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Satellite Workshop 04
This area of Yorkshire is farming country so I wasn't very surprised when a strong smell of cow entered my shop, closely followed by a fellow wearing wellington boots and a red nose. The latter had nothing to do with charity since it was firmly attached.
This burly fellow held an Amstrad VS1000 between thumb and forefinger and placed it on my counter with a look of distaste.
"Canst' fix it bah t'morra, lad?" he enquired. "Video's all raht but t' satellite's gone raht funny."
I asked him for more details of the problem. "What do you actually see on the screen?"
"Nowt" was his firm reply. I decided not to press him further and handed him the collection slip.
My first job was an Amstrad SRD510. A note on top said "Goes off when tapped."
Not a very helpful note since it didn't say what went off! On this model there is no "off"; it is either "on" (green LED) or in standby (red LED) so I wasn't sure what I was looking for. I plugged it in and tapped it. The red LED stubbornly remained on. I pressed the standby button then tapped the receiver again. the green LED stayed on. I decided to leave it to warm up and started reluctantly on the VS1000 combined Satellite/Video Recorder.
On the test bench the VCR section played my test tape without problem, once I'd cleaned out the straw! The satellite receiver, however, gave a completely blank screen. There was no LNB voltage so I began to strip the unit down.
This model is a pain to work on because the tuner board is in two halves, plugged together, and sits beneath a fully screened decoder board. I must have removed a dozen screws before I got the beast out. The "F" connector was loose in the tuner housing, although its centre connection was still intact. I soldered the body of the socket to the housing. The 13 and 17 volt supply comes from a seven legged regulator which seldom fails so I checked R515, a 3.3 Ohm resistor. Sure enough, it was open circuit.
Having replaced this resistor, I put the unit back together and connected it up. For about five seconds I could see the "Please Wait" message on a scrambled picture before it faded back to a blank screen!
With the help of my old Hameg 'scope I traced the problem to CV42 on the decoder board. This is between two hot transistors and had dried out. A new 47microFarad went in its place. As I was replacing the decoder I noticed a charred mess near the centre of the board. This turned out to be R537 and, despite its appearance, it measured exactly 680 Ohms. The only reason I could see for its overheating was a capacitor next to a TO220 transistor which was mounted flat on the board. The capacitor sleeving had split and its metal body was touching two legs of this transistor. The value was either 22 or 33 microFarad (I don't have a service Manual) so I put in a 33.
Reassembled, the receiver now worked perfectly. I made a note to tell the farmer not to mess with the connections while it was plugged in to the mains.
Back to the SRD510 which still remained on, even when thumped, I began to wonder if this was really worthwhile. You can buy one of these second hand for less than fifty pounds. I removed the screws and took off the cover. The inside looked very brown. I rated this as "medium well done." (I have a friend who repairs satellite receivers and he always gives them a "steak rating.") Why do Amstrad owners keep their equipment in an oven?
I connected the LNB feed and tuned my TV to the receiver. The picture was remarkably good considering the appearance of the components. C54, normally green, had turned jet black. Since this is a cause of many intermittent problems, I removed the board to change both C54 and C55. Having done that, I reconnected it to check the picture. The screen was blank! However, as I flexed the board to look underneath, the picture returned. It seemed that there was a dry joint or a cracked track.
The oscilloscope indicated a good baseband signal entering the graphics I.C. until the board was flexed. The signal then became very low. Checking on the opposite side of R60 showed no change in signal so I replaced this resistor but with no affect.
I checked the signal on C55 positive leg and discovered that the DC voltage rose to ten volts when the fault occurred; here was a clue. A little poking about revealed that the wire link, J64 next to C55, was bent over on to an adjacent track. Easing the wire up with the tip of my iron solved the problem. I just hope that this was what the customer meant by "goes off." As a precaution, I added the ground wire which is recommended in the "Satellite Secrets" book from Davenham Satellites*. The power supply connector is often poor and bypassing the ground connection can eliminate a lot of intermittent faults.
Unfortunately, with this type of repair, I couldn't charge a lot because the receiver wasn't worth a lot and there was always a risk of argument if the fault I had fixed was not the one in question!
* "Satellite Secrets Revealed!" is available for 9.95. Order from SatCure.