It has previously been reported that the Ramsey AM-25 AM transmitter when adjusted according to the manual exceeds the final amplifier power input limit for part 15.219 and produces audio distortion above about 80% modulation. A fix is presented here.
(For background see posts http://www.part15.us/node/938 and http://betatest.part15.us/node/1896)
By permanently shorting across R29 and adjusting the voltage at the R-34 test point using R 23 to approximately 3.33 VDC the power input to the final amplifier will be 100 mW. This is affected by the antenna so the setting should be made with the antenna attached and may differ according to the particular antenna used. By measuring the power input a proper value for the test point voltage can be found for a particular installation.
To meaure Pin, measure the DC voltage from Q3 drain to source. Then measure the voltage across R-34. The input power is (Q3 drain to source V) x (V across R34/10).
With a 50 ohm dummy load antenna this setting produces practically distortion free modulation up to 100%. Distortion appears though for loads above about 100 ohms.
Neil
Thank you Neil. I will do this mod to my AM25 and I appreciate the info.
I'm a little bit confused by everything because it's past sleep time, but with fresh morning coffee the instructions will come clear.
I have previously reported observing distortion on the modulation envelope of the AM-25 output and the nature of the distortion has baffled me. With a resistive load, the voltage observed on a scope at the transmitter output appears normal with little or no distortion however when connected to an antenna there is distortion. It turns out this is a MEASUREMENT ARTIFACT and is not actually distortion.
Here's the rest of the story. I have successfully constructed and calibrated a RF current transformer which allows me to display the antenna current on the scope. I noticed that the voltage waveform was severely distorted yet the current waveform was normal. Since the antenna is linear this cannot be accurate. It needs mentioning that the received audio sounded normal.
Here's the answer: The drain of the final amplifier is connected to the low pass output filter through a .1 uF capacitor (C1) and the filter passes DC. The reactance of C1 at 1 KHz is 1500 ohms and since the antenna has no DC path to ground what is seen on the scope is the sum of the audio at the drain and the RF which passes through the filter. A dummy load of 50 ohms used for testing effectively shunts the audio to ground and the RF looks normal but with the antenna and the 1 Mohm resistive load of the scope input audio is displayed with the RF.
It is my fault for not catching this sooner but I was applying standard testing procedures for an AM transmitter and most transmitters have a protective choke from the antenna output to ground and a band pass RF filter rather than a filter which passes DC or audio frequencies.
So, the lesson is to be very careful in interpreting measurements and don't assume anything. I was reminded of this after spending a lot of effort tracking this down.
I am also very encouraged with the performance of the Ramsey AM-25 now that I have lowered the input power to 100 mW and have managed to get my antenna system to operate at resonance. The signal looks and sounds very good and the power transistors which have a reputation of running hot now run cold to the touch.
The real power delivered to the resonant antenna system is surprisingly (to me anyway) high and I will report the numbers when I have had a chance to assure they are accurate (not going to assume again).
Neil