Many thanks Leapfrog....i guess my mind is getting a little feeble with age! I have a tendency to think in auto terms, but it pretty much shakes out the same. Auto coil no more than a step up transformer.........12 volts in, however many volts out depending on wire size & number of turns. Bias voltage on a diode is about .6 volts, so running some in series would create a voltage drop to whereever you wanted to be.....am I pretty much in step so far?? Like i say, dont have much of an rf background, and am really learning a lot here using the tools i already have. So heres maybe a stupid question....are the transformers on the collector leg or the emitter. My simple mind sez mike switches relay, relay works base leg, transistor controls power out to antenna, modulated. Yeah, maybe not quite right, but trying to relate. Too much output from radio on base of amp transistor and all the magic smoke gets out. Am really gonna make an effort to get Lou's (?) book and start my rf education, i hate not knowing. My oldest phrase to my auto students for 10 years was "you cant fix it if you dont knoe how it works"
The Cobra 25 and Cobra 29 do not use a relay to switch TX and RX they use a transistor/fet coniguration instead.
Most amplifiers on the other hand that are solid state will use a RF sensing circuit to trigger the relay automatically so you don't need to run any special key up wire between the amplifier or the radio it just senses when a carrier is sent into the amplifier through the coax jumper, the relay automatically switches the signal path using the RF sensing circuit in depending on whether or not an input signal is coming into the amplifier. ( the relay is triggered automatically to switch the signal path )
*An idea would be to switch diodes in to lower the carrier, in a pinch to run an amplifier for an experiment on the dummy load / test bench.*
For every diode that's in the circuit between the transformer and the anti-spiking diode lowers the voltage on by 0.7 volts but when you apply (audio)forward voltage
and the diodes conduct
audio passes (gets added to the carrier) most of the maximum voltage can still "flow through".
There is little to no forward voltage drop with the diodes in line when they are "conducting"
and the affect is you modulate the lowered carrier the voltage jumps up based on the amount of audio put in.
So you will still see most of your high peak numbers (but you lose some p.e.p. wats) some guy use diode mod to run amplifiers and the possible (over)modulation can be "problematic" so you really need an oscilloscope to make sure that you're not creating a spatter box radio in the final coniguration.
(it works though, and can be used on any transformer modulated radio to run an amp in a pinch)
You will need the following:
* Several Diodes ( I use the same type that's present in the radio already (get the same kind), if it's a 1N4001 I use those, sometimes it's 1N4003 )
* Scrap wires long enough (2 or more dependin on the # of switch positions) to reach the switch.
* Solder, plus solder iron
* Patience & tiny bit of skill to wire and solder up the diodes inline and the wires/switch together in the right configuration ( you could read a picture on how to wire the switch ) depending on where you put the wires on the string of diodes and how many are in line on any given switch position is going to affect your carrier so add more diodes in that same switch position line to lower the carrier for that position.
This has been called:
* the diode super mod
* the dial-a-watt mod
* the super modulation mod
Some of these names have been applied to other methods.
To lower the carrier in an a.m. only Cobra radio my preferred method
is a variable dead key with the tip120 transistor and add a potentiometer
(use delta tune spot and just tape that pot off and tucked away inside the radio.)
Re-place the stock knob on the new potentiometer
so you get a knob on the front of the radio that looks original and it lets you set your carrier whenever you need to very easily without taking the lids off.
but like I said you have to check your (modulation) w/ an "RF sampler" on the oscilloscope if you want to make sure it's not a splatter box final tune you can try adjusting the AMC variable resistor, and L10 to clean it up, also you get to see how the microphone gain adjustment can affect your output waveform when you use these methods to lower the carrier.
Best Regards
-LeapFrog
