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"Burn up" radios or cause distortion of the output signal |
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By jumping this diode you will get approximately .7 volts more to the final, resulting in more power output. However this diode is ABSOLUTELY NECESSARY to keep spike voltages coming from the modulation transformer from reaching and blowing the Final Amplifier. At least 7 of every 10 Cobras and Unidens that come into my shop with blown finals have this diode jumpered! In order to get more voltage to the final, some "techs" parallel the spike diode with one or two more of the same number on the bottom of the board. However, a silicon junction drops approximately .7 volts across itself and the voltage across any number of diodes in parallel will still be approximately .7 volts. Therefore the addition of a second diode, or more, is of no use in gaining a higher voltage. |
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Anyone that teaches that "PowerMod Kits" are not good for the radio and that they will burn them up has no understanding of the How or the Why of them. We all know that "KITS" is not a good term for them as they are not kits. We need to quit calling them a kit when in reality they are simply a "circuit conversion". And A Damn necessary conversion if we are to cause the radio to run cooler and have a higher percentage of modulation without clipping! However the modulation conversion that is being done to the Galaxy Radios and most of the other Ranger-built radios using two resistors (1.2k and 4.7k) along with two 1N4001 diodes to replace R240 needs to be done away with. It is causing considerable distortion of both the positive and negative audio peaks and the conversion can be accomplished in another way that will not have any bad effects in the following way: Prepare a 1N4001 diode by cutting the wire on the cathode end to 1/16 inch long. Do the same thing to one end of a 100 ohm 1/8 watt resistor. Carefully solder the two short ends together. Now bend this combination into a hairpin shape and solder this in place of C189 with the cathode end of the diode pointing to the back of the radio. Solder C189 across the same pads on the bottom of the board being careful to lay it flat enough to avoid being hit by the cover. Now the radio can have the limiting circuit disabled without causing clipping. The circuit should be tuned for maximum carrier attainable before clipping of the positive peaks. The negative peaks will be compressed at approximately the 98% modulation level due to the inverse curve of the 1N4001 diode rather than clipping from over-driving. |
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When the jumper JP36 is taken out and replaced with an electrolytic capacitor with a 62-82 ohm resistor in parallel with it on the trace-side of the board, this will lower the Dead-key while still maintaining the same peaks during modulation that were attainable with JP36 in place. However, some techs offer a variable carrier front-panel control, instead of the resistor, which requires long wires to make the connection. This is not in good practice for two reasons. The pot will almost always burn out leaving the customer with either no output during the transmit mode or it will only work if it is turned wide open. Also the wires required to reach from the capacitor to the front panel mounted pot is a source of possible Audio/RF feedback. If the customer insists on variable carrier power, the wiser choice is to use an am regulator type transistor in place of the resistor with a front panel mounted variable bias control that will adjust the collector current therefore varying the voltage to the driver and final which will in turn set the carrier level. There is very little chance of audio/RF feedback using this method. |
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