Satellite Workshop 06
A local shop sold an Amstrad satellite system to a customer from
Baslow (a rather posh sort of place). A few weeks later he came into
the shop wanting to buy another dish & LNB. The shop, of course,
sold him one, and away he went. Next of course, was where I became
involved. He rang the shop with a question and they told him to ring
me!.
Anyway, after I had quizzed him for a while, I got all his story. He
has a holiday cottage in France, and had bought the system to use
there. However, he enjoyed it so much, he decided to buy another dish
for his UK home and take the receiver back and forth with him.
So, now knowing his story, I asked what his problem was. He said he
wanted to know how to set his dish up. Being the sort of person I am,
I enquired how he aligned the dish in France.
He replied that, from his cottage in France, he could 'see the
satellite' and merely had to point the dish at it. My next question,
fairly obviously, was what did the satellite look like. He replied
that 'it was a bright light in the sky', but that from England he
couldn't see it.
At this point, I mentioned to him how far away it is, and that it
doesn't have headlights.
Anyway, how lucky can you get, just pick a star at random and point a
dish at it!.
Yesterday, a PRD900 came into the workshop with a curious fault. The
picture was perfect from all Scart sockets but very dim from the RF
output. Tapping the receiver, I noticed, would change the
brightness.
I did all the usual checks around the modulator, looking for broken
joints and cracked tracks. Visual inspection failed to reveal
anything so I resorted to the 'scope. The video level reaching the
modulator section was very low, but why? It was a fairly simple task
to trace the signal back to the buffer transistor, Q105, which feeds
the modulator. This is a surface mount NPN device near the centre of
the board. A good healthy signal going in contrasted with a very low
one coming out of the emitter. Replacing this transistor cured the
fault but I still can't explain why tapping the unit should have had
an effect?
Since then I've had two PRD900 Plus with the same problem. One gave
no video at all from the RF socket and the other would work for a
while then would begin to modulate the picture from bright to dark as
if someone was scrolling through the contrast settings. In each case,
replacing Q105 effected a cure. A call to Pace elicited the
information that this fault had just been flagged in their repairs
database. At present it seems to affect only receivers more than a
year old (how convenient!) which have been installed with no
ventilation. Our summer weather seems to have precipitated the first
failures.
This morning I had a call out from a lady in distress. She couldn't
receive satellite pictures at all and her video recorder wouldn't.
Her TV had been "playing up" or, to be more exact, "giving funny
pictures". Consequently, she'd tried swapping it for the portable but
couldn't figure out how to tune the channels and had swapped it
back.
When I arrived, the aerial coax was connected directly into the back
of the TV and the VCR was connected to the satellite receiver with
two RF leads! A quick swap of the leads got the satellite pictures
back. The TV was an ITT model CT1611/2. When tapped, it would produce
maximum volume and lose vertical hold. A dry joint which will, no
doubt, be found in the workshop later. For the moment I've left the
portable connected.
On Wednesday three weeks ago I had an evening call-out while I was
listening to Eric Wiltsher's "Satellite Workshop" (Astra radio
frequency 7.56 on the JSTV transponder, 8.10pm for those who are
interested). I was a little annoyed because I couldn't very well
listen to it in the car!
The problem was a "dead" Technisat 4000ST receiver which I brought
back to the workshop for investigation. It didn't take long to find
that the mains transformer primary winding was open-circuit. As
Technisat no longer have a UK subsidiary I dusted off my German
dictionary and phoned the factory in Daun. The girl who answered said
that the transformer for that model was obsolete but, if I faxed an
enquiry, she thought that a replacement part might be available (at
least, I think that's what she said!)
I faxed the information in my best schoolboy German and waited for a
reply that didn't arrive. However, a week later my toes were crushed
by a battered jiffy bag which arrived unannounced through the letter
box. It was a transformer, free of charge, with the compliments of
Technisat! Impressed by this service I faxed a letter of thanks.
Inside the jiffy bag was a note explaining that their UK agent is now
J. D. Electronics in Sudbury, Suffolk. I rang Joe at J.D. and he
confirmed that the transformer was now a stock item, together with
Echostar parts. Handy to know.
The gentleman who telephoned last week was happy to let me take away
his Connexions CX95A receiver for repair and I made the mistake of
assuming that he would also be happy to pay for the repair!
