Troublesome telco line problems
Ever had one of those days when nothing seems to work according to theory? Usually, it is something simple, and we technicians often tend to overlook the simple problems in pursuit of the more complex and weird stuff.
After working many long hours for several weeks in an effort to meet a startup deadline for our new centralized dispatch center, my assistant, Hilton Crews, and I were beginning to see the light at the end of the tunnel. Little did we know that the light would turn out to be a speeding freight train. After all, the most difficult part of the journey was behind us, and the remaining tasks were fairly simple in comparison_or so we thought!
The Problem Part of this project included the installation of quite a few base stations and all that goes with that (antennas, control lines, grounding, lightning protection, etc.). At one location we installed three base stations with associated control lines leased from telco to allow remote control from the central dispatch point. The remote control system used the standard remote control audio tones (2,175Hz for transmitter keying).
After getting everything installed and wired up on both ends, we were ready to run initial tests on the three stations. The basic block diagram is shown in Figure 1 below.
Ready, set, go! It works!
No, it doesn’t.
Yes, it does!
Simply put, it worked intermittently. With 10 transmitter keying signals sent down the line, the transmitter would not key 100% of the time. The keying percentage would vary from 30% to 70%, with each series of 10 keying control signals.
Before installing the base stations at the remote sites, we had checked each one with the dispatch console to make sure each one worked. This included transmitter keying and modulation. The only thing different now was the interconnecting telco line_or was it? We decided to check the telco line for loss and frequency response. Maybe there was a severe attenuation notch at the 2,175Hz keying control frequency. It can happen, but on all three lines?
A fairly complete frequency response test did not indicate such a problem. From the test results, it would seem that the telco line should work. Hilton and I were using two identical-line testing instruments with a built-in intercom so that we could communicate and coordinate our test procedure. It was during one of these voice messages that I noticed that Hilton’s voice sounded as if he were talking through a fan. I communicated this to him, and he also noticed the “fan-voice” sound. It sounded much like a broadcast radio with a bad filter capacitor in the power supply, with the voice riding on top of the 60Hz hum (120Hz in the case of a full-wave rectifier).
As it turned out, this was the clue that ultimately led to the cure. However, another twist would lend more confusion to the issue. Having tried almost everything we could think of, it was almost desperation time! If you look at Figure 1, you will see a line protector that is used to protect the equipment from lightning surges on the telco line. Given our past experience with these devices, we never suspected that this device could play even a minor role in the problem we were experiencing. In a last-gasp attempt to find some common denominator to the problem, I asked Hilton to disconnect this line protector and to wire the telco line straight into the base station tone-termination panel.
Though neither of us was expecting to see any positive results from this procedure, we had nothing to lose. After Hilton made the change, the dispatch console sent 10 consecutive keying signals down the line to the transmitter.
“Okay Hilton, how many times did the transmitter key?”
“Ten for ten,” came the enthusiastic reply from Hilton. Then a few seconds of dead silence on both ends_neither of us could believe that we had overlooked this innocuous device for so long! Hilton made some ac voltage measurements and found the ac voltage (60Hz) on the line to be high. This accounted for the “fan-voice” effect. Apparently, the ac voltage was causing the line protector components to “fire,” which caused distortion preventing the tone-termination panel circuitry from recognizing the keying signal.
The Solution Obviously, we couldn’t leave the base station hooked to the telco line without any lightning protection device connected, so a means of eliminating the 60Hz ac voltage had to be found. In a conversation with some of the telco personnel, Hilton learned about a special filter that telco technicians use on occasion to eliminate certain line interference problems. Telco loaned us three filters to use on a trial basis. Unfortunately, the filters did not eliminate the interference problem, but they did lead to the solution. Hilton contacted the manufacturer of these filters only to learn that better filters were available to combat the 60Hz “power influence” problem.
Having made the call to SNC Manufacturing, Oshkosh, WI, Hilton soon had the proper filters in hand. In each case, the filter was installed between the line protector and the line itself. The result was amazing. The transmitters would now key reliably even with the line protector in line. Figure 2 on page 8 shows a block diagram of the circuit after the installation of the SNIX filter (Single Noise Interference Xterminator) from SNC Manufacturing, as shown in Photo 1 on page 8.
Summary Something can be learned from this_never be surprised at what you might find on a leased telco line. Expect the unexpected! Check for any induced 60Hz ac voltage on the line. Check for any dc voltage. Measure across the line and from each leg to ground. Check the line loss over the audio frequency range and especially at each of the control tone frequencies. Where bidirectional amplifiers are used, the loss and response tests should be conducted in both directions and compared. In short_be thorough.
Further information on these filter products can be obtained from SNC Manufacturing, 101 W. Waukau Ave., Oshkosh, WI 54901, or call them at 1-800-558-3325. Information is also available from SNC from their website at: www.sncmfg.com.
Until next time_stay tuned!
Kinley, a certifiied electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is a member of the Radio Club of America.Je is the author of Standard Radio Communications Manual: With Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. Kinley’s email address is firstname.lastname@example.org.