Post by HighMountainRadio on Mar 17, 2018 19:51:18 GMT -6
Greetings From High Mountain Radio !
The age-old question that has been posed and pondered by many in this community has been, 'How do I modify my Chinese manufactured FM broadcast transmitter in order to set it for 75us pre-emphasis ?"
After doing extensive research, I was able to finally determine exactly how to modify basically most every Chinese manufactured FM transmitter for 75us pre-emphasis, the USA broadcast standard ! Without having the luxury of actually viewing the circuit boards of many available FM broadcast transmitters manufactured in China, I can assume with a high degree of confidence that in all likelihood they all utilize the commonly utilized BH1414K Audio Link integrated circuit in their designs. Since the vast majority of these Chinese manufactured transmitters are set at the factory for 50us pre-emphasis, the Asia and European broadcast standard, they of course do not match the 75us pre-emphasis USA broadcast standard and although they will work and sound pretty decent despite this fact, they can be modified to sound even better with a careful, steady hand, the correct type of soldering iron, a good magnifying glass and patience !
Based on the few Chinese manufactured FM broadcast transmitters I have been able to physically examine in my shop, the BH1414K Audio Link IC is typically housed within a metal RF shield enclosure somewhere on the transmitter's circuit board. A small amount of detective work will reveal just exactly where this is located on your FM transmitter. It will be necessary to access the area near the BH1414K IC where the RC components (resistor and capacitor) are located in order to perform the magic.
Once accessed, the components that will need to be changed are connected to pins 5 (Left Channel Audio Input) and 7 (Right Channel Audio Input) of the BH1414K IC. The op-amp and R1 are inside the IC. R1,2 and C3,4 are outside of the IC on the circuit board. Depending on the board manufacturer the actual two capacitors that need to be changed may be marked on the circuit board as something other than C3 and C4 but careful observation will reveal which two capacitors they are. Reference the included schematic diagram picture to determine how they are connected within the circuit. Two 1000 pf (picofarad) capacitors will need to be removed and replaced with 1500 pf (picofarad) value capacitors in their respective locations on the circuit board. These capacitors are of the surface mount variety. For a pre-emphasis value of 50us, 1000 pf value capacitors are utilized, for 75us pre-emphasis, 1500 pf value capacitors are utilized. I have included a few images to aid in component location and circuit description.
The pre-emphasis treble frequencies boost capacitor is 1000pF for 50us and is 1500pF for 75us. The audio input coupling capacitor determines which low frequencies are passed and is 1uF polarized for the BH1414K. Actually ANY values for R1,R2 and C3,C4 can be utilized as long as their values equate to the correct value required for 75us pre-emphasis.
E.G., Values utilized on circuit board are R1,R2=51K Ohms, C3,C4=1500pf for 75us pre-emphasis or 1000pf for 50us pre-emphasis. The formula used for calculation is: R X C = T (51,000 X .0015 = 76.5 (Close enough to 75us). Multiplying 51000 X .001 = 51 (Close enough to 50us)
***NOTE:If your FM transmitter utilizes the BH1417 Audio Link IC instead of the BH1414K, it appears that you may be out of luck since the necessary components that will need to be altered in value are contained within the IC itself and can not be accessed.
The information provided should prove helpful in understanding the voodoo that occurs in that small FM transmitter !
***NOTE: I cannot take complete credit or any liability for the use of this information, it is completely at your own risk and peril if things don't go quite right during surgery. The information is accurate to the best of my knowledge and independent research conducted. Credit and kudos go to new member Bruce for two included images contained in this thread, I 'pirated' them elsewhere in keeping with the true spirit and nature of this forum.
73... Spooky...
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