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Satellite Workshop 05
Although I don't install satellite systems, I frequently receive calls from the public about installation faults. Sometimes I can help and sometimes I offer the number of a local installer. On this occasion I was intrigued because the system in question had been installed by a firm which always does an excellent job.
The customer explained the problem to me over the telephone:
"I can't get Sky One but everything else is all right," he said. I asked him to explain exactly what he saw instead of the Sky One picture.
"Nothing," he replied firmly. Now some customers exasperate me and I could tell that this was going to be one of them.
"I need to know exactly what you see on the TV screen when you select Sky One," I explained as patiently as I could.
"Nothing," he confirmed. Now, I knew he wasn't blind so I tried another tack. Sometimes a woman's eyesight is better than a man's. Having established that his wife was in the room, I asked to speak to her. He was suspicious but passed the handset to her.
"Your husband has problems in describing colours," I stated, kindly, "so I want you to tell me exactly what colours you see on the TV screen right now."
"Just black and white lines and the whole picture just keeps jumping," she explained.
"Any decoder messages?" I asked, hopefully. "Does it say Please Insert Card" or anything like that?"
"No oh, yes but it's gone again."
"And the other channels are all right?"
"Yes, but he wants his 'Star Trek' so you've got to fix it, quick."
This particular customer lived only half a mile away so I asked her to bring the receiver to me for test. Her husband arrived within minutes, clutching a Nokia SAT1700 to his chest. It was still warm. I connected it up and selected Sky One. Immediately the message Please Insert Card appeared. I did so and the picture cleared; a nice stable picture with no obvious fault. Since it was almost lunchtime, I offered to call at his house to see the actual problem. I locked the shop and followed him in my car.
On arrival, we reconnected the receiver and, sure enough, that particular channel gave every appearance of having a very weak signal. The rest of the channels were almost perfect, with just a few sparklies on Sky News. This was unusual because that is one of the strongest signals in the U.K.
I connected my spectrum analyser and a marked dip in the signal was apparent on a few transponders.
"When did the problem start?"
"Last week after we moved it from the Dining Room to here."
"Who actually moved it?"
"Harry. The cable wasn't long enough but he used the proper stuff with Satellite written on it."
Now I could visualise the problem. I knew exactly what I was going to find before I entered the Dining Room. Sure enough, someone had extended the cable by soldering an additional length to the original. The joint caused a massive impedance mismatch. I was amazed that any signal was reaching the receiver at all.
As a temporary measure I fitted an in-line threaded barrel with 'F' connectors to join the two cables. That improved the picture enormously. I advised them to get the original company to replace the entire cable if they were still unhappy with the picture quality.
Thankfully, with only the first five minutes of Star Trek missed, the customer was happy to pay my call out fee. I wondered if I should mention the new Sci Fi channel which was due to be broadcast from "Hot Bird."
I had just arrived back at the shop when the telephone rang again!
"You've got to help me or my neighbour's going to belt me."
I explained that I did not handle domestic disputes and that satellite was my game.
"That's the problem" said the distraught voice, "Each time I move my dish, it interferes with my neighbour's TV." After a lengthy interrogation I gleaned the information that the dish actuator motor seemed to be causing interference on both his TV and that of his neighbour.
"Are you sure that you've connected the cable screens to ground on the back of your positioner?" I asked.
"Of course. I've done everything right!"
The receiver/positioner combination was, he said, made by Uniden. I offered to inspect the installation for a nominal call-out fee. We arranged that I would call in the evening for our mutual convenience.
One of the repairs that I had was an old Maspro SRE90S. The customer complained that the pictures had been sparkly since January. The fault had appeared quite suddenly, she said.
On the test bench the poor pictures looked for all the world like a symptom of a faulty tuner. Black and white sparklies dominated the weaker channels. Now, my test bench has several test points fed from a Global Mini Magic. As it happened, another receiver was on soak test so the LNB was receiving power from that. I noticed that, as I disconnected the LNB feed from the Maspro, the picture remained because the signal was still reaching the tuner by capacitive coupling. Strangely, however, the picture was almost sparkle-free! As I moved the connector until it was in contact with the tuner input, the sparklies increased.
The indication was, therefore, that the signal was too strong, rather than too weak, for the tuner. I tested this theory by connecting a 100 metre reel of cable in line. Picture quality was much better.
On a hunch, I fitted a Global Astra 1D filter in line. Perfect pictures! Clearly, the additional signal from 1D was swamping the tuner and causing its AGC to reduce the gain.
The next repair was a Pace SS9200IRD. This unit would flash its LEDs in sequence but that was all. Disconnecting the LNB and removing the decoder slowed down the flashing, indication a possible current overload.
This problem is usually caused by a high resistance R13 which is supposed to be 0R22 fusible. People often fit a 2R2 by mistake. However, this unit had not been repaired previously. I replaced R13 for good measure but this had no effect. I measured R11 which was exactly 4R7 and checked all the diodes as well. C11 sometimes causes this problem so I replaced it and made sure its connecting tracks were all right. Next, I measured the 100R surface mount resistor which takes current from the feedback winding to the chopper control chip. That was fine. I measured all the tracks on the primary side; disconnected the diodes on the secondary side. Still the power supply pulsed. I remembered one occasion when this symptom had been caused by an open circuit feedback winding so I replaced the transformer, although it measured all right. No luck.
I thought hard about this. If it wasn't a short on the output and the feedback circuit was definitely working, the only other possibility was the resistor which sets the output voltages. This consists of a 1k and a 15k surface mount resistor in parallel. I measured the combination which was 1k8. Bingo! Replacing the 1k (R7) cured the fault.
After tea, I drove to the house of the man with the Uniden system. When I arrived I was somewhat surprised to find that the nearest neighbour's house was some twenty metres away. Quite a distance to be radiating interference from screened cable. However, inspection of the offending wiring showed the cause to be obvious. The owner had used four-core caravan cable for the motor and reed switch feeds, with twin bell wire for the polariser.
"It loses its position, too," said the man, helpfully. I wasn't surprised. With no sign of any screening, not only would the cable radiate motor interference to any nearby equipment; it would also interfere with the positioner pulse count circuitry.
I offered to replace the entire cable run about 50 metres with properly screened motorised ribbon cable. The owner quibbled about my price until he spotted his neighbour peering over the hedge. As I don't do installations, I arranged for a reputable installer to do the job and still made a profit.
The following week, the man had the decency to ring me to say that the new installation had cured all his problems and the picture was actually better than before. I couldn't say why, although screening the polariser wires had probably helped. Dithering the skew at 50Hz doesn't do anything for the picture quality!