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You may recall that I repaired an SRD6 for Tom in the next town. The symptom was low video level and no decoder messages. The cause turned out to be transistor TV04 which is connected to what I thought was the video level adjuster, PV01. In fact this adjuster controls the deemphasis, as I discovered once I'd replaced TV04.
Anyway, the same receiver "bounced back" to me just two weeks later. The fault report, unhelpfully, said "same fault again" but it wasn't. This time the pictures were "washed out" and the decoder messages were present but heavily distorted. The decoder was working perfectly. My oscilloscope showed that the video level was correct until it came out of the graphics generator (pin 8 of IV02) at which point there should have been 2v p-p composite video imposed upon 1.5v dc. In fact the dc bias stood at 3 volts and the video waveform was being clipped by TV22 on its way to the TV Scart socket and the RF modulator. Now the graphics generator is a TCE10117180 and, curiously, I didn't happen to have one in stock. My final solution was an empirically derived "bodge". I soldered a 330k resistor from IV02 pin 8 to chassis. That fixed it!
According to Tom, the customer indicated his grattitude, as customers do, by whining about being charged twice and muttering about "Trading Standards." I really must try this myself; next time I get an engine fault I'll take my car to the garage which replaced my exhaust pipe. "It's the same fault" I'll say. "It makes a noise again." (Wonder if they'll fix it free of charge?)
I've just bought ten "scrap" SVS250 receivers for spares. Of course, I won't end up with many spares because I've just mended the first two "dead" ones (fuse melted) and jumped up and down on the third in frustration! Well, they are frustrating machines!
I had a letter from Peter Thorneycroft, a dealer in Telford. He described the problems that he had with his customer's SVS250:
"The original fault was the EEprom faulty" he says. "Symptoms were "OUT OF RANGE" bars on display, the LNB offset read 25.38 and if corrected it couldn't be stored. I replaced the 24C04 with a used one, fitted the SatCure upgrade kit (Relkit 17) and the customer left happy. An hour later he phoned to tell me the receiver was the same as before. It turns out he had used it for about 2 years via RF and wanted to Scart it. As soon as he connected the TV Scart, the receiver went feet up. I had previously checked it on our TV Scart socket and it was OK. I checked his TV and there's a 12 volt output on pin 10 of the Scart socket! The SVS250 offset reads 25.38, indicating that the EEprom is dead once again, so I shall cut the necessary wires before re-installation. The TV is a BEKO, by the way."
So beware Scart connectors! I have come across similar problems with decoder Scart connections on Amstrad receivers but never with the TV Scart. It seems that the SVS250 has a data line conected from the 24C02 to pin 10 of the TV Scart socket.
Left to themselves, these Pace receivers will give trouble-free operation for years but the Pace designers failed to consider the ingenuity of the average customer who believes that equipment should be kept hot! And, in my experience, it seems that the more money they have the less common sense they use.
This particular receiver belonged to a gentleman whom I shall refer to as "Lord Potts" to save him from embarrassment. He designs clothing--or, at least, he employs others who design it. Consequently he has, so he tells me, "a feel for quality."
The MSS1000 had been squeezed into a custom-built brick fireplace arrangement, together with a 28 inch TV, two video recorders and a Hi-Fi audio system. Just to ensure that there was absolutely no possibility of nasty draughts (or "ventilation" as we call it in the trade) the chimney stack had been blocked and the whole affair was hidden behind velvet drapes.
The fault symptom was obvious. The audio from both left and right channels was very faint and extremely distorted. In addition, the vacuum fluorescent display flickered in time with the audio. Very pretty.
Back in the workshop, I discovered that the fault vanished when the Dolby board was disconnected. Before I plunged in head-first, however, I tried another working Dolby board. The distortion returned. Hmm.
I decided to replace all the power supply electrolytics since these looked decidedly black. I see little point in trying to trace a dificult fault when such an obvious clue is staring me in the face. Relkit 10 contains all the high reliability capacitors required, apart from the 100 F/400v which seldom fails. I replaced each capacitor in turn, reassembling and testing after each change. Since this symptom was new to me, I was interested in determining which part or parts were contributing to the fault. In fact it was C10, a 100 f/35v electrolytic but I replaced the complete kit as a precaution. In addition, I fitted a miniature fan kit since Lord Potts was determined to install the receiver in its original 'oven'.
The end result was an excellent picture and sound for Lord Potts and a nice big cheque for me!
This receiver had been "pizzafied". If you've ever left a pizza in the oven for slightly too long, you'll understand exactly what I mean! Anyway, it didn't light up.
I spent some time scraping away what looked like burned, melted cheese from the board then fitted the PSU repair kit. The receiver remained stubbornly dead until I discovered two cracked tracks near the fuse. The power supply board is wedged quite tightly in place and has to be wiggled and twisted for removal. I don't think I had been heavy handed, however, (yuk! aliteration) and assume that someone else had "had a go" before me.
With the receiver now working and giving pictures, I discovered that there were no decoder messages. Another half hour passed while I replaced all the decoder capacitors and other bits supplied in the kit. Usually these kits save me a lot of time but this one didn't seem to work until I heated the decoder board with my hair dryer. My impression was that the board was damp so I scrubbed it with isopropanol then dried it thoroughly with hot air. Finally, since the PTV111 area seemed to be the most senistive to moisture, I dripped hot beeswax around it, the bench and my trousers, leaving nice white stains. (My wife gave me a funny look later on. I can't imagine why they were very old trousers).
Next morning was cold and damp but the receiver worked perfectly.
The fault report from a local dealer was "dead", although the receiver lit up when plugged in. However, my first impression was that the tuner might be dead since I could get nothing but "snow" on most channels. Tuning was tricky because the menus did not appear on screen. Clearly there was more than one fault.
I fitted a second-hand tuner with no effect and scratched my head in confusion. Flicking through the channels produced a few pictures but only on horizontally polarised stations. Aha! The power supply noise was switching my Universal LNB to high band. I'd seen this before and the cure was to replace all the power supply electrolytics. I did, but the fault remained!
Looking at the circuit, I traced the LNB supply to the tuner and noticed that C128, a 100 F, was used to decouple the LNB supply as it reached the tuner. Replacement of this capacitor solved that problem and vertical channels now appeared. However, there were no decoder messages, channel idents or menus. I guessed that there were no sync pulses (yes, I should have used my 'scope but sometimes guessing is quicker if you are right!)
I was right. Replacing the TEA2029C sync separator IC restored normal operation.
Finally, for good measure, I replaced C125 with a 1000 f electrolytic in order to get rid of some horizontal streaking on decoded pictures. Sometimes a lot more capacitors need to be replaced to effect a cure but I was lucky this time.