- June 23, 2019 at 9:02 pm #111693
Total posts : 496
Well, my trusty Landmark FM-350 finally gave up the ghost. It was manufactured in 2003, purchased in 2006, and used extensively since then.
For those that are unfamiliar with this now unavailable FM transmitter, it is both Part 15 and Industry Canada RSS210 certified. It contains an industrial grade CD ROM so that it can play prerecorded messages, as well as the transmitter itself. The antenna is a wire, attached via an F connector (similar to the Talking Sign, which was attached to the transmitter with an RCA connector). Yes, I know that certified transmitters are not supposed to use commonly available connectors, but there are plenty of exceptions to that rule – I think that the intent (which is always more important than the wording) of that restriction is to prevent someone from attaching a gain antenna via a long (and radiating) cable.
The Landmark is one of the best FM transmitters I have used, and is comparable to the Decade MS-100 (at much less cost). The sound quality was on par with the Decade, and it had absolutely no hum or noise in the signal. Range was not that of the Decade, but then each Decade is supposedly individually tuned at the factory to get the highest output – I guess you do get something for all that extra money. But needless to say, the Landmark was one of the staples of my transmitter collection.
I decided to investigate the cause of the failure (it was intermittently ‘dropping out’ transmission, and then finally stopped, period). The guts of the transmitter were very interesting and rather simple and elegant.
They consisted of a fairly high quality AC power supply, which I determined was the source of the failure. The power supply outputs 5 volts to power both the transmitter board and the CD ROM. The transmitter is contained on a single board, and uses a standard computer 4 pin connector for power. The live audio input comes from a standard 1/8 inch stereo plug, which connects directly to the transmitter board via a standard 4 pin connector (as does the audio output from the CD ROM for prerecorded material).
Now, I’m comfortable working with electronics on a board level, but not on a component level, particularly when the components are tiny. So the power supply is a lost cause to me. I suspect at least one or maybe more capacitors are gone, but I don’t have the test equipment or the know how to properly debug and fix the hardware.
However, I’m thinking that I may mount the board itself, untouched, inside a computer tower and see how (or if – I may be wrong about the power supply) it works. It will just plug into a spare power connector, and all I’ll need to do is to supply audio through the existing plug. The only challenge will be the mount and I have some ideas about that. Since all I’m doing is removing stuff (the cabinet and CD-ROM, and replacing the power supply), and not modifying the transmitter board, I think that it should be OK from a rules point of view. Nevertheless, I’m keeping the cabinet with the certification information on it, just in case.
One of the potential problems I see is noise getting into the signal from the various computer circuits. I’m hopeful that there is sufficient on board filtering and that hope has some basis in reality, given its noise free operation (when it worked). But the only way to know is to try it out.
- June 24, 2019 at 4:11 pm #111723
Total posts : 489
DON”T WRITE IT OFF YET! The only components that are time sensitive are electrolytic capacitors, not the other caps, just the electrolytics. After 25 years even less if not used much they deteriorate.
The power supply ones are the most susceptable and an easy fix and easy to identify as they are the largest and near the transformer and rectifier diodes. Just look at them to get the capacitance and and the working voltage(this can be higher than what’s there). See what happens and then start to look at any others. To check them they have to be out of the circuit and use a capacitance meter but after all that trouble just out in a new one. The only other thing that can go wrong is the output if a spark from your finger when touching the antenna blew it. If the output blew it would just not work at all, not be intermittent. But if you have discrete transistors it’s easy to see the number and get more on ebay. If you do get it working to protect the output from electric shocks etc. get a 2.2uH coil and on the circuit board install it across the antenna to ground. Don’t do what you are thinking and try to run it from the desktop computer. You don’t
know the voltage of the transmitter power supply.
To find this out connect the transmitter to power and measure the A/C voltage at the secondary side of the transformer on the circuit board.
- June 24, 2019 at 5:50 pm #111724
Total posts : 496
Actually, I do know the voltage of the power supply. A plug run in parallel with the transmitter board supply also powers the CD-ROM. A computer power supply will work (as long as it doesn’t introduce noise).
Sorry, but I just don’t have the knowledge, equipment and patience to fix the power supply. I probably could if I wanted to and wanted to spend the time, but I don’t. I do have my Decade CM-10, and I really do want to try to put the transmitter board inside a computer. If it doesn’t work, that’s life.
I got more than my money’s worth out of the Landmark over the years. Just a little sad to see it (possibly) go.
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