had the infamous 18K resistor flameout in my ranger, so, I pried the vfo apart, including removing the front panel cuz I wanted to get rid of the OA2 and put a Zener in for better voltage regulation.
I installed an 18K 5 watt resistor under the chassis, and then replaced the OA2 with the Zener. Put everything back together and when I started to recalibrate, the trouble started.
7 mhz calibrated fine, spot on. But when trying to calibrate for 160/75 metere, no matter how I try, I cannot get the low end the get closer than 75kcs above the low marker, 3.5khz.
What could have changed. Before the resistor flameout this rig was spot on calibrated. Well, as close as a 50 year old boatanchor can be gotten.
4.0 mhz comes in perfect, but for the life of my I can't get the danged lower band edge to drop beloe 3.575 or there about.
I'm using an IFR 1500 service monitor.
Any suggestions fellers? Thanks in advance - W5SUM Ronnie
I am not saying this is your problem concerning the calibration but the 18k resistor may not be the correct value for the Zener diode you used to replace the VR tube.
Look up the zener and find out whats its IZT current is based on the specification of the zener your using. This is the current that when flowing through the zener will produce the rated voltage of the zener including the tolerance voltage of the zener. The second current you need to know is the load current that your pulling from the zener circuit (VFO). Take the Zener IZT current and add to that the load current of the VFO circuit. That is the current that the series resistor feeding the zener needs to supply. To calculate the resistor value, take the source voltage feeding the circuit (under load) and subtract the zener voltage and then calculate the resistor value based on the total current which is the determine from my comments above. As the zener get closer to its knee, it will produce some great wide band noise components. If you bias it at the IZT point and make sure it stays at that point then the noise products are minimized. I usually use a .01, .1 and 1ufd parallel combination across the zener. As the load current changes from the the VFO the bias point on the zener will change. Based on the static and dynamic impedance of the zener, a small shift in the zener voltage will occur.
Zeners make great replacement for VR tubes but some folks don't go to the trouble of doing the calculations. Once you know all the currents you can do your power calculations to assure good circuit de-rating.
Joe, W3GMS
