Improving the Ramsey AM25C
Posted on October 18, 2007
There are basically two issues with the Ramsey AM25C. The first is that the included Wal-Wart ™ power supply is unregulated and delivers WAY too much voltage to the unit. This is because it is designed to supply 15 Vdc at 1 Amp, per the nameplate label (on my unit, anyway). I have read criticisms of Ramsey elsewhere that the hum problem many users encounter is caused by a “cheap, underpowered supply”, when actually the opposite is true!
The result is that the power supply delivers too much voltage to the transmitter, causing several bad things to happen. First, the on-board modulator transistor runs extremely hot because the unit draws excessive current when operated at this abnormally high input voltage. As a matter of fact, I think that if you ran it continuously it would eventually burn the PCB (I admit I did not try this). Second, the RF power input is WAY too high. Besides being out of compliance with Part 15, this screws up the modulation. Third, since the power supply is unregulated, it introduces hum on the carrier. The amplitude of the hum is exacerbated by the high current drain.
So the first thing you need to do is power the transmitter from a filtered, regulated 12V power supply. This will completely eliminate the hum, stop the regulator from overheating, and drop the power input into the proper range for 100 mW operation. I really think that for this transmitter, a regulated supply is an absolute necessity. Otherwise, you will not be able to get rid of the hum completely. I noticed that on my unit, the power input was still a little higher than 100 mW, and Q5 was overheating (actually it was too hot to touch). Therefore I needed to reduce the RF drive level slightly. I padded down the drive into Q5 by putting a 1000 pF capacitor across R16. You will probably need to determine the exact value experimentally, because it will depend on your power supply voltage and also to some extent on your operating frequency.
Once you make these changes, you should be able to achieve reasonably clean modulation at the proper 100 mW power input level, with no hum and no overheating (of anything on the board). This design does not support positive modulation greater than 100%, but you can still have a very “loud” and “punchy” sound if you use a good audio processing chain with it.
One thing I do like about the Ramsey is that the RF output is well filtered and that it is designed for a nominal 50 Ohm load. So, you can easily match this to a base loaded shortened monopole antenna– no tricks required. This should work fine with the “Antenna Guy” coil antenna. You can also easily measure the RF output power.
I’d like to call your attention to a minor indiscretion on the part of Ramsey. In the manual, they seem to confuse input power with output power. Note that FCC rules impose a limit of 100 mW input power to the final amplifier– not 100 mW output power.