Servicing FM Receivers – Part 2
By Jim Arcaro WD8PFK
In the last column we covered some of the basics in two-way FM receiver servicing. This issue we'll look at some special problems in FM systems.
Continuously tone coded squelch systems (CTCSS) have been around for many years. I discussed them in a past column, but let's review for a moment. Whenever two or more users want to share the same frequency, each can Code their transmissions with a low frequency or tone. This acts like a key, which unlocks the squelch of any receiver requiring that tone.
Thus, user A does not have to listen to user B's traffic, and vice versa. Most systems will allow monitoring the frequency before transmissions are made, so users don't end up talking at the same time. This tone is usually set at a fairly small deviation – typically + 600 hertz. With or without CTCSS, the peak deviation of the audio should be no greater than +5kHz. This is where some problems are misdiagnosed.
Receiver or transmitter?
For example, you are talking with WN8XYZ, and the audio pops in and out. You might think your receiver is at fault. Many times this is the fault of the transmitting station, which is over deviating. In one case, a group of handhelds for a police department had been set for + 8kHz deviation! Probably an inexperienced tech used an uncalibrated monitor to set his levels.
Let me comment here that excessive modulation will not make up for weak audio, or poor coverage. Cure weak audio by adjusting microphone gain.
Poor coverage is usually due to a bad antenna or coaxial Cable. Another problem at the transmitting end occurs when the CTCSS tone encoder drifts off frequency. The tone should typically be within several tenths of a hertz and proper deviation.
Wrapping it up
When testing receivers, it is important to set the generator in the communications monitor to the proper frequency, CTCSS tone and deviation. Some may allow you to set the CTCSS deviation first, and then add a 1kHz audio tone to bring the full deviation to . about ± 4.5kHz for a SHNAD test. Some receivers do not accept an Overmodulated signal, so make sure the transmitter being used meets specs. If all else fails, read the service manual.
Test 9V batteries too
This column is being written during the fast weekend of October, when those of us on EDT switch back to EST. At this time, local fire departments want you to check the battery in your Smoke detector. Most use a Common 9V battery, which is often six small 1.5V cells connected in series. They can be tested in many detectors by simply pressing a test button.
If no test button is available, try using a common #47 bulb as a load. It should light up fairly bright, and battery voltage should measure around 7 volts. If lower than 4 volts, replace it. Remember, when it gets low, you may be out of the house — and miss the telltale chirp-chirp of a dying battery.
The only thing worse than no smoke detector is one with a dead or missing battery. Check it now!