However, after I'd ordered and fitted a new decoder board and
telephoned him to offer the good news ("only 75 plus vat"), he became
quite stroppy and insisted that it should not have gone wrong "after
only two years."
Without extended warranty there was little I could do about this. It
was well outside the 12 month warranty and I'd been lucky to get a
brand new Thomson decoder board so cheaply.
Call me stupid but, to try to reduce the cost, I decoded to try to
fix the old decoder. The symptom was the message "Card Invalid" each
time I inserted a valid card. I measured the Vpp 5 volt supply to the
card reader. It would appear for an instant as the card was inserted
but would then drop to zero. This implied to me that the Vpp circuit
was all right but that there was a problem with the card reader
itself. However, it took an hour to track down the culprit - a leaky
5.6 volt zener next to the card holder.
I compromised on a 45 charge - expensive for a zener but it helps to
defray my petrol costs and the fact that I've now got a new, unwanted
Thomson decoder board in stock (fits most Echostar and Palcom
models).
When I returned the receiver to the customer he asked me to install
it back in the cabinet on top of his VCR! I compromised by squeezing
them side by side on the shelf. The arrangement still looked tidy and
allowed far better ventilation. He paid me without a murmur and
seemed grateful.
Although I don't do installations, I'm often called out to sort out
some problems. In this instance, the customer had had his 60cm dish
swapped for an 80cm Lenson Heath with two LNBs on a bracket. The
problem was very sparkly pictures on some Astra channels, plus
picture and sound interference on UK Gold.
The "expert" installer had aligned the dish on 13 degrees East for
"Hot Bird". Ths meant that the signal from Astra was probably about
6dB down. In addition, side-lobe pickup was giving the interference
and increasing the sparklies by swamping the tuner with unwanted
reception of other satellite transmissions.
I realigned the dish directly on Eutelsat at 16 degrees, using just
one LNB on the standard bracket. Then I fitted the Lenson "Multi LNB"
brackets - one on each side of the arm - and screwed the LNBs in
place (a fiddly task requiring ultra-thin fingers!) Setting the dish
like this can reduce losses on each satellite to just 1.5dB.
Moving each LNB along its bracket produced pictures of a sort. The
final alignment required a spectrum analyser to get the best
compromise between maximum signal peaks and minimum noise floor. Most
"experts" use an ordinary signal strength meter which is about as
useful as a piece of wet string for this. If you align for maximum
reading you are certain to get maximum noise, too - the meter can't
tell the difference!
The LNBs had to be twisted slightly off-vertical to match the "skew"
of each satellite and I had to bend the brackets for best readings on
the analyser and to reduce co-satellite interference from side-lobes
which was quite visible on the spectrum.
Once this was done, the pictures from both satellites were nearly
perfect, although I had to warn the customer that he would still see
sparklies in bad weather. There was no need for a Link Budget
calculation to predict that an 80cm dish was going to be a compromise
in Yorkshire!
One of the most recent repairs was a Mimtec Premiere. This arrived,
dead, just as Mimtec in Scotland began their summer fortnight
shutdown! Luckily, Martin Pickering of SatCure, (formerly Technical
Manager at Eurosat Midlands), is offering an on-line help service for
satellite receivers.
I contacted him via Internet at his CompuServe
address:-(INTERNET:100613.2105@compuserve.com)
It transpired that he can supply a power supply repair kit for this
model. Since there are about fourteen components which can fail, it
saved me a lot of time simply to order the kit and follow the
instructions.
Unfortunately, the customer had omitted to send the remote control
so, although the receiver now lit up, I was unable to test it. I sent
it back, knowing full well that it would return. Sure enough, the
following week I received a call from the customer to say that there
were no horizontally polarised channels. Back came the receiver (this
time with remote control!)
It took only a few minutes to trace the problem to a leaky TR4
(BC547) in the LNB supply circuit. I don't know if this is a common
problem but it's worth checking all functions after power supply
failure. The last Mimtec I repaired needed a new power supply and had
also killed off TR16 and TR17 on the decoder interface board. Those
Sorensen power supplies really go bang!
The new replacement power supply is made by Nokia and is said to be
more reliable. Unfortunately, satellite owners simply don't want to
pay and we will inevitably end up repairing the old ones.
*Technisat Customer Service we thank you!