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#1
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MRX-40 modification
Hello OM,
I have build the MRX-40 form a kit many years ago. It work fine but ... Now I would like to make a small modification to this receiver. I would like to replace the tuning capacitor with some kind of diode. I manage to add a small potentiometer with the shaft getting outside and I remove the initial capacitor. Now I wonder how and what kind of diode I could use. I dont have also any idea about what to expect in the range of tuning. May be someone else had the same idea Any help is will be greatly appraciate. Happy day 72 de Jean (VE2GHI) |
#2
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MRX-40 modification
Reposted.
wrote in message ups.com... Hello OM, I have build the MRX-40 form a kit many years ago. It work fine but ... Now I would like to make a small modification to this receiver. I would like to replace the tuning capacitor with some kind of diode. I manage to add a small potentiometer with the shaft getting outside and I remove the initial capacitor. Now I wonder how and what kind of diode I could use. I dont have also any idea about what to expect in the range of tuning. May be someone else had the same idea Any help is will be greatly appraciate. Happy day 72 de Jean (VE2GHI) Try VU2UPX's excellent circuit: http://www.qsl.net/vu2upx/Projects/dc_rx.htm This uses varicaps for tuning in a NE602 based receiver (although this is for 20m). Also http://www.hanssummers.com/radio/varicap/index.htm for G0UPL's testing of several common diodes as varicaps. Red LED's came out top! Does anyone else have info on using them as varicaps for tuning? 73s Klop |
#3
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MRX-40 modification
On Sat, 2 Sep 2006 11:52:41 +0800, "Klop"
wrote: Also http://www.hanssummers.com/radio/varicap/index.htm for G0UPL's testing of several common diodes as varicaps. Red LED's came out top! Does anyone else have info on using them as varicaps for tuning? When using a semiconductor in a transparent package, you should remember that the ambient light will generate a voltage across a reverse biased junction. When using a high resistance potentiometer to generate the tuning voltage, the voltage generated by the ambient light might upset the voltage across the diode and hence, the capacitance (and resonant frequency) varies with the ambient light level. At least keep the ambient light out of any diode with a transparent package, when used in a high impedance reverse biased circuit. Paul OH3LWR |
#4
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MRX-40 modification
Also
http://www.hanssummers.com/radio/varicap/index.htm for G0UPL's testing of several common diodes as varicaps. Red LED's came out top! Does anyone else have info on using them as varicaps for tuning? When using a semiconductor in a transparent package, you should remember that the ambient light will generate a voltage across a reverse biased junction. When using a high resistance potentiometer to generate the tuning voltage, the voltage generated by the ambient light might upset the voltage across the diode and hence, the capacitance (and resonant frequency) varies with the ambient light level. And, if the ambient light was from a mains-powered bulb (fluorescents would be worse) there would be a significant amount of 50 or 60 Hz light-flicker energy. This might be sufficient to inject hum into the signal... flicking light modulates junction voltage, which modulates junction capacitance, which modulates the RF oscillator frequency. So, the LED-varicap-tuned receiver might be both drifty, and hum-prone, if the LED were exposed to light. At least keep the ambient light out of any diode with a transparent package, when used in a high impedance reverse biased circuit. Yup. A dab of black paint would be a good idea. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#5
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MRX-40 modification
Klop wrote:
. . . Also http://www.hanssummers.com/radio/varicap/index.htm for G0UPL's testing of several common diodes as varicaps. Red LED's came out top! Does anyone else have info on using them as varicaps for tuning? It's too bad he didn't think of trying zener diodes. By using zeners, you can choose a wide range of capacitances -- low voltage zeners have the most C and higher voltage zeners have less, high power zeners more than low power ones. I've used them for decades for such things as RIT adjustment. A reverse-biased transistor emitter-base junction is also basically a zener, with typical breakdown voltage of around 5 volts, so this works fine as a varicap when a fairly large amount of C is needed. Any reverse biased diode will behave like a varicap, although as G0UPL found out, small signal diodes (intentionally) have a very small amount of C. The diodes sold as varicaps are doped to provide a particular and repeatable C-vs-V characteristic, but otherwise are the same as other diodes. The real problem with using varicaps of any kind for tuning is the relatively high and nonlinear temperature coefficient. Roy Lewallen, W7EL |
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