View Full Version : FM25A modifications
09-07-2001, 12:41 AM
I am new to this board...great idea. I would
like to add some info on my experience
with the FM25A transmitter. The audio is
great!, but I have found there is some
problem with the RF getting back into the
circuit which causes hum and interference.
I tried wrapping the unit in foil (grounded
to the audio input) and this really helped.
I then replaced the plastic box and mounted
the circuit board in a metal box. Fantastic
improvement! There is barely any audible
noise now even when listing on headphones.
I also have found over the years that the
RCA pin jacks are not reliable for RF. I
used a BNC connector for the antenna on the
new box. SO239 will also work. Hope this
info is of use to the readers.
Mounting the FM25A in a metal case does work wonders for reducing, or in my experience, eliminating hum.
While some metal cases are pretty fancy, and pretty expensive [such as some of the fancier Hammond cases] and rack mount cabinets, there are some decent cabinets available at good prices. Radio Shack used to carry a steel cabinet - black top and sides with a beige bottom, front and back. One of these cases was about 1 1/2 inches high 6 inches deep and 8 inches wide. The other was the same size except about 3 inches high. They were $12 CDN and $14 CDN respectively. When my local Radio Shacks decided to get out of the parts business, they were clearing the 3 inch high case for $1.99 each. They only had 2 left, and I grabbed them both. I wish I could get more! The steel is a bit hard to drill, but otherwise they're great.
A properly grounded metal case is an excellant upgrade for the FM25A which makes it a lot more pro - e.g. no hum!
As I fall into the Industry Canada maximum one watt provision, [thank goodness for suburban sprawl!] I've found that mounting an LPA1 [Ramsey one watt linear amp]along with a FMLP1 [Ramsey's neat FM low pass filter] in a metal case [I used a low profile aluminum cabinet for a rural satellite TV decoder] works very nicely along with the FM25A exciter in the RS steel case I described earlier. Don't use the LPA-1 in the US though. And, only use it in Canada if you are certain that you won't violate Industry Canada Regulations.
I liked your suggestion about installing a more pro connector - and hope to do this in the near future. Right now, my project is to install a stereo / mono switch so I can experiment with mono operation [reduced 19khz and 38khz based spurs]. I'm hoping that the combination of a metal case plus shielded wire for the switch will make this idea work out in practice.
Regulated power supplies are another nice upgrade. The old wall wart adapter will come in handy for other Ramsey kits, or just general experimenting.
The STC-1 is also a good product and enhances the FM25A considerably. However, I also use a home brewed unilevel amplifier which offers much the same effect as the old Gates `Level Devil'. A good unilevel amp keeps all your average volume levels the same without destroying dynamic range - great for automated systems or busy operators! Combined with a 10 band per side EQ and the STC-1 peal limiter/brickwall 15khz filter, this gives great loudness and punch with good sound quality.
The FM25A is a dependable, reliable building block for the experimental orienteed microbroadcaster.
Now - if I could only eliminate the TVI my rig causes on our TV! While the FMLP1 virtually eliminates harmonics, the antenna preamp for our TV overloads with the strong FM signal, then creates its own internal harmonics which mess up Channels 7 through 13. Bear in mind that my transmitting antenna is only 15 feet away from the TV antenna. Perhaps an FM trap between the TV antenna and its preamp would do the trick!
09-08-2001, 12:24 AM
Nice to see your response to my info. Glad
you agree about the metal box. Check out
MECI in Dayton, Ohio. They have some
wonderful project boxes that measure about
8" by 2.5" by 6". They are stripped
aeronautical battery charger boxes and they
sell them at $2 each. I don't know if they
are in their catalog or not...I live close
enough to drive there and browse. You might
call Ed there and ask him about these.
I wish I had your problem with TVI. Here
I have to watch my power. I am an amateur
radio operator (KB8Z) and have only had one
TVI complaint. My neighbor had a preamp on
his TV antenna and, as it turns out, the
coax shield was broken. Fixed and everyone
I bought my son a FM25A kit for Xmas. He
assembled it and had the same RF problems I
mentioned. I put it in one of the metal
boxes, and all is very well.
12-11-2001, 05:00 PM
<BLOCKQUOTE>quote:</font><HR>Originally posted by Phil:
Now - if I could only eliminate the TVI my rig causes on our TV! While the FMLP1 virtually eliminates harmonics, the antenna preamp for our TV overloads with the strong FM signal, then creates its own internal harmonics which mess up Channels 7 through 13. Bear in mind that my transmitting antenna is only 15 feet away from the TV antenna. Perhaps an FM trap between the TV antenna and its preamp would do the trick![/QB]<HR></BLOCKQUOTE>
If you could increase the distance between your receiving antenna and your transmitting antenna along with raising one antenna to a height that is higher than the other..say by ten feet,that may reduce or completely elimate your interference.
Hi Bob - your right - distance certainly helps. I found that careful selection of the transmitting frequency helps a lot. Also, the FM25B works better in this regard than the "A".
09-01-2002, 04:25 PM
I was thinking about using aluminum foil on my FM25, I figured that annoying hum was due to RF feeding in to the power supply or audio stages. I came to the Forum here, and sure enough! There is advice to do this, or to use a metal case.
That's what I am going to do, get a little aluminum case. The complete line of Budd metal enclosures (100's of styles, some nice and cheap) is handled by HLK & Associates www.hlkassoc.com (http://www.hlkassoc.com) -sold online, very convenient.
I see from the the little filter that Ramsey put on the DC input of the FM25 as an add-on that RF on the DC input is an issue; so I will for sure by-pass the DC to the case where it enters with a tantalum cap or something, get rid of that nasty RF coming in. I may also see if making some little shields of copper sheet to try and shield the RF stage from the audio section does any good.
Aside from the hum, the FM25 sounds pretty good- not as good as my Broadcast Warehouse exciter with 250 watt amp, but not bad!
Thanks to Ramsey for providing this forum.
09-01-2002, 04:42 PM
I'd like to say I'm glad to hear other's have used the "tin-foil method."
I don't know if others do this, but I also use tin-foil when splicing coax together. I'll solder the inner and outer connectors, then wrap tin-foil around the cable, making contact with the outer one. I figure this helps shield the inner connector better.
09-05-2002, 08:33 AM
I did the same thing, only with an fm-10a and it worked wonders. I took the guts out of my old Heathkit HD-1420 VLF converter kit and replaced it with my fm10a board which fit nicely, I even put a hole in the top for access to the main tuning coil. Fully sheilded out through an SO-239 connector. This feeds a magmount 2 meter fm antenna on the roof, cut for the middle of the FM broadcast band. No hum, or noise of any kind. The box is equipped with a ground lug and dc power connector, and is fed from a very stable switching supply at 13.8 volts. With the coils I added to the circuit, it is very stable, with performance very close to an FM25. smile.gif
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