- October 20, 2008 at 4:14 am #7180Carl BlareParticipant
Total posts : 1540
It’s been awhile since this transmitter has been mentioned so I guess opening a new forum is appropriate. Being the owner of two SStran3000s I was, oddly, left wanting something more, given the fact the SStran is a perfect finished product which needs no “improving.” I wanted something to tinker with. I’d read about modulation and hum problems with the Ramsey, so it sounded like a candidate for experimentation. How perfect it is for that!
Once constructed from the well documented kit I connected the supplied 1-Amp 15 VDC wall-wart and a blasting hum came out of the radio. Yes, there was a hum problem. The two output transistors got frighteningly hot and even smelled. I began holding a series of talks with a technician at Ramsey who was absolutely wonderful in his frankness and patience in discussing things with me. He acknowledged it had hum and gave the opinion the power supply was the cause. I mentioned that I’d counted a large number of ground loops in the way the circuit card was laid out (the SStrans has absolutely no ground loops).
I rigged up a 2.5 Amp power supply variable from 0-25 VDC, added 4x .1 ceramic capacitors across all the rectifier diodes, which keeps them from carrying AM signal noise, and found exception to the manual’s claim of a voltage range of 9 – 15 volts. I found that at 6-volts the unit immediately begins excitation with a loud, clear, hum-free modulation, but weak power. At 7-volts the power is up, but modulation significantly down. Readjustment can almost recover that loud clear sound heard at 6-volts. Above 7-volts the power keeps rising but the modulation becomes muddy and suppressed. At 11-volts the output transistors begin heating.
Thinking maybe a bad standing-wave ratio from the short 3-meter antenna was clamping the modulation, I made a loading coil on a round oatmeal carton and there was some improvement in antenna matching.
The Ramsey AM25C is an ideal transmitter for experimentation and contains a clever circuit.October 20, 2008 at 5:40 am #16815radio8zGuest
Total posts : 45366
I have an AM25 (not sure about the C on the end) and it was my main unit for a while. I did quite a bit of experimenting with it and found it to be a good “tinker” unit.
I never had a hum problem but the modulation transistor did get pretty hot. I mounted it on an external heatsink. This works since the only signals on it are DC and audio and the RF is bypassed. The RF output FET doesn’t get hot enough to be a problem as mounted on the board.
Using a resistive load, I ran a series of tests and found that the output circuit is not sensitive to load from 10 to 100 ohms and the transmitter will drive most antennas with no problems. The input and output power does change with load with the maximum output power happening at 23 ohms resistive load.
The modulation does not go to 100% without distortion as observed on a scope, but is sufficient to give a decent audio signal with lower modulation.
Though I had no hum problems with the wall wart, I ran mine from a 13.8 VDC 3 amp switcher I had in my junk box.
I switched to the SSTRAN primarily because I found the final input power on the Ramsey greatly exceeded the 100 mW limit for part 15.219 operation and because the SSTRAN has the built in limiter/compressor and dual audio input with internal mixing for stereo sources. The measured final input power on my SSTRAN with a tuned antenna was 93 mW.
Here’s a link to a post I made some time back regarding the input power on the Ramsey:
If you use the search function for “AM25” you’ll find some more posts on this board.
NeilOctober 23, 2008 at 9:10 pm #16826Carl BlareGuest
Total posts : 45366
Thank you Neil/radio8z for the link to your previous findings on the AM25 transmitter. I’ve printed and filed the information because it tells significant facts about the unit.
Consider the circuit description, page 5, “…transistors (Q8 & Q9)…are biased to be operating in the linear region with a no signal output level OF EXACTLY ONE-HALF THE SUPPLY VOLTAGE as set by pot R23. Yet on page 19, ALIGNMENT PROCEDURE No. 7 specifically instructs setting the bias for 7.1 VDC. If this bias is to be EXACTLY ONE HALF of supply voltage, two-times 7.1 = 14.2 VDC. But the manual throughout specifies a supply voltage of 12 VDC, one-half of which would be 6 VDC. Yet the kit comes with a 15 VDC supply, one-half of which would be 7.5 VDC. You, Neil, used a power supply of 13.8 VDC, one-half of which would be 6.9 VDC (close to 7.1 VDC). I am operating mine at 6 VDC supply power, and in fact have the bias set at 3 VDC, which gives me the loudest, clearest modulation I can get, as I explained in my first post.
I’m curious to know what anyone thinks about this apparent confusion in the Ramsey Manual.October 24, 2008 at 7:25 pm #16827radio8zGuest
Total posts : 45366
You are digging farther into this than I did and that is good. What it means is that I have little to add yet am interested in your observations. My comments which follow are just meant as information as you ponder this.
One thought about the “half the supply voltage” comment in the manual. For amplifiers there are two load lines, the DC and AC load lines. These are usually different though they intersect at the Q point which is the 7.1 volt point in the manual. To get maximum undistorted AC output the Q point may have to be modified away from the intuitive value of 1/2 the supply.voltage in order to center it on the AC load line. Q8 and Q9 form a Darlington amplifier which serves both as a DC and an AC amplifier in that it supplies both the audio and DC power to Q3. If Q3 would not operate down to 0 volts then the minimum AC voltage would not be 0 volts but something higher. It could be the 7.1 volts accounts for this. The statement in the manual about 1/2 supply voltage doesn’t explain this.
On page 6 of the manual, at the top, it says “…a battery pack consisting of eight 1.5 volt cells, a 12 V sealed battery, or other external 12 V DC supply may be used.” So it would seem that 12 VDC is the intended supply voltage, yet all sealed batteries I have used operated at over 13 volts. Perhaps it is not that critical.
Another thing I find interesting about the design is that the bias voltage set by R23 comes from the power supply which would introduce any noise or hum from the power supply into the audio signal going through U1. Since low cost wall warts are usually not regulated and have ripple this seems to be a weakness.
I also noticed that with the bias voltage reduce to 4.7 V (the lowest I could adjust this) the modulation improved. Maybe operating on 6 volts supply not only improves the modulation but brings the unit into the 100 mW compliance as well. I have not tried this.
The RF filter after Q3 seems to be very effective in that I have never observed other than a good sine wave at the carrier frequency from this unit. The modulation envelope (using a sine wave 800 Hz audio test tone) does distort for higher modulation percentages. It would be useful to know if lowering the supply to 6 volts improves this.
Hope these comments help.
NeilOctober 28, 2008 at 1:50 am #16843Carl BlareGuest
Total posts : 45366
Radio8z you have introduced the difference between AC and DC within the Ramsey circuit. I was only looking at DC. But, of course, the main output RF signal is AC, as is the audio input, and certain stages in-between. While I ponder this I’ll change the subject a little bit.
At sundown a station in the background beats against my signal and I find that the variable C16 is not able to perfectly zero the difference between my transmitter and that other station, resulting in a pulsing of several Hz per second. This adds a grinding sound to the music or voice modulation that could be mistaken for distortion, but it is a frequency beat. During other times the beat goes away because the distant station fades out.
When I have my SStran 1640KHz station turned on, using the windowframe antenna I’ve written about in another post, the audio on that transmitter (when it is different than the audio on the Ramsey at 1550KHz) gets back-modulated into the Ramsey through the antenna. When the same audio is fed to both transmitters there is no difficulty. The antennas are 12-feet apart.
The input to the final stage using 6vdc is 118mW. By the way, I have bypassed R33 & R34 (total 20-ohms) so as not to attenuate the modulation.
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