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Nothing to do with Satellite TV but you *must* look at Steve Pendlebury's informative web site about the repair and refurbishment of old TV sets:
This is a sad tale with an unhappy ending so you might want to get your tissues out now. For reasons unknown, the owner of this expensive digital satellite receiver had decided to screw something to the top cover. That would have been all right except that one screw touched the heatsink on the TOP202 power supply I.C. and the receiver died spectacularly.
The mains input filter was completely destroyed and the 1N4007 bridge diode, DS04, was short circuit. Luckily, the mains filter from an old Thomson SVA1 "Sky decoder" fitted perfectly so I replaced the diode, the TOP202 (KS01) and made several checks before standing well back and plugging in.
The power supply now sprang to life giving 5, 12 and 25 volt supplies. However, there was no front panel display and nothing appeared to function. At this point I decided to cut my losses, since I don't have a service manual and I didn't have the slightest chance of locating the cause of the fault. It is quite possible that the power supply, in its death throes, had driven a massive voltage spike through all the I.C.s.
I think that the cost of repairing these digital receivers is going to be very high - assuming it's actually possible. If anyone out there is able to offer a repair service, please let me know!
Arrived with "intermittent blue screen/no signal" report.
D18 had a bad solder joint so I fixed that and left the receiver on test all night. Next day it was still working fine but, by afternoon, I noticed a very pronounced interference pattern on the picture. It took the form of "wavy coloured lines" running down the picture - three or four curves in length and around 20 lines across the screen. Unplugging the receiver then powering up made the interference disappear but it would return as a great number of fainter horizontal lines about 2mm apart. The pattern would change quite suddenly, jumping from almost no interference to the pattern described initially. The interference was present on all channels and could even be seen on the encrypted channels on top of the mess of black and white lines. It was visible from TV and VCR Scarts and, slightly softer, from the RF output.
I located the cause with a hair dryer: it was U3, a 7812 regulator at the front left corner. This already had a 10uF electrolytic strapped directly across two legs. (I had measured the ESR of this capacitor and it was fine). Replacing the capacitor with a 1uF/50v multilayer ceramic cured the fault. These regulators are very prone to parasitic oscillation and it was well known (at least in my design days) that an electrolytic capacitor had the wrong impedance characteristics to damp the oscillation reliably. We used to use tantalum bead capacitors. I imagine that a 1uF (or higher) tantalum rated at 25v (always double-rate tants) would do the job quite nicely but I happened to have the 1uF/50v MLC in stock.
I had to go out to deliver a receiver and, when I returned, I discovered a GRD200 resting inside the porch. It bore part of a cigarette packet with a scribbled note that read:
"Dead. Collect later. Dennis."
Dennis, I should explain, is a local aerial installer who has no concept of thieves. He is so trusting that he will leave his wallet on the bar. Only once has he had anything stolen and he blamed Jackdaws for that!
When I connected mains power the receiver failed to light up. Quite often these Grundig "GRD" models can be fixed simply by replacing the fuse. The design is excellent in that the fuse quite often protects the power supply from mains power spikes by melting. Sure enough, the fuse wire had melted and all other components measured OK. I replaced the fuse and was rewarded by the green standby LED segment. Unfortunately, the receiver was stuck in standby and would not respond to button presses. Fearing the worst, I allowed myself to be fooled into thinking that this fault had been caused by the mains power surge, too. Consequently, I began to replace first the EEprom, then the microcontroller followed by the tuner. All of these are connected by the serial data bus. The job was made more difficult by the fact that Grundig made several batches in different ways. This one had two SDA2526 EEproms with links 603, 604, 609 and 610 fitted. The only pre-programmed EEprom I had was in a scrap GRD200 (cat pee damage) which employed a 24C16 and no link wires.
I also checked for broken tracks near the tuner - common in the earlier GRD150 and GRD250 models - but without success. Even with the parts replaced, the receiver stubbornly refused to come out of standby. I decided that this fault had nothing to do with the melted fuse and that the cause was probably so simple I would kick myself. It's very common for crystals to fail in the GRD models so I replaced the 12MHz crystal next to the microcontroller. (These parts have no silk-screen markings on the PCB). The fault remained. On a whim, I connected my TV to the receiver with a scart cable and was rewarded by a scrambled Sky picture from all three outputs. That was interesting, considering that the unit was supposedly in standby! It should not have been routing video through the STV0030 mounted beneath the board. I placed my finger near this I.C. and the screen went blank. That rang bells in my head. I remember seeing this previously. I replaced 4.433619MHz crystal X3 and applied power. Yippee!
When Dennis returned I said, innocently, "receiver... what receiver?"
"I left it in your porch!"
"Some thieving sod must have pinched it", I grinned.
"No Signal" was the reported symptom. I checked and confirmed that a blue screen bearing those words appeared. The LNB voltage was present and tuning right through the band made no difference. I pressed "F" then "Store" to toggle the blue screen generator off. Now I had a completely blank screen with no evidence of a video signal at all. However, the audio was present and I could get scrambled Sky pictures and flickering German channels from the Decoder scart socket.
I had seen this symptom previously and wasted twenty minutes in guessing, first that the ceramic resonator was faulty, then the 1uF capacitor next to it and, finally, the PTV111 sync separator I.C.
After this, I decided that it would be a lot quicker to trace the cause properly!
A quick 'scope probe on pin 14 of the PTV110 showed that no video signal was reaching the decoder circuit. Q62 is the buffer transistor so I checked the signal on L19, which precedes it in the circuit, and that was fine. Replacement of Q62, a BC846B, cured the fault. So much quicker than guesswork!