.. well, it's not loaded properly. I tested the cathode resistor (5K), and it showed 4.56K. I replaced this. No dice.
OK.. so what else could it possibly be? AHA! Stray capacitance caused by the probe being connected! This theory I tested by inserting a 1200pf mica cap across the cathode resistor. Now, you would think that such a large capacitance would swamp the incoming signal from the LO.. BUT IT DIDN'T! Instead, Band 5 started working beautifully! From the bottom of the dial to the top, everything was loud and clear.
All right.. what did that do to the other bands? I ran though each and found that each was still working very well as it already had. Of course, the calibration is off, but I can't do a lot about that (and it was already off) until I get the rubber mounts in for the tuner and can properly set everything up. I suspect that it will be even better once calibrated and properly aligned.
Now, I'm going to see if I can still make this work with much smaller shunt caps, which should in theory improve the S/N ratio due to more LO injection.
I'm also going to try the same trick on my other 16b's and see if that solves the same issue for those. Would be great if it did.
Now, my question is, why should I even have to do this? I've replaced every component around the LO circuit in all of these radios, but this is the first one I've been able to make work below 14 Mhz on Band 5. Any ideas?
