FM transmitter troubleshooting

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soft2
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FM transmitter troubleshooting

Post by soft2 » Mon Dec 22, 2008 3:00 pm

I have an FM transmitter that I depend on to use my phone. In fact, I can't use my phone without it! I'm hearing impaired and the transmitter transmits the audio signal to receivers on each of my hearing aids. The reliability of the device is poor and it costs $300+ to get it serviced, and usually they just replace it (looks like an inexpensive gadget). I'd like to try and repair it myself this time so I don't have to wait 3 weeks to get it back but I have no experience with FM transmitters. It looks like a very simple device - a small, low density 2 layer pc board with a phone jack and power receptacle (for 9V AC) on the edge. It's made by Phonak in Switzerland.

A new unit costs only a little more than the repair fee so I don't have much to lose. Plus if I learn some things I might be able to build some other devices to work with this system; there are other gadgets I'd like to hear that Phonak doesn't make a tx for. I have experience programming microcontrollers and some electronics but no audio devices or FM. I have an oscilloscope.

So my question for this group is where can I learn more about this type of thing? And how would i start troubleshooting the device? You can set the FM frequency with dip switches between 214.0 Mhz and 220.0 Mhz. My unit is set for channel N12 which is somewhere in that range.

Thanks!

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jwax
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Post by jwax » Mon Dec 22, 2008 5:29 pm

Welcome to the forum soft2!
Since you sound familiar with electronics, I'd suggest you take a good look at the transmitter, looking for defective battery contacts, loose wires, cracks in the circuit board- basically a visual inspection to see if the defect can be seen.
Can you determine that the power for the transmitter is adequate?
You didn't mention what the problem with the transmitter was specifically, but does it transmit at all? Is the antenna intact?
Getting into circuit troubleshooting would be easier with a schematic of the transmitter. Is one available?
Can you read any part numbers on transistors or IC's?
Fortunately, audio FM transmitters are fairly simple circuits, although I'm not familiar with the 214-220 MHz band. 220 MHz is a ham radio band, as I recall. A good reference for ham radio and beyond is the American Radio Relay League (ARRL) Handbook. Let us know what you find.
John

soft2
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Post by soft2 » Mon Dec 22, 2008 8:53 pm

jwax thx for your ideas. The problem with the tx is a loud AC hum and sometimes crackling static like a bad connection. The AC hum drounds out the normal sounds. I discovered something else: The tx also has an input jack for audio from your computer or TV. I don't normally use it (I have a different tx for that). I tried that and the AC hum isn't there, but comes back when using the phone! That is good information - the problem must be in the early part of the phone circuit since both phone and aux use the same FM tx circuitry (I assume). I tried bypassing the jack and looking for shorts to the 9VAC power input jack but they seem fairly well isolated.

The digital signal processing in my hearing aids is fairly advanced and will often filter out an AC hum - recognizing that it's not part of normal speach (don't ask me how). This processing makes it hard to know what's really being transmitted sometimes. But this is quite loud.

The question remains for me to find out where the noise is being introduced. What type of component would introduce a hum and static if it failed?

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jwax
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Post by jwax » Tue Dec 23, 2008 2:35 am

Nice diagnostics! Does the static/hum get worse or change when you wiggle the connector from the phone? Do you have a different phone to try? Can you just use one of the other inputs to the transmitter instead of the phone input?
Usually a bad connection or failed capacitor would cause hum.
You say you had this same problem before? Try and find out what they did to fix it last time.
John

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Post by soft2 » Tue Dec 23, 2008 8:20 pm

jwax wrote:Nice diagnostics! Does the static/hum get worse or change when you wiggle the connector from the phone?
There is no change. That was the first thing I tried. I also tried replacing phone cord, different house, and finally connecting directly to the pcb bypassing the jack altogether.
jwax wrote:Do you have a different phone to try? Can you just use one of the other inputs to the transmitter instead of the phone input? Usually a bad connection or failed capacitor would cause hum.
You say you had this same problem before? Try and find out what they did to fix it last time.
John
Don't know what they did last time it broke. Phonak is terrible when it comes to getting information out of their repair service. They just tell you to contact your Audiologist who sold you the thing. He had his secretary call but she wasn't very savvy or assertive and she got no info. I once sent one of the FM receivers back for repair 5 times before they finally fixed it.

The only other input on the tx is for audio out from tv or computer or the like. I don't know any way to use that with the phone line.

How would I test for a failed cap? Thinking about it, it seems like a failed diode could do it, too? There's also a small transformer on the board, very similar to the one you usually see on telephone modems. I can't think of a good way to test any components without removing them. Maybe I'll trace the audio signal with my oscilloscope, and then do the same with the phone signal. Not sure what to look for though.

I remember now that it went bad slowly. The noise got louder and louder over about a month and the static was always intermittent.

Thanks for your help!

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Post by Jim Barrett » Wed Dec 24, 2008 6:04 am

Sounds like an impedence mismatch twixt the phone ckt & the tx amp ckt.
I'm guessing that most if not all of the audio & RF are on a single chip & what's left is input & output conditioning.
What are the odds that the transformer is for Z matching? 600 ohm to something lower?
Excellent trouble shooting technique soft2. Sectionalize, localize & isolate. :grin:

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