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Satellite Workshop 07
The customer described his symptom as "no satellite pictures" after decorators had been. This sounded straightforward but, when I inspected his installation, all seemed to be well. The receiver, an old Amstrad SRD400, had popped its 630mA fuse, indicating a possible short circuit in the LNB or cable. A replacement fuse melted instantly, confirming the diagnosis.
However, his terrestrial TV pictures were also very poor and this brought up a suspicion in my mind. I traced the aerial and LNB cables through the wall and went outside. They'd been swopped! The decorators had removed the plugs in order to pull the cables into the wall to allow the new wallpaper to lie flat. Unfortunately, they'd made a superb job of re-fitting the plugs - on the wrong cables.
An MSS1000 arrived "dead" and the customer wanted an estimate. At first I thought that it was simply the front panel display not lighting up but a quick voltage check confirmed that it was indeed dead.
Replacing the main switching transistor with the recommended STP5N90 MOSFET appeared to be all that was needed. However, at switch-on the power supply simply ticked, with pulses appearing on the secondary.
This symptom is often caused by a shorted amplifier I.C. on the audio board but disconnecting the supply to this had no effect. However, my diagnosis was quite close since I traced the problem to D54 - the power diode which feeds the audio board. Removal of the diode allowed the receiver to work nicely, but without loudspeaker function. I couldn't tell if the audio output I.C.s were damaged so I quoted the customer accordingly and ordered the diode, fully expecting the estimate to be accepted.
The customer complained that my price was far too high for a receiver which cost only three hundred in the first place and insisted on taking it away! I charged him my standard amount for an estimate and he marched off with the receiver under his arm. I'd taken the precaution of putting back the faulty diode but removing the mains fuse as well.
I do a lot of repair work for local TV shops and a week later an MSS1000 was brought in. Sure enough, it was the same one. I gave my estimate and this time it was accepted. Replacing D54 was all that was required - the audio board having survived intact. I still don't know what the owner paid in the end but I bet he wished he'd accepted the original estimate!
The Amstrad SRD510 has a download facility which allows you to connect it to another, via a Scart lead, to transfer the memory contents. The transfer is initiated by holding the Standby button whilst applying mains power, when the red and green LEDs will begin to flash alternately.
The unit which arrived at my workshop did exactly that when plugged into the mains but I didn't need to press the Standby button. A quick inspection revealed that someone had already "had a go" but they'd been looking for a microprocessor fault.
Thinking that one of the two Standby buttons might be damaged, I removed the front panel and looked. The buttons operated freely but measured less than 1k when off! There was a lot of glue spread across the little printed circuit board and it had gone completely black with heat. Scraping off this carbonised glue cured the problem.
The receiver had another fault (don't they always!) The picture was fine but the screen remained blank for four seconds each time the channel was changed. In addition, the menu graphics were not superimposed on the picture but on a grey background instead.
I checked the microprocessor pin 5 for horizontal sync pulses but there weren't any. In this model, the micro "knows" a good picture is present when it receives good, clean sync pulses but in this unit they were not reaching it.
The synch-separator circuit is fairly simple, comprising individual components taking the signal from a TEA2029C. The fault was traced to TR16 but I've known the same symptom being caused by TR13 and TR14.
This receiver came all the way from Scotland. According to the owner, he had taken it to a local repair shop who had removed all the power supply components, which they had found were faulty, then returned it to him minus components and all screws and brackets! That's how it arrived on my doorstep but the owner had done an excellent job of packing it (not many do) so it was in perfect condition.
I acquired most of the Power supply components from David Poole* of Davenham Satellites but one capacitor eluded us. It's a 22nF rated at 630 volts. Amstrad don't stock it but helpfully suggested trying the "usual distributors". Farnell don't stock it; RS list it but no longer stock it. Finally, in desperation, I "emailed" all my friends and Martin Pickering suggested "Tardis Electronics" at 30 Station Road, Sandbach (01270 763029). This is ostensibly an ordinary TV and video repair shop but it is aptly named for when I visited, Terry Boyd, the owner, showed me around a vast labyrinth of rooms stretching far back into the building! A radio Amateur's paradise with every component under the sun. (How about a 15,000 F capacitor rated at 150 volts or two unused Quad II amplifiers?). Terry says he does "mail order on request" which apparently means "if you know what you want, we'll look for it". I noticed computer boards stuffed with Eproms; boxes of resistors, capacitors, valves, video heads, TV spares....even a butter churn!
But I digress. We found a box of 20nF capacitors rated at 1500 volts. I bought the lot.The size was "extra large" but I persuaded one to fit by glueing it neatly (well, if Amstrad can use glue...) at the rear of the power supply and using short, insulated wires to connect it to the board. After the parts were fitted the unit fired up immediately and everything worked - except that the D2Mac board gave audio, and messages on a blank screen (didn't I say that faults come in pairs?) - I spent a long time with my oscilloscope trying to find why no video was coming out before a friend at Eurosat reminded me that the SRD600 model suffers from this problem. Can you guess the (simple) cause? *
By the time you read this, "Telepart" at Wolverhampton (01902 773122) should be supplying a PSU kit for the SRD600.
* Sadly, David Poole died of cancer in June 1997
This model has been part of a Sky promotion recently. I can see satellite receivers going the way of mobile telephones; subscribe and get one free!
People think that, because a receiver is low-priced, it will cost pennies to repair. Unfortunately, nobody is subsidising the poor repairman.
However, this one was brought to me for repair because the customer wasn't prepared to wait even three days for the Pace warranty repair! Well, his loss was to be my gain.
The symptom was described as horizontal streaks on the picture of all satellite channels. Just to make it more interesting (!) the fault was intermittent. By tapping the unit gently I was able to reproduce the symptom. It affected only the picture from the RF modulator. Scart outputs were fine.
By poking and prodding with a plastic knitting needle I discovered that the track to C51 in the modulator had broken. I made a neat repair and the customer got it back in slightly less than three days and at a very reasonable price.
* SRD600: The colour, brightness and contrast were set to zero in the menu!