OK, I could not resist… I ordered a Ramsey AM25C and put it together over three lunch hour sessions. There were no major problems assembling the kit. The coils were easier to wind than I expected.
When I turned it on, there was no smoke, but there was no output either… until I found I had made an error setting the DIP switches, and corrected that. Then I got a strong carrier on 1500, accompanied by a massive amount of hum! Keeping the unit powered on for very short intervals, I made all of the adjustments according to the manual. When I was finished, the hum was still very strong, and the output power measured 23.9 dBm into 50 Ohms (over 200 mW)! I also noticed that the regulator transistor (Q9) and the driver transistor (Q5) were both hot.
Thinking to reduce the power a bit, I padded down the drive into Q5 by putting a 1000 pF capacitor across R16. The power went down a few dB, and the hum dropped significantly. I bumped that up to 1500 pF and the power dropped to 19 dBm. The hum is essentially gone, and Q5 is a lot cooler now. The 2nd harmonic is down just shy of 40 dB. I do notice some close-in spurs at around +/- 250 kHz, which I will have to investigate. They are pretty far down, but I would like to eliminate them. I am guessing that they may be caused by instability in the audio circuitry.
The overpowered ac adapter supplied with the kit appears to be the reason for the hot regulator transistor. Even after padding down the RF drive, I am still measuring almost 18Vdc into the board. The adapter is rated at 1 Amp at 15 Vdc, and since it is unregulated, the voltage goes even higher when the current drain is less. So I think the problem here is not that Ramsey supplied a cheap adapter; rather, it is over-rated for this application.
I like the 50 Ohm output. It simplifies designing the matching network for a base-loaded antenna, and making measurements on the transmitter’s output. The audio section of this unit doesn’t look like much to write home about. I have not made any measurements of it with modulation yet, but have read that it’s difficult to get it to hit 100%. I’ll write with more details after further tests are completed.
Ramsey AM transmitter
Weak,
You might be interested in reading the contents of these two links. I have a Ramsey AM-25 and I don’t know about the “C” model.
http://www.ramseyelectronics.com/cgi-bin/ultimate/ultimatebb.cgi?ubb=get_topic;f=1;t=001765
http://www.ramseyelectronics.com/cgi-bin/ultimate/ultimatebb.cgi?ubb=get_topic;f=1;t=001794
EDIT***OOPS! I messed up and posted one of the wrong links, but I trust you will still find it is a good read. The one I meant to post is:
http://www.ramseyelectronics.com/cgi-bin/ultimate/ultimatebb.cgi?ubb=get_topic;f=1;t=001871
This will give you some hints and I trust good ideas about the heat situation.****end edit and I hope it is fixed,
I have done some pretty extensive testing on my Ramsey unit and have a lot of data regarding power input and output and load Z on this unit. If you have any specific questions about this and I can provide some data to save you time and trouble, let me know.
Also, hum has never been a problem with this unit, but I must confess that I am using a 13.8V 1.5 switcher for power and I have mounted my unit in a metal box. I don’t recall any hum problems when I used the stock plastic box and wall wart.
Neil
Thanks
Neil,
I see that the engineers at Ramsey are a bit touchy about compliance! To their defense, if you attach a few feet of wire to the output, the unit will only go a few feet and will be in compliance on a field strength basis. However, it won’t be terminated properly in that case, and the output filter won’t work properly since it is designed for nominal 50 ohm loads.
I would rather adjust mine so it is in compliance on a power input basis. And my initial observations would indicate that if you do that, the supplied ac adapter should probably be OK, although I’m still unhappy about the high voltage. I think I will investigate the switcher that you recommended, because I prefer things to run cool.
The SSTRAN, by contrast, runs stone cold and Phil even recommends that you just leave it on 24/7 since it only draws about 2 watts. Even in these days of high electric rates, that’s not going to amount to much. Probably the computer running the automation will cost a lot more to run than the transmitter!
WEAK-AM
Classical Music and More!
AM-25 load
Weak,
Yes, that post about compliance did not elicit the information I was seeking and I guess it will have to stand on its own.
Again, my unit is a AM-25 and I don’t know anything about a 25C so yours may not be the same as mine.
I ran a series of experiments using various resistive loads and found the maximum power occurred with a load of 24 ohms. This is not bad, since if one is using a base loaded antenna over buried radials, the expected feedpoint impedance at antenna resonance is not far from this number.
Here’s the data I collected:
VDC at R34Top = 7.12V.
VDC at R34Bottom = 4.47V.
VDC at Q3Drain = 1.78V.
Load Resistance at antenna connection = 24.3 ohms.
Vrms across load = 1.87V.
Load power = 144 mW.
Input power to final amp. = 470 mW.
Efficiency = 31% (peak power out and also peak efficiency).
I used loads from open to 10.6 ohms to find this peak.
I checked the schematic for my unit and I see no on board regulation of the supply voltage which would indicate that the power supply to this unit could very well affect the operation of the device.
Hope this is of interst to you.
Neil
No on-board regulation!
Closer inspection of the (very difficult to read) schematic for the AM-25C revealed that there is no on-board regulator even in the current “C” revision of the transmitter! So, you are quite right that the voltage applied to the board will likely have a significant effect on output power as well as power dissipation. I will need to run more tests to determine the optimum voltage for use.
I am speculating that with 12V applied to the board, there may be no need to pad down the RF drive to get the transmitter operating at a more reasonable output. I’ll report my findings as soon as I find time to check this out.
WEAK-AM
Classical Music and More!
AM-25C power, etc.
WEAK,
You probably noticed that the final amp. drain voltage is set by U1A operating through the darlington pair. U1A DC setpoint is controlled by R23, so it appears that if the unit is powered by a particular power supply and R23 is adjusted per the manual that this will set the final amp. input power rather than the supply voltage. You could use R23 to throttle back the power.
I have found that the setting of R23 not only affects the signal power but also affects the modulation. You may find, as I did, that changing R23 will change the quality of the modulation. In my unit, the best modulation was not at the setting from the manual.
The scheme using R23 and U1 to bias the final amp leaves the amplifier wide open to noise on the power supply since this noise will go right through U1 to the final drain. Not the smartest way to do this is it?
My unit has been retired from service for a couple of years so I don’t do much with it anymore and my memory about some of the quirks of this unit are dulled, but I hope with what I posted you get some ideas.
Neil
Thanks!
Neil, thanks for your comments and observations. To me, it looks like some aspects of the design are not that well thought out, while others (mainly in the RF section) are good. Although I don’t think 31% efficiency is very exciting (even though that seems to be about what the SSTRAN achieves). It is possible to get much better efficiency than that (over 90%), and I have done it.
I will continue my testing as soon as I can get time to return to the bench. I intend to optimize for the best audio and then do what I have to do to bring the power level into compliance. I know that those two are interrelated, so it will probably be an iterative process. For this unit, I would expect to achieve at least 60% efficiency when properly optimized for the desired operating power. There’s no reason to accept anything less.
WEAK-AM
Classical Music and More!