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:
http://welcome.to/oldtellys
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!