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Satellite Workshop 46


E-mail in a box

The observant amongst you will have noticed that my e-mail address changed last month. This annual change helps to reduce the amount of junk mail that I receive. It also forces readers to buy at least *one* copy of TELEVISION each year if they want my free advice. You don't need a computer to use e-mail, by the way. An Internet Set-top box is available for just 399 from Satellite UK in Derby (01332 812588, fax 01332 850300). This box looks like a satellite receiver with a cordless keyboard. It simply plugs into your TV and 'phone line and is ready to use.

Nokia SAT1700

A nicely dressed lady brought me this Nokia receiver for repair. It had "gone off" suddenly. She had taken it to Wossname up Church Street but he hadn't managed to find the cause of the fault. However, he HAD managed to lose two screws! On test, the channel number appeared in the display, LNB voltage was present and on-screen graphics worked OK. However, there was no picture or sound on any channel, apart from an audible hiss. I deduced that the 5 volt supply must be OK as the microcontroller was working. It was likely that the 12 volt supply was at fault. Amazing what you can deduce when your head is kept cool by lack of hair!
I consulted the circuit diagram. According to my multimeter, 12 volts was going into the BC327 PNP transistor, TP08, but not coming out. I replaced this with an FXT749 that's rated at 1 Amp. It wasn't easy because the lead configuration is different! Now the PSU was simply pulsing. The capacitors all checked out with my "Genie" ESR meter. After a few minutes, I traced the problem to TP13 - also a BC327 - which switches the 28 volt supply to the decoder board. Removing TP13 brought back picture and sound and the decoder worked perfectly so I didn't replace it. The high voltage used to be used by Sky to "zap" out of date cards but they don't do that nowadays. How did I trace the fault to TP13? Easy, I used my eyes. The board around it was darker than the rest. Sometimes you don't need expensive test equipment!

Pace MSS300

The receiver arrived courtesy of "Parcel Farce". Despite this fact, it was intact and had been in transit for less than a week. I was impressed. The sender had packed it in a box inside another box and had included a two-page letter which detailed the problem. In brief, he'd reconnected the receiver on return from his holiday and now it simply made a ticking noise. He had enclosed a RELKIT 9 which he had purchased but had not dared to fit.
I dared - it is simply a matter of being methodical and replacing every part, one by one. The receiver now lit up and pictures were excellent. Unfortunately there was no left or right audio from any output. I was undaunted by this fact as the MSP3400 audio processor is a common failure item. Unfortunately, replacement of this and its associated crystal failed to produce even a whisper of sound. In desperation I phoned Pace and a helpful young man suggested that the 3R3 resistor labelled "LK140" might be at fault. I searched in vain for this, although I recall seeing it in an MSS500.
However, a glance at the circuit diagram indicated that R619 was a 3R3 resistor feeding 5 volts to pin 18 of the MSP3400 I.C. This resistor turned out to be a surface mount device beneath C148. It was open-circuit so I replaced it and was rewarded with good audio. I measured the 5 volt rail as overvoltage here can cause the 3R3 protection resistor to fail but the measurement was 5.18 volts which was within specification. Presumably the failure of the PSU "start-up" capacitors had caused a surge which destroyed the resistor but all was now well.

Pace MSS100

A stooped, bearded gentleman shuffled through the door and handed me his Sky smart card.
"It's an invalid" he muttered. "Fix it!"
Taken aback by such a fearsome attitude I retreated quickly to the workshop. The card worked perfectly in another customer's receiver so I handed it back.
"That was quick. Is it fixed? How much?"
"There's nothing wrong with it, sir."
"But the telly told me 'Your Card Is an Invalid'!"
"I think you'd better bring your satellite receiver in."
"Huh? What's one of those?"
"The thing with the card slot?"
"Ah, right, yes, bathroom scales, right."
He shuffled out and returned an hour later with a Pace MSS100 receiver.
On test, the receiver gave the "card invalid" message even without the card inserted. Sometimes this fault can be caused by the PTV113 card verifier I.C. and sometimes by the PTV115. Before "jumping in at the deep end", however, I tried a simple test. Beneath the board I dropped a solder blob between the pins of the card-detection switch which is a simple leaf spring arrangement inside the card slot. The on-screen message now gave the name of the programme and requested the smart card. This indicated that the switch was permanently open-circuit. Replacement of the contact assembly provided a simple cure and, since I took it from a scrap PRD800 it cost me nothing.
"How much?" demanded the fierce little man on his return.
"Just thirty five pounds, please," I replied.
"Darn!" he muttered. "Hope it reads pounds instead of kilogrammes now."
"Bathroom scales. Never been right sin' I got 'un."

Amstrad SRD510

Despite the fact that digital is "free" and analogue repairs have decreased, there are still people willing to pay for repairs - even if they are not necessary! I've had three SRD510s this week. The first was reported to "ignore the remote" but I found nothing wrong with it. The zero volt wire had been fitted; the 5 volt supply was correct at a fraction under 5 volts; the capacitors from "Relkit 3" had been fitted to the power supply and main board. As a precaution, I made sure that the ground wire was still connected beneath the card-slot board (cowboys disconnect this for some reason) and I cleaned the power supply connector contacts.
The second one also showed no fault so I fitted "Relkit 3" and the ground wire then adjusted the 5 volt supply with RV600. RV601 needed a fraction of a turn to bring the 13/17 volt LNB supply back into specification. Unfortunately, the receiver now produced perfect pictures but no audio - apart from a loud hiss! Fearing the worst I resoldered the TDA6160 FM demodulator IC which is beneath the board. No better, drat!
I decided to replace it with one from a scrap receiver. To remove the suspect IC, I removed as much solder as possible then used a needle-tip iron to lift each of the IC's legs, one by one. A twist to break the glue bond released the nasty little beast from the PCB. The "good" IC had to be removed from the scrap board in a different way. I put a knife blade beneath the IC and twisted. It broke free with a bang, ripping tracks from the scrap board. The tracks clinging to the IC were removed easily with the iron. I was then able to solder the IC to the first board but - still no sound!
It took quite a while to realise that in scraping away the glue to desolder C86, I had inadvertently broken the track that connects the microcontroller pin 15 to the tuner via link J18. As this data line also controls the TDA6160, it was not surprising that there was no audio.
The third receiver had been bought in a while ago from a "digital upgrade" customer as "perfect". I fitted the parts as above and left it on "soak" over night. In the morning there was no picture or sound and the LNB voltage measured 3 volts. The board around TP303, inside the little metal screening box, was blackened. I replaced TP303, a 2SB1143, which brought the receiver back to life, but I could find no reason for the failure.