Again, truly fantastic explanation sir!!! I'm a newb RF Telecommunications Technician and found this series on the receiver most excellent!!! Thank you so very much for taking time out of your day(s) to build, evaluate, video, edit and post your videos!
Thank you for the feedback and thank you for watching, Dane. My goal is to present everything in a way that would have made sense to me if it was the first time I heard it.
Can you elaborate on why there is only 3.5V on the cathode of the varactor diode? If that 7.4V is a DC bias, I don’t understand why the voltage drops across R29 and R30. Those two resistors, along with C26 and C27, look like low pass filters that should just pass the 7.4V DC coming from the discriminator. I am trying to troubleshoot the FM side of this radio myself and your videos have been a great reference. I have 7.48V out of the discriminator, 7.20V between R29/R30, and 6.94V at the cathode of the diode. Some drop, but not nearly as much as you observe. This is the first big discrepancy I’ve found in my test measurements versus the ones shown so far in your video series. My entire FM circuit is assembled in case that makes a difference, but it’s not obvious to me why it would. Any advice would be greatly appreciated. Thanks for the very informative content!
Hello Adam, Good question, I should have covered that in the video. The Fv1043 is a crusty old varactor that has a DC current leak, usually around 5uA. The designer took advantage of this to get the voltage from cathode to anode that he/she wanted. Todays varactors have leakage current in the nana amp range so this kind of voltage drop across those two 390K ohm resistors wouldn't occur.
I have made the same Observations. Taken into account the great explanation of the leak current my FV1043 behaves more like a standard diode. So it might be a counterfeit part ? Unfortunately i haven't found a source for the varactor diode here.
I had the same problem, I added a 1Mohm resistor between the cathode and ground (to add some "leak"), and got the desired voltage. However I am not sure whether it's working properly without an oscilloscope
I want to first thank Adam for bringing this up, the author for the explanation, and ak48disk for the solution. Adding that 1M resistor will bring the circuit back in line with the manual and these videos. I am so happy you all contributed, I was starting to assume I had a bad varactor, and in fact, bought replacements before I read this. That would not have helped and I most likely would have just left it as incorrect. Again, thanks to you all.
Very nice section by section explanation and theory. And I have read and heard several. Yours is among the best. However... image rejection. On my kit, it really stinks. I also get some bleed from adjacent stations. And the AFC doesn't seem to work the way I expect. I am used to AFC sticking solidly to a station when going up and down the dial, then popping into the next one with no dip or fade. I guess on this kit it's not the best but it does keep it in place. Yet I do get some imaging. I notice on some FM radios there is a coil at the input of the antenna. Would that help with image rejection? This one seem a bit too broadband in my mind.
![Electronic Circuitry Diagnosis And Fault Finding! [Repair]](http://i.ytimg.com/vi/LC7OjuYAOQE/mqdefault.jpg)
Again, truly fantastic explanation sir!!! I'm a newb RF Telecommunications Technician and found this series on the receiver most excellent!!!
Thank you so very much for taking time out of your day(s) to build, evaluate, video, edit and post your videos!
Thank you for the feedback and thank you for watching, Dane. My goal is to present everything in a way that would have made sense to me if it was the first time I heard it.
EXCELLENT PRESENTATION,YOU ARE THE BEST ON YOU TUBE.
Classical theory Sir really impressed by your way of explaining
Thank you for the lessons. Please do some on transmitters
I was giving that some thought lately. I will try to find one that is common and inexpensive so everyone can work on it at home.
Can you elaborate on why there is only 3.5V on the cathode of the varactor diode? If that 7.4V is a DC bias, I don’t understand why the voltage drops across R29 and R30. Those two resistors, along with C26 and C27, look like low pass filters that should just pass the 7.4V DC coming from the discriminator. I am trying to troubleshoot the FM side of this radio myself and your videos have been a great reference. I have 7.48V out of the discriminator, 7.20V between R29/R30, and 6.94V at the cathode of the diode. Some drop, but not nearly as much as you observe. This is the first big discrepancy I’ve found in my test measurements versus the ones shown so far in your video series. My entire FM circuit is assembled in case that makes a difference, but it’s not obvious to me why it would. Any advice would be greatly appreciated. Thanks for the very informative content!
Hello Adam, Good question, I should have covered that in the video. The Fv1043 is a crusty old varactor that has a DC current leak, usually around 5uA. The designer took advantage of this to get the voltage from cathode to anode that he/she wanted. Todays varactors have leakage current in the nana amp range so this kind of voltage drop across those two 390K ohm resistors wouldn't occur.
I have made the same Observations. Taken into account the great explanation of the leak current my FV1043 behaves more like a standard diode. So it might be a counterfeit part ? Unfortunately i haven't found a source for the varactor diode here.
I had the same problem, I added a 1Mohm resistor between the cathode and ground (to add some "leak"), and got the desired voltage. However I am not sure whether it's working properly without an oscilloscope
I want to first thank Adam for bringing this up, the author for the explanation, and ak48disk for the solution. Adding that 1M resistor will bring the circuit back in line with the manual and these videos. I am so happy you all contributed, I was starting to assume I had a bad varactor, and in fact, bought replacements before I read this. That would not have helped and I most likely would have just left it as incorrect. Again, thanks to you all.
Thanks a lot, the manual of this kit explained nothing of this.
Very nice section by section explanation and theory. And I have read and heard several. Yours is among the best. However... image rejection. On my kit, it really stinks. I also get some bleed from adjacent stations. And the AFC doesn't seem to work the way I expect. I am used to AFC sticking solidly to a station when going up and down the dial, then popping into the next one with no dip or fade. I guess on this kit it's not the best but it does keep it in place. Yet I do get some imaging. I notice on some FM radios there is a coil at the input of the antenna. Would that help with image rejection? This one seem a bit too broadband in my mind.