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Satellite Workshop 43


Amstrad SRD510

Sometimes I wonder why I bother! This morning I came into the workshop to discover that the receiver I had spent an hour in "repairing" last night now had "rolling picture" and absolutely no sound. It's lucky that I had left it on soak test.
The original report fault had been "wavy pictures". When I first tested it, there was sound but just a blank screen. Someone had already been inside and replaced C54 - a common cause of picture faults. They had also lost three screws. I should have given up right then but I'm a glutton for punishment. I traced the initial lack of signal to R9 (470R) which had suffered from the corrosive effects of black glue. Now there was sufficient signal to give weak inverted video but this receiver had been well "cooked" during its service - another reason why I should have jumped up and down on it!
I carried on testing and measuring. More components were replaced but the video level remained stubbornly low. It was only when I got a brief flash of good, strong picture as I touched C54 that the reason became clear. Whoever had replaced this capacitor had broken a track nearby.
So, this morning I'm left with a choice of still more work or of cutting my losses. Do you hear that satisfying crunching sound? That's size ten Doc Martins improving the shape of a zero-value SRD510. I can use the power supply for another repair.

Amstrad SRD520

My first thought was "Oh, no! Another."
This receiver had severe "lines on picture". In fact it was rather like the Pace SS9000 tuner capacitor fault symptoms. My first move was to replace the "Relkit 3" capacitors but that didn't help at all. In fact now I had a blank screen! I got a weak picture back (still with lines) by removing the "C-Band" switch and fitting a wire link. TR7 had glue around it so I replaced that. Now I was back where I started half an hour previously - a picture heavily obscured by lines. Just for good luck, I replaced ALL the power supply capacitors with low ESR types. No good. Working blind is not a good idea so I decided to do the job properly. A quick prod with the oscilloscope showed a serious high frequency ripple on the 12 volt supply from TR304. This transistor, in combination with the IC300 Op-Amp circuitry, is supposed to give a smooth output. Measurement of all associated resistors showed R8 (1k) to be open-circuit. Fitting a new one didn't cure the ripple fault. Nor did the replacement of TR304. In desperation, I replace IC300 (LM392) and achieved success at last. The picture was clean, stable and the audio was good. Dare I "soak test" it, I wonder? Nah, let the customer find the next fault!

Pace Apollo

(applies equally to MSS200/300/500 etc.)

You may recall Frank at the "Lion and Swan" whose PRD800 I repaired previously. Well, no more than a week after I fixed it and made it reliable, he had the urge to "trade it in for a better one" from a customer. This turned out to be a Pace Apollo which apparently suffered from a sharply defined vertical white line on "Sky Sports".
It took a long time for the fault to appear and, when it did, I discovered that it affected all decoded channels (including Sky News and Ch.5) but no unencrypted channels.
The line was 2mm wide and approx 100mm high on a 14" screen.
It was located about 1/3 from left edge of picture.
It was very intermittent, occurring only at a specific temperature but it could be forced to appear by using hair dryer and freezer in the area of the PTV decoder I.C.s

By replacing each I.C. in turn, I traced it to U9, the PTV114 decoder graphics inserter. This was, of course, the very LAST PTV I.C. that I replaced!
It was not possible to narrow down the cause specifically to the PTV114 by heating/freezing because changing the temperature of *any* PTV I.C. would cause the line to appear or disappear.
Anyway, I get free beer for the next week. (Don't expect me to get much work done!)

Pace MSS200

This receiver arrived in the van of a local installer. His customer had complained that the screen flashed "LNB Short" but, when I connected the receiver up, it would not come out of standby. The fault report was useful, however, since it prompted me to check the LNB current-sensing resistors. On this model there are two 1R8 surface mount resistors connected in parallel, just below the front edge of the board. They were open-circuit.

The installer was in a hurry so I confess to a "bodge". I soldered a 1R fusible resistor across the burned out resistors and the receiver worked perfectly. The installer paid up with good grace and I warned him to find the short-circuit in the cable or LNB before reconnecting the receiver!

Amstrad SRD510

I don't do many customer call-outs nowadays. Since I started quoting "a pound a minute" the time-wasters stay away in droves. However, a little old lady just round the corner tempted me with a cup of tea and a bun so I trotted round immediately.
The Amstrad SRD510 worked perfectly but, after warm-up, the voltage on pin 8, TV scart, would not drop below 1 volt when switched off. This caused the TV to stay in AV mode, with the receiver switched off. Voltage was 12.2 volts when switched on so I felt that the TV was being very uncondescending in treating a 1 volt level as "high". I soldered a 10k resistor from the Scart pin 8 to ground. This extra load pulled the "off" voltage down to 0.85v and the problem went away. I went away when the teapot was empty.

Pace PRD900

My wife was complaining again. (She likes to keep in practice).
"It won't record. The timer is stupid." I agreed to take our PRD900 to the workshop, later, to test it but she wanted it done NOW.
"And if it DOES record it plays the wrong program with a flashing clock or else just a blue screen."
Well, I hate to admit it, but she was right. I tried setting a number of timed events and the results were somewhat random to say the least. Sometimes it remained on the wrong channel but flashed a clock symbol on the screen and a "t" on the LED display indefinitely until manually cancelled by pressing "F" then "Standby". Sometimes it seemed to change channel but left a blue screen as if the desired frequency had been overshot and lay outside the "capture range" of the phase lock loop system associated with the "Nicky" I.C., U9.
I fixed this particular fault by pressing "F" with the LNB menu displayed so the AFC was disabled. Then it was simply a matter of retuning a few channels manually to remove "sparklies". I measured all the power supply voltages and checked the ESR of all the capacitors around the power supply. Everything was perfect. However, the timer function was still erratic so I replaced the microcontroller, U2, and that cured the problem. I have never known a micro to develop a fault like this and I doubt if I ever will again. It's extremely unusual, but my wife thinks it's typical.

Nokia ACU8152 dish positioner

I love decorators! Mr Winstanton is a typical satellite enthusiast. He has an enormous motorised dish, a Nokia receiver and an Antenna Control Unit (or 'positioner' as we call it). Nokia equipment is generally very reliable but Mr Winstanton had "had the decorators in" while he was at work. He hadn't considered labelling his cable connections so, upon his return, he found that the decorators had disconnected everything and left him to sort out the mess.
Apparently, he stuck wires into connectors in a random fashion and the result was a non-working positioner. It would send the dish East or West with a few jerks then stop. Normally I turn away anything associated with motorised systems because the customer always wants me to spend four hours reinstalling the system for no wages. On this occasion, however, Mr Winstanton had brought a local installer along so I knew I wouldn't be lumbered.
There was a screw missing from the ACU8152 and the customer saw my worried look.
"Oh, Wossname up Church street had a quick look but he couldn't find anything."
I agreed. He clearly couldn't find all of the screws either! Luckily, the printed circuit board did not appear to have been touched. I inspected the pulse input protection circuit which comprises two 6v8 zeners, DA27 and DA27, and two low-resistance 10uH inductors, LA05 and LA05. Of these, LA05 was clearly burned and DA28 was discoloured. My ESR meter indicated that they were both short circuit. I confirmed this with a multimeter after removing both components. It seemed that somebody (I glanced at my customer) had connected the wires wrongly!
Replacement of the faulty parts appeared to be all that was required. The installer confirmed that the repair had been successful when we met in the "Lion and Swan" that evening. I charged him two pints and a tenner. That must have been too cheap because he grinned.