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February 16, 1992
LPB AM-5 TRANSMITTER MODIFICATIONS
PRE-EMPHASIS Install small 103k caps in series with modulator tubes (between pins 7 and GND.) Ground pin 2 on both tubes. Change resistor between pin 10 and modulator xformer pins 3,4 to 100 ohm. Change resistor between pin 9 and GND to 33 ohm.
RF TUBE Bias resistor in oscillator between pin 7 and orange wire should be 2k ohm. Move orange wire and all wires on secondary side of modulation transformer to pin 12 (was on pins 9,10) Move green wire on modulation transformer to pin 12 (was on pin 7) On RF tube, pin 10 (grid) remove resistor (was 15k ohm) Install 210 ohm to ground from pin 10. Connect wire from pin 10 to .2 mfd cap. Connect cap to pin 7 on modulation transformer. Install resistor (modulation bias) 22.2k ohm across cap connected to modulation transformer.
CLIPPER ASYMMETRICAL Install diode (ok to use small silicone generic type) on modulator tubes across pins 8 and GND. The diodes go in OPPOSITE DIRECTIONS since the tubes are 180 out of phase. On V1, cathode of diode goes to pin 8. On V2, cathode of diode goes to GND.
TUNE UP Tune Pi network for best combination of RF output AND frequency response. (This network will attenuate audio above 4kHz if set to maximum output) Use low ohm settings on matching box. 1 ohm setting works great. (because the lower settings cause less impedance change at the 50 ohm end. Also attenuates hum slightly better) Check for loose connections on Fuse holders, all solder joints. (This is a very important step. I have found loose connections too numerous to mention due to heat and dust) Check all capacitors. They are frequently bad from heat fatigue. Check capacitors in matching box too. Install cooling boxer type fan in transmitter. Run at all times. Start with dead carrier and tune all floors for maximum penetration. (A signal strength meter on a receiver is almost mandatory) After getting RF into building, run audio into transmitter and (make sure it is clean first) adjust modulation level just below point of negative peaks as seen on oscilloscope. At this point you should have positive peaks of at least 100% (I peaked out at an average peak at 130% with a few 150% at times. Check frequency response with tone generator sweep and scope pattern. (Should show slight pre-emphasis at 3-6kHz.)
HUM NULL Install a .1mfd cap to diode junction on power supply. (to get ripple minus DC) Connect resistor (choose value upon building conditions) to pin 8 on one of the modulator tubes. One tube will ADD to the hum (in phase) the other will null it (out of phase)
HISTORY I have spent 5 years on this and have heard of MANY solutions all of which were just impractical due to the restrictions of the buildings and beaurocrocy. This isn't the best solution, but it does a fair job of nulling out 120Hz hum that is created when the transmitter is loaded by the AC wiring. This results since the load to the transmitter changes twice per AC cycle. I have spent 25 hours re-designing the 1971 vintage tube transmitter that I have. Knowing so much more about electronics now, I am able to successfully modify the existing transmitter to bring it close to broadcast standards. Untill now, the old transmitter has been plagued with low modulation, distortion, hum, and a very bad frequency response. I was able to correct each of these problems by tackling them one at a time. Before I started my modifications, I replaced all bad components. There were 3 capacitors which had shorted due to age and heat fatigue. There were also a few resistors which had changed value slightly. All 3 tubes checked out to be in brand new shape. I recall we had replaced these tubes once a year. Here is what I did.
Modulation - The transmitters are now equipped with a modulation clipper. Two diodes in the modulator are forward biased and thus clip any level at that point exceeding .7 volts. These diodes are arranged to only clip NEGATIVE peaks. This way, the modulator can be run higher and there is much less distortion on the output carrier.
Distortion - All 3 tubes have undergone radical changes in screen grid bias. The modulator tubes are now running class A (they were class B). The oscillators bias is lower and thus the output is no longer clipped. This along with the new resistors and capacitors greatly improved the output of the modulator section. In addition, the final tube which was running class C was originally plate modulated. It is now grid modulated. Though this reduced the power output originally, by altering the bias and installing the clipping diodes in the modulator, I have regained all the original power. The distortion level is about 1/4th that of what it was. One can clearly hear the lower distortion as well as improved frequency response. The output of the transmitter was originally limited to 200-2.5Khz. It is now running 50-8Khz + or - 2dB. The difference is like night and day.
Hum - There has always been a problem of hum in the carrier. Some have explained that that's just the nature of carrier current. While there are better impedance matching networks that interface the transmitter to the buildings AC wiring, these units cost money and I know that WECB is on a tight budget. I have therefore used a unique way to greatly cut down on the hum. Instead of further attenuating the hum passed on by the matching network, I have decided to inject a hum of my own into the carrier. I inject it 180 degrees out of phase with that of the incoming hum. When the two meet if they are of same frequency and amplitude, they cancel each other out. I have found there to be two hums in the carrier. They are 60 Hz and 120 Hz. The 120Hz tone is much more noticeable on most radios and I have decided to tackle it over the 60Hz. 120Hz is already inside the transmitter at the output of the bridge rectifier in the power supply. I simply sample some of the ripple and inject it into the modulator and it is then superimposed on the carrier until it meets the incoming hum.
Frequency response - I have also installed some high pass filters in the modulator. These filters act to pre-emphasize the audio to overcome the de-emphasis of the receivers. This transmitter now sounds almost as good as a commercial broadcast station. I have compared our audio to that of WEEI, WRKO, WHDH etc. We sound every bit as loud and clear as the big guys. With the exception of WBZ. (possibly due to their Stereo status requiring higher levels of tolerance)
For further information; Greg Weremey - WECB General Manager Emerson College Boston, MA. Class of 1985 Electronic Service Professionals 13 Kildee Rd Harwich Port, MA 02646 (508) 432-8831
02/11/92
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